- Welcome to the Huberman Lab Podcast, where we discuss science and science-based tools for everyday life. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, my guest is Dr. Susanna Soberg. Dr. Susanna Soberg completed her doctoral thesis work at the Center of Inflammation and Metabolism and the Center for Physical Activity Research at the University of Copenhagen in Denmark.

Her research has focused on how deliberate cold exposure and deliberate heat exposure can be used to enhance human metabolism. She is the first author of a seminal study which discovered the minimum thresholds for deliberate heat and deliberate cold exposure for increasing brown fat thermogenesis, which is essentially a mode of increasing heat production and metabolism in the body, and for establishing actionable protocols that can be used outside of the laboratory to improve metabolism and human health.

Dr. Soberg's research was published in the journal Cell Reports Metabolism in 2021, adding to a long and important history of research focusing on the role of cold and the role of heat in altering various aspects of the body's physiology, including hormone health, metabolism, and changes in neurotransmitters such as dopamine and epinephrine.

In fact, today's discussion with Dr. Soberg focuses on the role of deliberate heat and deliberate cold exposure on metabolism, but it also includes discussion of the effects of cold and heat on things like neurotransmitter production, namely dopamine and epinephrine and norepinephrine, the so-called catecholamines, which strongly impact mood and metabolism.

In addition, Dr. Soberg answers many common questions about deliberate cold and deliberate heat exposure, including, for instance, the difference between cold showers versus cold immersion up to the neck versus total body cold immersion, including whether or not going back and forth between heat and cold changes fundamentally the way that heat and cold impact the metabolism, hormones, and neurotransmitter production.

And we talk about almost every single nuance and variation on deliberate cold and deliberate heat exposure protocols as it relates to the underlying science, in particular, how cold receptors at the level of the skin are impacted versus cold reception and perception at the level of the brain and how all of that impacts systems of the brain and body relating to mental health, physical health, and performance.

Based on her scientific research and academic training, as well as her understanding and use of deliberate heat and deliberate cold exposure protocols, Dr. Soberg is considered one of the world's leading experts on these topics. In fact, she is the author of a recent book entitled "Winter Swimming," which is, I have to say, a terrific book because it breaks down chapter by chapter the different aspects of deliberate heat and deliberate cold into its various constituent parts, including cold acclimatization, the cold shock response, dangers and safeties of cold water, the impact of cold and the impact of heat on various aspects of human health, as well as specifics relating to sauna versus ice versus cold swimming, showers, et cetera.

It's a very thorough read and a very easy and accessible read that if you are interested in deliberate cold or deliberate heat exposure or both, will allow you to embrace those protocols with the greatest degree of confidence that you're going to obtain the specific endpoints that you're interested in and to do so safely.

Before we begin, I'd like to emphasize that this podcast is separate from my teaching and research roles at Stanford. It is, however, part of my desire and effort to bring zero cost to consumer information about science and science-related tools to the general public. In keeping with that theme, I'd like to thank the sponsors of today's podcast.

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Once again, that's hubermanlab.com/tour and use the code HUBERMAN to access tickets. I hope to see you there. And now for my discussion with Dr. Susanna Soberg. Dr. Susanna Soberg, welcome. - Thank you. - So great to have you here. I feel like I should give a little bit of the backstory of how we got connected, which was that for many years, I've been interested in cold thermogenesis.

It was the topic of my senior thesis in college. And I've, of course, followed the popularity of Wim Hof. And we've had Dr. Craig Heller, my colleague from biology department at Stanford who works on cold and its impact on physiology and sports performance. So for a long time, I've been interested in this area, but there's been a real lack of new, let's say high profile quality scientific information in terms of how, for instance, cold plunges and sauna, how that impacts human physiology.

I know there's been some information out there, but it's been sort of scattered. And then a little over a year ago, I see this paper in Cell Reports Medicine and was immediately struck the, first of all, the fact that it was in Cell Reports Medicine, I've been on the Cell Press editorial board for a long time now.

So press journals are, of course, phenomenal journals. And the title and the content of the paper was directly in line with the sorts of practices that people are very curious about and then are starting to emerge, things like sauna, cold plunges. And there was your name first on the author list.

And I reached out to you through social media and we've done a little bit of live content there together. And I've been tracking what you've been doing in the world in terms of your book and talking about the results in your manuscript and talking about the science and impact of deliberate cold exposure and sauna.

And I have to say that it's been a wonderful and remarkable thing to see. And you're bringing so much quality information about this area that for a long time, I think was kind of niche and is now becoming more and more mainstream. So I'm going to start off with a thank you for being here and a thank you for the work that you've done.

And I'm looking forward to talking to you about it today. So my first question to get things started is what is happening when we get into an uncomfortably cold environment? So for instance, if I'm really hot on a hot day, jumping into a cold pool feels really good. But if I'm already kind of at room temperature, I'm a little bit chilly, getting into that same temperature of water doesn't feel so good, right?

There's a shock there. So if you could just walk us through what happens when we get into uncomfortably cold water, whether or not it's by way of shower or cold plunge at the level of our physiology and if you'd like our psychology, I think that's a good place for us to start because I think it will orient people to their own experience if they do that.

And for those that haven't done it might start to peel back some of the layers as to what the underlying mechanisms of cold are. - Yeah, thank you for that question. It's really good to just address what actually happens in our physiology when we get cold. And you can get cold in many ways.

So you can just head out for the one that gives you the most potent stressor, which is submerging into cold water. And, but you could also go in outside in the cold wind. That's also gonna activate your sympathetic nervous system. So get all these neurotransmitter going in your body and so your catecholamines.

Let's just address that we are taking a cold plunge, for example. So if you are very hot, for example, before you go into the cold water, it's gonna feel less stressful, but the temperature difference from your skin to the cold is definitely gonna give you a shock, but your core temperature is warmer and that's gonna feel a little bit better.

So that's why when people go into a sauna, for example, and go out then into the cold water, they can do it easier than if they were cold beforehand. - Could I just ask you a few questions? So you mentioned the sympathetic nervous system, which for people listening who aren't familiar with that is that the branch of our nervous system that's responsible for creating accelerations in heart rates, feelings of alertness, it's accompanied with stress and the stress response, but it's accompanied with waking up in the morning for that matter.

So it's not always about stress. And then you mentioned the catecholamines, which are dopamine, epinephrine, and norepinephrine. So maybe a little bit later, we'll talk about those individual neurotransmitters, but you raise a really important point, which is something I get asked about a lot for people that are curious about using deliberate cold exposure, which is how cold should the water be?

And I know it's very hard to give a straight prescription for that because I think it boils down to what you just said, which is it's really the difference between your current temperature and really the temperature of the surface of your skin and the temperature of the water. So if you're very warm, getting into cold feels good.

If you're already cold, getting into more cold feels stressful. Is there any way that we can start to gauge what is the best way to approach a deliberate cold exposure protocol? I mean, should it feel uncomfortable? And that leads into the question of how do we balance the discomfort with the amount of time that we spend in?

So for instance, if it's just a little bit uncomfortable, will spending more time in the cold get us the same benefit as getting into very uncomfortably cold water for a very short period of time? - Yeah, it's really good question. And I definitely think that this could be future studies on this as well to really unravel what kind of protocols are the best way or also for which outcomes, of course.

So if the temperature is very cold and you also feel very cold, then you should stay in the water a little bit longer. So I think it's just, you should get uncomfortable cold. So as long as you get uncomfortable cold, it's cold enough and you get this, what we call the cold shock.

So the cold shock is activation of your sympathetic nervous system and these activation of the catechola mines, which you just mentioned before. - Does the shock mean that I'm having trouble controlling my breathing? Is that a good gauge? - Yeah, you can say so because that's kind of like how we define it.

So you hyperventilate, so you have a faster breathing rate. So that increases also because you activate your gasping reflex if you are new to this, but if you are adapted, it kind of subsides with time with adaptation. So what you can do is that you can train this cold exposure and you can kind of like get adapted to it.

So you don't have this hyperventilating response every time you go out in the cold water. So this is like building up your resilience, building up your adaptation. It's gonna make the shock like subside a bit. So it's always harder in the beginning, but you should do hard things, right?

It's not something that we, you shouldn't think about cold water and cold water immersion as something that is comfortable. It should be hard because that's the point of it, right? If you enjoy it, then yeah, then I'm thinking something is wrong. It's not right. You should not enjoy it.

- Well, this is an important point that you're making because I think that many people shy away from deliberate cold exposure because it's uncomfortable in a way that at least from my experience is very different than the discomfort of exercise. Because with exercise, for instance, if running fast and breathing hard is uncomfortable, you can slow down or walk.

If lifting weights is uncomfortable, you can remove some weight or reduce the number of repetitions or stop. With deliberate cold exposure, I suppose you can be sort of halfway in, halfway out of the water or partially underneath the cold shower, but it's very hard to titrate and adjust the level.

It's kind of all or none. And I've seen, I should just, I can tell this by anecdote. I've done some work with military special operations. I won't say which country. This was outside the US. And these are very tough individuals. They're used to going without sleep and doing hard, high consequence, high risk kind of work.

And they were asked to do some cold water exposure training. And I was there that day and it was remarkable. About a third of them just went straight in and just kind of grinded through it. They looked stoic anyway, to me. There were a few whimpers, no cries. About a third talked a lot and got really, you could tell that they were agitated and anxious, but they made it through.

And then about a third of them just simply would not get in past their knees or thighs. Were just, it seemed like they were just dreading the whole experience. Some actually didn't actually go in completely, which was really surprising to me. And you couldn't tell based on their physical appearance or anything else about them.

They're all high performers as to who would have this response. So it seems like people vary tremendously in terms of their ability to embrace the discomfort of the cold. Is that, from your studies, is that your experience as well? Or are there these weird mutants who seem to just love going into the cold for the first time?

- So some people just feel better in the cold and some people dread the cold even more. And you can say the more people are pushing the cold away, they might feel the cold pain even more. So they would definitely, people who are maybe the soldiers you just talked about, some of them might be already adapted to the cold.

So if they are not scared of the cold, they go out and they embrace the cold in a better way. It could also be that some people have a more sensitive nervous system. And when you are a bit sensitive to the cold, you will, of course, try to get away from it, right?

And you will also have the cold pain more, feel the cold pain more if you avoid it. So the more you avoid the cold, the more painful it will feel when you go into it, so yeah. - You mentioned being outside in a t-shirt versus cold immersion up to the neck versus shower.

I think this is something a lot of people wonder about. What are the differences in terms of impact, short-term and perhaps even long-term, between cold showers, cold plunge to the neck, so that could be an ice water or just very cold water, immersion with dunking one's head and then coming up, because obviously people have to come up for air at some point, and then simply being outside on a cold day in shorts and a t-shirt or something of that sort.

- So there are different outcomes because they are very different exposures of the cold to your cold receptors in your skin. So the more you can say you cover your body in the cold, which you would do in cold water, because they're, of course, covered totally and the molecules are closer to your skin, you will have a more potent activation of all your cold receptors in the skin.

So that one will definitely activate your autonomous nervous system more and rapid compared to going out in a t-shirt in the cold wind, just go for a walk, but that is also something that's gonna activate your sympathetic nervous system, meaning that you will have an increase in norepinephrine and you will activate something called the brown fat.

So this is a healthy kind of fat tissue that we have in our body, and when you activate that, that's gonna increase your metabolism. - Before we talk about brown fat, and I'm so glad you brought it up because there's so much to talk about there, what about cold shower?

I mean, obviously cold shower is somewhere in between being outside in the air, cold air, versus being immersed up to the neck. - If we had more studies on cold showers, we would learn more about how does that activate our metabolism, how does that increase our neurotransmitters in the brain, which could also have an impact on our mental balance.

So I think that would be interesting for the future. But what we do know is from activating brown fat and both from rodent studies, but also in humans, is that as soon as we get cold on our skin, we will activate our brown fat. So it is kind of like our first responder in the body to keep our temperature up.

So our muscles is like the second tissue in our body. We have two tissues which can increase our thermogenesis. So the brown fat, which is like always like temperature regulating our body, and then we have the muscles, which will secondarily start to shiver, and that's gonna increase our temperature in the body.

But as soon as you go into a cold shower, you will activate your brown fat also immediately. So it could be good also for increasing metabolism, in theory, because we haven't really any studies showing how much does it actually activate the brown fat. So if someone out there wants to do a study on that, that would be great.

- I've thought about why there are fewer studies of cold showers than cold immersion. And I think the answer to my mind is that from a methodological standpoint, it's just harder to do because if people are getting into cold water up to the neck, they're getting into cold water up to the neck.

Whereas if people are getting into a cold shower, some people are larger or smaller. Some people are gonna stand under the shower with it hitting their head. Some people, the back of the neck. You could direct people to do it, but it's a little bit more difficult. Also, I think for, you and I are both research scientists, there's a little bit of a methodological challenge that might seem silly to people, but it's a real one, which is if people are in a cold shower, also the water is going to be, I'm kind of pushing their clothing against their skin.

There's a certain vulnerability for most people coming to a laboratory in the first place, let alone being absorbed while they shower. Whereas when you get into cold immersion, you're getting under the water. And some people might roll their eyes and say, "Okay, really, is that the barrier?" But science exists in these real world contexts.

And this will vary by culture and things of that sort, but we run human subjects in my lab. And I'll tell you just the process of getting people to the laboratory and having them park and find the lab. And it's a whole new environment with people in lab coats and people moving around and where's the restroom?

I mean, there's a certain amount of stress just associated with taking part in a study for most human subjects. So I totally agree, however, we need more studies of cold showers. It's just a harder environment to control in my mind. So it sounds like any form of cold to the skin that people register as what you call the cold shock or uncomfortable, like, oh, like this is kind of jarring, activates the brown fat.

Do we know what the pathway is from cold receptors on the skin to the brown fat? I mean, how does the brown fat know that we're cold? - Yeah, really good question. And it seems that I think that, of course, in the future, we will know much more about these pathways.

But what we do know is that the co-receptors will send a signal to our temperature regulating center in the brain, so hypothalamus, and that's going to be taking in this message. And we have so many co-receptors in the skin. So it's going to be very fast, as you can say, if you immerse the body into cold water, this is going to be so rapid.

So it will have a rapid increase in neurotransmitters in the brain to no adrenaline, adrenaline and cortisol, which is not that much, but it's still there. So you have this increase in no adrenaline, which will then immediately activate the brown fat because you can say the activator is the most potent one, cold and no adrenaline, that's going to activate the brown fat.

But there's also a direct pathway from the co-receptors in the skin to the brown fat, which really shows that if, because of these different pathways, it shows that it could be that this tissue to keep us warm was developed in our evolvement as humans to keep us warm and to save us whenever the temperature on our skin varies just a little bit to keep us in that right homeostatic balance so we don't get hypothermic, but also so we don't get hyperthermic, but because it seems that the brown fat is also activated when we get warmer in our skin.

So it's also maybe a temperature regulator in our body, but the pathways is different. I think it's also a third pathway from directly from the muscles. So the brown fat is also activated even though the muscles are starting to shiver. So there's an extra pathway that way to keep our temperature up.

So muscles and brown fat are working together to keep us warm so we don't suffer too much in the cold water. - That's super interesting. And what I hear you pointing to is the existence of three parallel pathways. And this notion of parallel pathways comes up over and over again in biology, as you and I know.

And I think it's important for people to know about because as you said so eloquently, when something is very important to our survival and/or evolution, the brain and body install multiple mechanisms for it, not just one. And so it sounds like it's cold skin, cold on the skin triggers a response in the hypothalamus, which then activates brown fat, cold receptors in the skin directly to the brown fat, and then shivering in the muscle to the brown fat.

I want to talk about brown fat in depth and learn from you more about brown fat. Before that, however, I want to ask about shiver. I've heard that shiver causes the release of succinate, which then activates the brown fat. Is it known whether or not inducing shiver is important?

And when should people shiver? I mean, I've gotten into cold plunges and shivered while I was in there. And then I've also had the experience of getting into a cold plunge or a cold shower, then getting out, or even standing outside on a warm day after swimming in a pool and then starting to shiver.

So the shiver comes later. So how important is shiver and does it matter when shiver happens? - Yeah, well, shivering is good because that increases your metabolism and that's going to burn some calories in your body. You shouldn't be so afraid of shivering, I think, because the shivering, as long as you don't get too hypothermic, so if you don't sit in the cold water for too long, and what you just said by shivering after you get up, that is because of the after drop.

Something called the after drop is when your core temperature decreases even after you get out of the cold water. And it always does that, your body, because as soon as you get into the cold water, all your blood vessels is going to constrict because you need to keep your blood in your core and keep your vital organs warm.

So as soon as you get up, those blood vessels will open again and the warm blood will flow out and get colder and then flow back again into the core. And that's going to decrease the temperature in your core, of course. - So that's the drop. - So that's the drop, yeah, after drop.

- Got it, oh, I'm so glad you explained that. I've heard, years ago, Wim Hof, I heard him talk about the drop and I've heard colleagues of mine talk about the drop, but that's the first time I've ever heard it explained clearly. Let me make sure I understand this.

So I get into cold water, obviously, I'm cold. Vessels constrict to keep blood near the center of my body, keep me alive. I get out, the warming up of my body allows those vessels and capillaries to dilate again. The blood goes out to the surface, but the surface is still cold.

And so that blood is cooled and then my core body temperature drops. And that's what you're referring to as the drop. And that's what induces shiver. - Exactly. - Great, and then, am I right in thinking that then the shiver activates brown fat, which then warms me up again?

- Yes. - Got it. - That's why you should end on the cold. We can get back to that. - Yeah, let's talk about, yes. Ending on cold is, you know, it's what I refer to as, and what has now become known as the Soberg principle, which is a really important principle about the importance of ending on cold and not doing what I do, which is to get into a hot shower or back in the sauna.

But we'll get back to that in a few minutes. So that's wonderful that you can explain that so clearly, because I think that shiver is something that a lot of people do avoid. People think, oh, I don't want the chattering of the teeth. And it feels like a loss of bodily control, which really it is.

It's an autonomic response. - Yeah, but I don't think that people should avoid it that much. It's just like seeing shivering as a way of your body in, like it's training. It's training for all your cells. It's training for your muscles. It's training of your metabolism. And that's gonna increase your, what's called the insulin sensitivity.

So if you can like, in your mind, get used to the thought of shivering is just like when you go exercising in the training center and get that feeling of like, oh, this is tough. Now it hurts a little bit. Yeah, it's gonna hurt because that's what shivering also does, but it's just a different way of training your cells and your body.

It's gonna create what is healthy stress. It's called homeosis in the cells. And the more you expose your muscle cells or your brown fat cells to these kind of like healthy stresses, exercise, cold, and heat exposure, it's gonna make them better at like activating and also at keeping you healthy.

So as long as the cells get exposed to this, it's gonna keep them on its toes, you can say, because it becomes more robust, increasing these heat shock proteins and cold shock proteins in the cells to make you more robust for the next time. And that is also what happens when you go to the training center.

And I keep like drawing that parallel because people today know more about, we know more about exercise and what that is gonna do to your muscle cells. But the same kind of like training is also what you do when you go out into the cold water and submerge into cold water, because that is just your cold training center, you can say that.

And also your heat training center going into the sauna because the cells are getting stronger with hermetic stress. So it's the same process, just different practices. - I'm so glad that you brought up the fact that the discomfort or the embarrassment or both of shiver is still crucial to actually to reach for and try and experience the same way that with exercise, I think a lot of people don't realize this, but when we did our series with Dr.

Andy Galpin, it became clear to me what should have already been clear to me. And I think that most people don't realize, which is that if we were to measure heart rate, blood pressure, stress hormones, and inflammation in a human being during exercise, it would look as if they were ready to die.

Blood pressure would be high, inflammation is through the roof, but all of that is setting in motion and adaptation or set of adaptations that allow blood pressure to be lower at rest, that allow inflammation markers to be lower at rest, all the things that everybody is seeking with exercise.

In addition to, of course, the aesthetic changes that people are seeking with exercise. It sounds like the exact same things are happening with the cold. So the redundant message here seems to be that the more discomfort, provided it's done safely, just like with exercise, the more shivering, the more cold shock, provided it's not to the extreme and stop somebody's heart, right?

We can talk about thresholds for that a little bit later. It sounds like all of that is going to set in motion some long-term changes that will make people feel better and will improve health. Could you just touch on a few of the longer-term changes that are known to occur?

I mean, I'm well aware of the study showing that, I think it was European Journal of Physiology, it was the European Journal of Physiology, showing long-lasting increases in catecholamines, dopamine, norepinephrine, and epinephrine for many hours after deliberate cold exposure. What are some of the other things that happen at the level of metabolism and brown fat in, let's say, the hours and day after a deliberate cold exposure?

- As soon as you go in, of course, there's an activation, but it seems like, no, you're asking for the later outcomes like blood pressure and stuff like that. Is that what you mean? - Yeah, blood pressure, but also in terms of metabolism. I know that in your study, and we'll talk about brown fat in depth here in a moment, but that there were changes to the brown fat that equate to changes in, for instance, people's ability to be comfortable in colder environments when they're not doing deliberate cold exposure, or in the same way that I can exercise on an exercise bike or go out for a hard run, but then if I go hiking with the family on Sunday and it's a steep climb, I could do that steep climb more easily because I'm quote unquote fit as a consequence of the exercise.

What are some of the fitness adaptations of deliberate cold exposure? - Yeah, so what happens is that you get adapted a little bit every time you go. So you will, like exercise, get a little bit stronger. So every time you go into the cold water for every time, you will be more exposed to it.

You will feel more comfortable in the cold. So you're gonna build your adaptation, which happens on a metabolic level, which is gonna be the brown fat. So we have more activation of your brown fat. The mitochondria in the brown fat cells are gonna be, you have more of those, and they will be more efficient at heating you up because the body expects you to do this again.

So you are prepared in a way. The capillaries in your skin will also become better at like constricting. So you will have a better shield of your body to prepare you for the next time. So you will become better at going into the cold water in that way. So the body makes these mechanism and changes your body in a way so you can expose yourself to the next time, right?

And also you will have also your stress response will also be subside a bit. So you will have a less increase of your catecholamines with time. With time also you have, because of this activation of your brown fat or your muscles you will have an increase in your metabolism, which will then make your insulin sensitivity better.

And this is shown in studies, for example, there was this interesting study I found just before I started my PhD, which was from Gieber Stomer et al from 2016, where they measured metabolism not on brown fat, but they measured insulin sensitivity in middle-aged men and women during one winter swimming season.

So they were not very young like they were in my study, but they were middle-aged. And I think this is very interesting. So during these four or five months, they were winter swimming, they saw that they had a lower blood pressure after the season and they had a lower heart rate.

And they also saw that they have better insulin sensitivity. And I think that is very interesting because if you can have a better insulin sensitivity, you can prevent lifestyle diseases. So, and with lower blood pressure, which is a very strong outcome also for telling how much inflammation you have in the body.

And because it didn't measure brown fat, I figured that it could be, that was the missing link. That was one of the explanations to why we see this less inflammation in the body. So the longer outcomes, the long-term outcomes could be that you lower your blood pressure and have a lower heart rate.

You also have a better insulin sensitivity and a better glucose balance, but that is shown in my study. I'd like to take a quick break and acknowledge one of our sponsors, Athletic Greens. Athletic Greens, now called AG1, is a vitamin mineral probiotic drink that covers all of your foundational nutritional needs.

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Again, that's athleticgreens.com/huberman to get the five free travel packs and the year's supply of vitamin D3K2. - And we'll get back to the insulin sensitivity and glucose balance. That's an impressive list of benefits. Blood pressure, of course, most people are aware of blood pressure and what it is. It's what they measure when we go to the doctor.

And it's not very sexy nowadays, you know, blood pressure, people go, oh, you know, blood pressure. It's not, you know, people want to hear about the inflammatome and the microbiome and all of that stuff is really interesting. But I think that blood pressure doesn't get enough attention. And we have spoken to, on this podcast, to Dr.

Peter Attia, who is an expert in longevity and healthspan and things of that sort. And I was surprised to learn, again, I shouldn't have been surprised, that the number one reason people die, worldwide, is cerebral vascular disease and cardiovascular disease. And there are basically three things on the list of things to address.

One is not smoking, or vaping, by the way, not smoking. There are a few other things related to blood markers, ApoB and things of that sort. But then the big one is blood pressure. And so it's interesting because we don't think about blood pressure that much anymore as the kind of people interested in health optimization and health, but blood pressure is so vital to control.

So it's wonderful to hear that deliberate cold exposure is one way to control blood pressure, I'm guessing in concert with other forms of exercise. Let's talk about brown fat. And if you're willing, I'd love to drill into brown fat at a deep level. Again, my understanding of this is far more elementary than yours, obviously.

You're the expert. My understanding about brown fat is that it's located in specific areas of our body, maybe more widespread than when I learned in school. I thought it was, I was taught it was just at the clavicles in the back of the neck and upper back, but who knows?

I learned that there's more of it when we're children, maybe more distributed throughout our body, and that it's rich in mitochondria. But what is so special about the brown fat? Like if we could just go into the biology of brown fat a little bit, what does it look like?

You've measured it in human subjects. Where is it distributed really? Can it expand its distribution? Can we activate and expand the amount of brown fat as adults? And for those of you that are cringing already thinking we're talking about getting fatter, it's quite the opposite. We're talking about not subcutaneous fat, but fat located around the organs.

But please educate me, tell me where I'm wrong, and expand my knowledge on brown fat. - Okay, yeah, you are not wrong, but it's true that there are more locations of the brown fat than we previously thought. There's this very nice study from 2017 by Leitner et al. where they had made these pettities overlays of their subjects, where they could see where in the body do we have brown fat, and where can we grow more brown fat, so to say.

So the brown fat is very plastic, so it means that it can grow and it can decrease. And this is proven in studies where we have seen people with a failed cryocytoma, it's like a very specific cancer type, where from the '70s, where we can see that if they have this specific kind of cancer type, they have this tumor on the adrenal gland, so they have like a huge increase in noradrenaline.

And because of that, they have this continuous activation of the brown fat, and they have grown a lot of brown fat in the whole body, I don't know where it's located in these six different places, but it is like very much compared to like normal people. And what they then see, what we learned from this study is that brown fat can apparently grow if you have an increase in noradrenaline in the body.

It's not like you want that because when that happens, you have a high blood pressure, you don't want it chronically, right? You just want it on like a short amount of time, and then it can grow for a bit, but you don't want it chronically, of course not, but because it activates also your sympathetic nervous system so they have also showed they have high blood pressure, they had, they lost a lot of weight, of course, because this is activating your metabolism.

So they found luckily that when they removed this benign tumor, that the brown fat decreases again to normal size and they gained weight again, and they had normal blood pressure. So the story ends well, but it's kind of like proof of concept of the brown fat can actually grow.

So it's plastic in its way of like it can grow and it could decrease again. So that's very good, good studies to see what the body is capable of. But we don't, of course, want all that brown fat. We just want it to be, we just want to keep it actually and keep it activated because what we see in studies is also that after the age of 40, people, studies have shown that there is an association with having less brown fat, but increased obesity.

So of course we don't know yet whether brown fat decreases with AIDS and therefore we get obese or we get obese and therefore we have less brown fat, but as brown fat is an insulin sensitive organ in our body and we get obese, just like the muscles get less sensitive, insulin sensitive, the brown fat does as well, and therefore it maybe decreases.

It could be a theory that I think could be one of the reasons why we don't see that much brown fat in elderly people. Some have a lot, especially people working outside. There are studies showing this. - People who- - Who work outside. - Do physical work outside, farmers and, interesting.

- Yeah, they expose themselves to it, so they'll just keep it in that way. - And I suppose we should clarify for people in case they don't know that insulin sensitivity is a very good thing, you want that. You want your cells to be sensitive to insulin. Insulin insensitivity is type two diabetes and is associated with obesity.

So just a point of clarification there. Yeah, it's interesting to me, I usually work out at home, but I go to a gym once or twice a week if I can, because it's good if I see the outside world. And there are a few individuals at the gym who, they're not particularly large or muscular, but they are incredibly lean and their posture is great, presumably from the musculoskeletal work, and they're in their 70s and 80s.

I mean, it's remarkable, right? And I know all the telltale signs of hormone augmentation. I'm very good at spotting that. There are a few telltale signs. I've talked about this on other podcasts, and that's not why they're fit. They're clearly of that look, and you see this outside the gym too, of course, for people that look like they've done a lot of physical labor their whole life.

They're just moving a lot. They have strong hands and features, and they're not necessarily excessively lean, but you can tell that they've been using their musculoskeletal system. And I like to talk to these people and ask them like, not what are you doing now for your workout, but what did you grow up doing?

And I would say, and obviously I haven't run statistics on this, but more than 75% of them respond that they grew up on a farm or that they did some sort of manual labor or were a postman or a postwoman or doing something where they moved a lot for their early years and throughout middle age.

And most of them are now in retirement, but some of them are still working and they all still moving a lot. So the relationship between shiver and brown fat makes sense to me. But is it the case that as we're just moving around, I've heard of NEAT, non-exercise induced thermogenesis.

So if we're just moving around that we are activating brown fat, or does there need to be this stressor? Does there need to be shiver and a cold stimulus or a heat stimulus to activate the brown fat? In other words, is just staying active enough or do we need to do some sort of temperature shock type thing like deliberate cold exposure?

- Yeah, I think that is really good question because how, also why do we have this tissue? Then if it has to be extreme, then you can question what do we need this tissue for? But it seems that you can activate the brown fat with just a little bit of exposure to cold.

So cold is the most potent stressor activator of our brown fat because it's our temperature regulating organ in our body. So first responder to that. So the muscles will be a little bit too late and therefore we have maybe these two kinds of tissues. So actually just exposing yourself or hand actually just to cold water.

So studies have shown that if you just put your hand in cold water, not that you're gonna do that all day or every day or anything, it's not something you have to do, but it just shows that you can activate your brown fat just by getting a temperature change on your skin.

So you can go outside and t-shirt. That's why also we were just talking about, well, people who works outside or move a lot or get out in and out of it, like changing the temperature of their body all the time, they will have more brown fat and activating that is gonna keep your metabolism higher and your insulin sensitivity.

The study has also shown this. So the brown fat can be activated as soon as you just change your temperature in the skin. So going outside in a t-shirt, wearing cooling vests, also studies have shown this for 10 days, it's gonna also grow your brown fat. So you can get more brown fat if you expose yourself to the cold.

You don't have to start in a cold shower. You don't have to start in a cold plunge if you're not really ready for that yet, but just exposing yourself to the cold wind has also shown to activate your brown fat. Or if you don't wanna be like in this awake state, then you can also just sleep in the cold and you won't notice it that much maybe, but studies have shown that if you sleep in 19 degrees Celsius, then you will activate your brown fat and you will grow your brown fat.

So you have more of it. So this very nice studies from Hansen et al. from 2017 showed that a group of subjects who slept in a room at 24 degrees, and then they made this PET/CT scanning to see how much brown fat do they have from the beginning, so what we call baseline.

Then they measured again after a month of sleeping in 19 degrees, and they saw, I think it's remarkable, just one month at 19 degrees sleeping there. They had a duvet on, and they were still had clothes on when they were sleeping. - So they're under a cover, under a duvet?

- Yeah, under a duvet, yeah, yeah. The subjects were sleeping at 19 degrees for one month, had increased insulin sensitivity. The next month, they slept at 24 degrees, they measured this again, and they had decreased actually a little bit, and then they slept at 27 degrees. So quite warm room actually for the fourth month, and they saw even less activation of the brown fat and also insulin sensitivity.

So it seems that you can expose yourself and pretty rapidly the brown fat will respond to this because it's so sensitive to noradrenaline, right? So if you keep exposing yourself to a little bit of cold, you will also get a little bit adapted to it, but that's because the brown fat has grown these more mitochondria in the cells.

So these small energy fabrics that's gonna activate the cells, and that's gonna take up glucose and fat from the, fatty acids from the bloodstream to keep the thermogenesis up. And that's gonna clear up some sugar, and it's gonna clear up so in the bloodstream and some fat as well.

So the brown fat can in that way, decrease our unhealthy fat, which is the white fat. And the white fat is what we don't want too much of, but we still need some, of course. And it's our energy storage. So it's very important that it's there. We just don't need a lot of it.

So on our thighs and also around our inner organs, that's where it's located. So if we can have activation of the brown fat just by going out in the cold and just by sleeping in a cold room, or if you have courage for it, you can go out and expose yourself in a cold plunge.

Cold showers is also gonna do the trick. So you can do different variations of this. Just exposing yourself to various temperatures is gonna activate the brown fat because it was involved to keep us in a perfect homeostatic balance regarding temperature. So to keep us alive. - Incredible. I wanna just get a clarification around this 19 degrees Celsius room that they're sleeping in.

So they're under a comforter, a duvet, and you mentioned they had clothes on. The room is 19 degrees Celsius, but the temperature underneath their blanket might not be 19 degrees Celsius. So presumably it's the cold on their face that's activating the increase in brown fat that was observed. Is that a reasonable expectation?

- I think so, yeah. Because you have so many co-receptors in your face. So actually it's enough. And I think it corresponds very well with the studies showing that you can activate the brown fat just by putting a hand into a bucket of cold water. And I did this experiment myself in my studies just to see how well did they respond to cold water.

So it was four degrees Celsius cold water for four minutes. And then I just measured blood pressure and heart rate to see do they have like an activation of this. I actually also measured the brown fat during this cold exposure for four minutes with an infrared thermography camera to see can I see that the brown fat is activated.

And just to go back to the location of the brown fat. So usually you cannot really see activation of your brown fat because it's located centrally in your, around your central nervous system. And the biggest depot, as you mentioned before, is up here under the clavicular bones. So, and very close to the skin surface.

And because it's so close to the skin surface, I could measure it with this very expensive camera. And it's not very feasible for people to go home and do this. Don't because it takes a lot of practice, I can tell. But we measured the brown fat with this. And I could see that after a few minutes that the activation was there an increase in temperature arose from that activation, just four minutes.

So it's very rapid. And I'm also measured in my study, how deep was the brown fat under your skin? So it's very close to the surface, which also shows that it needs to be there to heat you up and heat your inner organs. - Well, I'm delighted to hear all of this and I'll tell you why.

One is by way of anecdote. I mentioned a little bit earlier that as an undergraduate I worked in a lab that studied thermogenesis and we were doing that in animals, but we had this room that was very cold. The whole room was cold. The guy who I worked for at the time, and I'm Harry Carlisle is a very accomplished physiologist.

He came from this lineage. I don't know if this literature is still discussed much, but it's a beautiful literature from Rothwell and Stock. They were the ones who discovered non-exercise induced thermogenesis. The fact that people bounce, who bounce their legs a lot and move around a lot and have a lot of kind of stochastic movement burn up to 1,800 calories more per day than people who sit more still.

- Fascinating. - Incredible. Just incredible. I don't think that work gets as much attention as it deserves. Published in journals like Nature, so very fine journals. But in any event, one of the things that I noticed when I started working in that laboratory was that I was cold because the room was cold.

And Dr. Carlisle, Harry, said, "Well, the key is to wear a t-shirt in here "for about two or three days and then you will cold adapt." I thought, well, wouldn't I want to put on a hoodie and get warm in there so I was comfortable? And he said, "No, actually what you want to do "is get yourself uncomfortably cold, "activate your brown fat." And indeed, when I did that, I think it was just two days of being in that cold environment.

Then I could come back on the third day and be perfectly comfortable because the brown fat had expanded or added mitochondria or both. And I was perfectly comfortable in that environment. I also got very, very lean in those days and weeks. Now, I've never been somebody who's very lean, nor am I somebody who carries a lot of excess adipose tissue.

I'm kind of somewhere in the middle. I'm sure I could adjust that with feeding if I want to. But it was striking what a powerful effect it had on my entire system of thermal regulation. And one of the things that I also delighted in when Cell Reports Medicine published your study is they had an accompanying press release that went out to those of us that receive press releases.

And it described a saying in Scandinavia, which is essentially, I'm not going to attempt to speak Danish, even though I have much of my family is in Denmark, believe it or not, from Denmark. We have a lot of Danes in my family. I won't embarrass myself by trying to speak Danish as I did before the microphones were rolling.

But that there's a saying that I think essentially translates to in the fall, when you're approaching winter, you want to actually wear fewer layers, not bundle up when you go outside so that you can prepare yourself for the cold of winter and be able to heat yourself up using your brown fat.

And that in the spring, as the temperatures are warming, rather than removing layers, you want to wear more layers in order to be a little bit uncomfortably warm so that in the heat of the summer, you're better at cooling your body. Do I have that right? And maybe, do you know the saying, and would you be willing to share it?

Only the Swedes and Danes will be able to understand. Maybe the Norwegians too. If you don't know it, that's okay. - Yeah, so I know the concept of it because we say it, you should wear less before winter and more before summer. - Oh, well, there it is in English, so it doesn't have to be esoteric, but okay.

- Yeah, and you're completely right. And I think this is just something that we know in the Scandinavian countries. I think that we intuitively know this, but if we just go back a little bit in history, I think that around the 1950s, the Russian government went out and said, well, we should do something about the tuberculosis and pandemic or epidemic, the worst at this time.

So they wanted to have the people be more resilient to the cold and also increase our immune system. So in Scandinavia, and actually also in Russia, we put our babies outside to sleep in the prom. And that is like to also to get more resistant to the cold, but also to increase our immune system.

And we still do that in Denmark. So we- - Do you really? - Yeah, we do. - Babies are taken out in the cold? - In the snow, in frosty rain, everything. My two boys have been sleeping out in winter of at least their first many three, four, five years, because it's like very good for them.

And they get a better immune system and get resilient to the cold so they will have less colds. And also they run around in a t-shirt when it's super cold because they have activated all the brown fat. I didn't understand at that time, I must say, but I kind of like intuitively also knew because we have inherited this way of doing things with our culture.

So, and I have heard people coming from the US saying, Danes are crazy. They put the babies outside in proms and leave them there. And then they go inside and drink coffee on the cafe. - Well, I don't think Danes are crazy. I adore the Danes, they're amazing culture and people.

I'm so fortunate to have family members from Denmark, but I did notice. So when we were in Copenhagen, and I know we saw you there not long ago, that was June, the water in the harbor was cold, even though the Pacific is close to here, which is very cold, it felt pretty cold, but it was summertime-ish.

So people were in summertime mode, right? T-shirts and shorts and things of that sort. But it did strike me that people in Copenhagen are dramatically fitter than they are in the United States. I mean, first of all, everyone's bicycling everywhere. - Yeah. - Not many people wearing sunglasses. So trying to extract as much photon energy from the sun as possible, which I support.

As everyone knows, I'm a big fan of getting sun. But also when we did see swimmers, they were swimming in this cold water and like it was nothing and the range and age of the swimmers was remarkable. You saw the kind of fit triathlete looking types, but also young kids, like really young kids.

And then people probably in their, again, their 70s, 80s, maybe even 90s, really remarkable. Vastly different than what you see if you go to the ocean here in Los Angeles or elsewhere. So yeah, you Scandinavians are onto something with this. I'd like to take a quick break and thank our sponsor, Inside Tracker.

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Again, that's insidetracker.com/huberman to get 20% off. I'd like to talk about your study. If you could give us a little bit of the backdrop about what motivated that study, and then walk us through what you did, who the subjects were, what you had them do, what you measured, in as much detail as you would like to share, because I think it's such an important, even fair to say landmark study, because it also explored not just cold, but sauna, and the co-use of cold and sauna as a way to probe metabolism and brown fat and other markers as well.

And as you do this, I'm hoping at some point that you might tell us some of the observations that you might've made that interested you that perhaps were not in the paper, 'cause that's one of the great benefits of sitting across from somebody who did the work in detail.

So yeah, if you could tell us about your study and what you did and what you discovered. - Thank you for that question, Andrew. I love to also explain a little bit what did we do, because when people read this kind of paper, they just see the numbers. They don't see what happened before that.

And human studies are very different from my study. My study, you can do a knockout of something, and then everything is perfectly matched and controlled. Doing human studies is very far different from that, because people are different even in the groups. So yeah, but when I started this research in 2016, I did not really know what the brown fat was.

So I started reading up on all this, and I was very interested in preventive medicine. Also the studies that I did before brown fat was also like very much in the preventive side, like how can we, that was about something else, but the sweet tooth and how can we lower our sweet tooth and stuff like that.

So, but after that, I wanted to do something new. So I looked into the brown fat, got hired in this fantastic research group where they, it's a cell group. So they mostly did cell studies and they didn't have anyone to do a human study yet. But they really wanted me to do that.

So I read upon a lot of research about how does the brown fat get activated, what have been done already. And I mentioned the paper before with sleeping in the cold. I found that particular paper very fascinating. And that was also where at that time I was like, okay, so cold exposure as an intervention of sleeping in the cold could be a good thing to go out and say, well, people do this.

But on the other hand is, first of all, it was already done. That was one thing. But the other thing was like, well, I wanted to see if we can do some kind of activity so we can have people move also, go and do something, do something together or whatever.

And the cold made us think about, well, what about winter swimming? And it was kind of like a bit of a joke in the beginning. It's like winter swimming, yeah, it's gonna activate the brown fat, right? But when we read the literature, we couldn't really find anything about activation of the brown fat with cold water besides hand in a bucket of cold water.

That really, that was already there. So we were just thinking, okay, so it should be very potent activation of the brown fat if it's cold water, but very different from cold air. So it was kind of also a new thing we were going into and we knew that we were gonna do like a more of a proof of concept study at the beginning of it because it was like winter swimmers must in theory activate the brown fat, right?

But we kind of didn't really know was this kind of stressor too much, too little, or what will happen actually. But we had this idea about, well, we always say that cold water and winter swimming will activate your metabolism, but do we know if it does that? No, we don't.

And while this idea was a little bit fun at the beginning, we kind of accepted it. It was like, okay, let's just try this out. But because we didn't have the funding for it, we was like, okay, let's do a proof of concept study. Let's go with a small number, but enough to see a difference between the groups.

So the power calculation of that study is done on what we know from PET CT scannings of the brown fat. So that's the main outcome of that, of course. So, and we wanted to go a little bit small on the numbers of participants because we wanted to dig a little bit deeper into the different mechanisms and also redo some of the days.

So I really wanted to do that to see if I can replicate also the findings. And that's gonna take a lot of funding, but it's also gonna take a lot of time to do it. So the proof of concept was just going small, but looking at different mechanisms. We also took fat biopsies, for example, and looked at the white fat to see if there was any differences between the groups before and after and stuff like that.

So that's kind of like how it started. And the first year was like a field study for me. So I was not a winter swimmer when I started this. It was just- - Oh, really? - No, I wasn't at all. I would say I was a bit afraid of the cold myself, bit of a cold sissy, always cold having big socks on and sweaters and stuff like that.

So I was like, I am so comfortable. I'm just like everybody else, very comfortable. I like being completely temperature neutral, but I started like playing with the storage, like, well, if this is so healthy in theory, I should not pack myself up. I should start not doing that. Yeah, but the first year observation of winter swimmers on the jetty, they kind of joked about it and say, "Come on, Zainu, you need to try this.

You cannot study this unless you have tried it." And I was like, "Ha ha, very funny. Of course I can do that." (laughs) But I couldn't. I read the literature. I understood in theory what happens when you go into cold water, but I completely understood it when I first tried it.

The first few times, not so funny, it felt painful. It was just like running too long after a long break and your muscles hurt the day after, right? You completely regret that you took that extra mile. - What about, when you say uncomfortable, you mean uncomfortable when you got in and when you were in or uncomfortable afterwards?

Because I find that on rare occasions, well, I should just, full disclosure, I do deliberate cold exposure every morning for about a minute to two minutes in a cold plunge. There are days that I miss, but when I'm at home, I do that. And when I travel, I do a cold shower.

I do finish with a warm shower. (laughs) So, and we'll talk about why that's probably not the best idea. But, and I've been doing it for some years now, on and off. But, so just full disclosure, I'm a devotee and I have family members that hate the cold, but have gotten into it and are starting to like it.

But they don't, and I don't necessarily like the experience in the cold water, but I love the way I feel when I get out. And I'm 100% on that statement about loving it when I get out. Occasionally, it feels good to be in there. It feels invigorating. And I think I've learned to control the gas reflex and the hyperventilation.

And I just have told myself what we know, which is that the forebrain struggles to engage for the first 20 or 30 seconds. But if you can get past that wall, it's far easier to push through. But when you say that it was really uncomfortable, do you mean the experience of getting in or you also felt lousy afterward?

- Yeah, and very important to clear that out. I only felt very uncomfortable doing it at the moment. But afterwards, the first time I went with a group and actually my husband was there as well, because I really wanted someone I knew coming along because it's very normal if you haven't done this before you feel a little bit anxious about it.

This is shown in studies as well, because blood pressure and heart rate goes up in those who are new to this kind of activity. So I was a little bit anxious about it. So it was really uncomfortable just doing it. But afterwards, as soon as I got up, I felt fantastic.

And we went into the sauna and I did three rounds because I just loved it. I loved the feeling afterwards, because you have all these neurotransmitters going in your brain and you feel more positive. You feel, I feel invigorated, I had so much energy. And I could totally see why people would do this to get energy throughout the day, because I definitely had that.

I didn't have to do three dips to get that. I think one would be enough. And I often do that also now today. I do one dip, sometimes I do two or three dips in one round, you can say in one day. But often it's like just one or two times a week.

For me, that is enough to get that energy and to get that positive feeling. And I think that that is also why I put up my study in that way. I wanted to study the lowest dose, you can say, the lowest amount that we can get away with, but still see health benefits.

So what I observed there on the jetty was that some did it a long time. They were in the water for a very long time. And to me, it seemed maybe a little bit extreme. - Could you give me an example of a long time? - Well, so maybe they were like really swimming and they could be 20 minutes or half an hour.

- That's a long time. - That's a long time. And there was like ice and people who came up. I mean, I just didn't really feel that this is something that I wanted to go out and recommend to people after my PhD. - You don't want any research subject dying either, because if you're not adapted, I mean, people can do that.

Also a 20 minute cold shower or 20 minute cold plunge. I know people do it, but it's probably not a good idea. - No, probably not. It's gonna exhaust your cells and make them age too fast. So exactly, that's when you pass that hermetic stress, the healthy stress level, that's what is happening.

The quite opposite is almost chronic stress actually in the cells. Well, what happened then was that I found out if I want to have this protocol get through ethical comity, I really needed to go like very like sleek with the not too long and make sure that they were also very healthy and to get approval, of course, of this study.

But what I did was to recruit winter swimmers who already have been swimming for two or three seasons. And I just observed them. I said, I'm not gonna do an intervention study yet. I did that after, but I wanted to do like a proof of concept where they were already adapted to the cold and then compare them to a match control group who were matched on, you can say diet.

So were they vegetarian or not? And one of them was in each group. Also- - They weren't all vegetarians. - No, no, no. Just one in each group, yeah. - I was gonna say with all the amazing fish and meat in Denmark, I'd have a hard time being a vegetarian.

The breads are amazing, the fruits and vegetables too. But okay, so there were a couple of vegetarians in each group. - Yeah, one in each, yeah. And they were matched- - token vegetarian. I have family members who are vegetarian, so I'm just poking fun. - Yeah, but they were matched on different things.

So what we usually match them on is also BMI. We chose one gender in this study and we would always choose both men and women normally, but we do see that there are different brown fat levels depending on gender. So women have more brown fat than men. - Really?

- Yeah. - Interesting. - Yeah. I think it's interesting. - That deserves study. - Yeah. - Yeah. - Why actually? I think it's interesting because women are also smaller. So in size, mass, right? But they also have a lower peripheral temperature, especially on hands and ears and- - Is that right?

That's documented? - Yeah. - Women do run colder than men? - Yeah, and there's also- - Physiologically, I didn't say psychologically. - No, no, no. - We won't go to the psychological cold heat. - Yeah, that's different, something else. That's a different podcast. - Yeah, another time. So women are just colder physically.

So on hands and ears, it's measured on that and feet as well, so compared to men. And men have bigger hearts than women and they can pump out more blood, peripheral than in a woman's body. So that could be an explanation for the colder hands, for example. Thermocompatible state is also different between genders.

So men are more comfortable at 22 degrees Celsius and women are thermo-comfortable at 24 degrees Celsius. And this is- - Okay, so the thermostat wars of home have been now validated. - Yes, yes, there's a study- - Two degrees Celsius. By the way, prior to starting recording, I made the executive decision that we were going to go with Celsius throughout the podcast, because the majority of the world uses Celsius.

So for those of you that think in Fahrenheit, the internet is your friend in making those conversions. So we're sticking with Celsius. So men tend to be thermo-comfortable at 22 degrees Celsius, women at 24. Okay, interesting. - It explains a lot about also some arguments in the homes where men are turning down the heater and women are turning up the heater and they cannot really.

So it's really, I'm on both sides here. I understand the men, we understand the women, but there is a difference there. Which was also one of the reasons why we in this proof of concept study chose one gender. So it is not like only because we wanted to study men.

It was just to see, to eliminate all the confounding factors which could have an impact on our results. So that was one of the reasons. But also because we, yeah, so women have more brown fat than men, and otherwise we would have to do four groups or something like that.

And not having funding yet, we were like, okay, we need to do just one group, just a control group, and then a group who were already winter swimmers. So I recruited winter swimmers who have been swimming for two to three seasons because I wanted them to be already adapted, but not going too long in the water.

So they told me, I did a lot of screening here, of course, beforehand and interviews to see, to ask them, how much do you do and how long do you stay in the water? And I monitored how long did they stay in the water and recruited based on that they only did like two to three times per week.

It seems reasonable for Denmark, at least, to do that. And they stayed only in the water for one to two minutes. So the coast subsides very quickly and you will get this activation of your rest and digest system, which is your parasympathetic nervous system. So the other branch of your autonomous nervous system.

And you get that activation because you submerge into cold water. And when you do that, you have an activation of your diving response, and that's gonna slow down the, you can say, the consumption of oxygen also in your body, and that's gonna slow down your heart rate. - Could I pause you on this?

'Cause I've heard this before, that when we get into cold water, shower or immersion, we get this sympathetic autonomic response. So increased blood pressure, increased heart rate, release of norepinephrine from the locus coeruleus in the brain, release of adrenaline, dopamine, adrenaline from the adrenals, dopamine presumably within the brain, but that the parasympathetic response is activated when we put our face into cold water or go underwater.

And that's a calming relaxation response. So this brings us back to, I don't want to take us off track from you describing the study, but this brings us back to the first question, which is if I go completely underwater for a moment when I start my cold plunge, does that change the physiological outcome as compared to if I just submerge myself up to the neck?

And actually nowadays there seems to be a little bit of a movement online of people putting a bowl of ice water on their countertop and submerging their face into it. Did you see this? I've seen more and more posts about this. So can you just touch on what the dive reflex is and why it perhaps activates the parasympathetic response, this calming response?

- Well, so the diving reflex is activated when you submerge into cold water. - Even just to the neck? - Yeah. - Or I thought you had to get your face under. I'm not arguing different. You're the expert. I just want to, yeah. - I haven't really, I haven't read that.

I've just seen that you can activate your diving response as soon as you go underwater with your body. So you don't have to do it with your face as far as I understand. I could be wrong though. Yeah. So when you activate your diving response, you will slow down your oxygen consumption in your body.

And that is because the body tries to preserve oxygen. So you will not get hypothermic too fast. So it's kind of like a survival system in your body. So this survival system is very important for us, of course. So that will be activated. And because of that, you will after maybe one minute or so, I can't be precise on that because maybe it also varies a bit in humans.

So one to two minutes, you will have full activation of the sympathetic nervous system, but also the parasympathetic nervous system. And that's gonna activate, for example, something like serotonin in your brain, which is also good for mental balance and people feeling in mental balance afterwards after they go up.

So that is measured on questionnaires and also measured on anecdotes. Of course, people tell all the time that they feel good afterwards. We need studies on this. So if anyone's sitting out there thinking that's interesting, then please do some studies on that to get more out on that. - Yeah.

- So you observed these winter swimmers who have done this for a few seasons. They're coming around for a new season of winter swimming and you've decided to recruit them as subjects. They are getting into cold water, climbing down a ladder or jumping into the water up to their neck.

- Yeah, climbing, yeah. - Okay, climbing down a ladder 'cause this is done outdoors. What a fun study to do. My graduate thesis was done under fluorescent lights with no windows in a building that, I mean, I had a ton of fun as a PhD student. I actually lived in the laboratory as a PhD student.

I loved it so much, but not something required to do a PhD, by the way. But they're climbing down the ladder, getting in up to their neck, staying in for one to two minutes and then getting out. And how many times a week are they doing this? - So they do this two to three times per week.

And for each time they go, each day they go, they take three rounds of, so three dips and two sauna sessions. So they start in the cold and they end in the cold water. - Okay, so it's get in for one to two minutes, then get out and get into the sauna.

- Yeah. - What is the temperature of the sauna? - About 80 degrees Celsius. - Okay, then how long are they in the sauna? - So they stayed there for 10 to 15 minutes. So depending on if they went two times per week or three times per week. - Okay, and then they get back into the cold for a few minutes, two minutes?

- Up to two minutes, yeah. - Okay, then back into the sauna, 15 minutes or so, then back into the cold for a third round, back into the sauna, and then they're- - Ending on the cold. - And then back into the cold again, and then ending on cold.

And we will talk about why it's important to end on cold, the so-called sober principle. How cold was the water in this particular, given average, 'cause I realized it's outdoor winter swimming. So it's gonna vary depending on windchill and things as well. - Of course, so it's a very uncontrolled environment to do this kind of study in, but I wanted to do something that was also very close to something people could do for free, going out in nature and use that, and also have the nature-like.

It's a very healthy impact on us. It lowers our stress level as well. So by doing so, I also measured the temperature every time they went. So I have this graph, and it's actually in the "Winter Swimming" book. It shows the temperature in Denmark going from October to April.

And it starts at 12 degrees. I think it's around 12 degrees Celsius in the water. And then it goes down to two degrees on average in January, and then up again. So it's within the spectrum of very cold water, I would say from around 15 degrees Celsius and down.

But it was actually not colder than two to four degrees on average when it was the coldest. So it doesn't have to be that cold to be good enough and enough to activate our metabolism. - And what time of day are the participants doing this cold sauna alternation? - So I think they did this throughout the day.

So I didn't control whether they wanted to go in the morning, in the afternoon, or in the evening. At the time where I set up the study, I was not controlling it in that way. I wanted them to go whenever they had time. And I also think that is the most important message to give to people is do it when you have time.

It's not, if doing it when you get home from work and it's six o'clock in the evening, and this is the time where you can do it, then try out if it's gonna impact your sleep or not. If it doesn't impact your sleep, then fine. But you have to try for yourself and find out what works for you.

It's the same for coffee, for example, right? Some people can drink coffee in the evening and go to bed and they can sleep. I can't. - Or exercise. - Or exercise, exactly. So I can't do that. And that's because the coffee exercise, cold water immersion, it's gonna activate your sympathetic nervous system.

You have an increase in stress response in your body and that's gonna make it really hard to fall asleep for some people at least. Maybe you are super exhausted anyways, and then you will just crash anyways, but yeah. But that's the only thing. So I just told them to do this if they can during the daytime.

And that's primarily what they also did. - And then all along you're measuring brown fat by way of this infrared camera, right? So what did you observe in terms of changes in brown fat? How quickly did that occur? And then I'd like to ask also about sauna a bit more because earlier you mentioned that you can activate brown fat with sauna as well, with heat on the surface of the skin.

How long did it take before you observed significant increases in brown fat? And was it increased density of brown fat or distribution? Was it showing expansion to different regions throughout the body? And maybe you could also touch on some of the changes in insulin sensitivity and metabolism. - Yeah, a very good question.

And I didn't mention this before, but besides measuring temperature as an outcome for brown fat activity, we also did PET-MRR scanning of the brown fat. So this is like the golden standard for measuring brown fat. And it's not very feasible for normal people to get a PET-CT or PET-MRR scanning of the brown fat.

It's super expensive. So we had both to see if we could have like a continuous measure of brown fat in humans because that was already not out there. So I wanted to see during both the experimental days, but also during day and night, what kind of like circadian rhythm do we have in our brown fat activity?

So that's why I wanted to have that as well. So the PET-CT scanning or the PET-MRR scanning was to see upon cold activation stimulation for some hours, do we have activation? Can we see the brown fat in this subject? And also during thermal neutrality or thermal comfortable state, how is that activated in each of the group, of course?

- Ah, so you want to see how comfortable people were away from the cold water and sauna just at different temperature environments. Is that right? - Yeah, so also measured that. How comfortable are you? I made this scale like visual analog scale and asked them how comfortable do you feel with this temperature and throughout the study days during cold exposure and thermal comfortable day.

I had a whole day where I just kept them thermal comfortable to see, do they activate the brown fat if they're just completely thermal comfortable? As good as we could get with that because you were asking people on a scale from one to 10 and five being thermal comfortable, where are you on this scale?

So one would be very cold and 10 would be super burning hot. Yeah, and so that was the way to try to figure out how do they actually feel also during the studies. I also measured electromyography of muscles to see, do they shiver during the cooling day? Sometimes people shiver before they know they're really shivering.

So I had this. - Oh, interesting. - Yeah. - So our conscious perception of shivering might not be the best readout of shiver. - Yeah, well, if you also get adapted to the cold water, you will have less shivering. They will be less vigorous. They will be very small.

So you wouldn't probably know that you are shivering because the shivering is so small and the mitochondria in the muscle cells will be so dense that it doesn't need to shiver maybe that much to get that thermogenesis going compared to when you're completely new to cold water exposure, you're not adapted, then the body needs to create these mitochondria, these energy fabrics to keep you warm.

And that's also what the exercise is in the beginning. But when we measured this, we did see that the winter swimmers were shivering less or having less vigorous shivering when they said I'm cold. So even though their perception of the cold was pretty similar in the groups, we could see that the activation of the muscles that we measured on were different and more vigorous in the control group.

- Were the subjects incentivized to be in the study? Were they paid or anything of that sort? Or they just happened to like doing cold and sauna. And so that's why they did the study. - Well, they got paid a little bit for it, but not much. And that's how we do this study.

- Sure, I was just curious. Yeah, I was just curious. There might be some folks that wonder. So what did you discover in terms of changes in brown fat, insulin resistance, or insulin sensitivity rather, and metabolism? - So what we saw was we had this kind of different measures to see what's actually going on when they are already adapted to the cold water compared to a control group who was matched on various parameters.

We did see that the winter swimmers had an increased insulin sensitivity. They produced less insulin on all the experimental days. So besides from just cooling them and measuring the brown fat on each of these cooling days, there were two cooling days and one thermo comfortable day, right? So I wanted to measure insulin when they were fasting, meaning that they hadn't eaten in eight hours before the study day.

And they were completely laying still, not moving just in a bed. And we measured insulin during the experimental day just to see what level are they on. And we could see that the winter swim had had lower production of insulin. And they also, when they had a glucose drink, so we give them that to see if they, to test before we enroll them in studies to see if they have diabetes, for example, and not knowing, for example, that that would ruin maybe the study.

So we test for that and see if they have a normal curve. So what we did see in that was that the winter swimmers had a faster glucose clearance in the bloodstream. So after two hours, we could see that they had a lower level and the curve went down faster than in the control group.

- So despite having lower insulin release, they have better blood glucose clearance, which is really what we all seek, right? Excessive insulin is bad. Insulin being more or less a chaperone for blood glucose. We can do all sorts of other things as well, of course. And having high blood glucose, obviously terrible for cells, especially brain cells.

I don't think people realize how toxic high blood glucose is, having high glucose is sort of great. If you want to kill neurons, you make their, put them in an environment where there's too much sugar. Oh yeah, very neurotoxic. I mean, and there are mechanisms like insulin that buffer that, keeping blood glucose in a reasonable range so that that doesn't happen.

I mean, I think that's why people will go into insulinemic shock. Hypoglycemic shock is also possible. So that range in which neurons are happy is not a tremendously large range. Incidentally, the range in which neurons are happy in surviving is much greater as one gets colder than when you heat up.

I mean, you can basically destroy brain cells by getting too hot for too long. - Oh yeah, yeah. - You can definitely destroy brain cells permanently by getting too cold for too long, but you have to get really, really cold for a really long time. - Yeah, yeah, very interesting.

- Yeah, we were thinking about doing an episode on sort of survival of the brain after death kind of things, which actually happens. We hear about these people who are declared dead and then come back. And there is actually now a lot of cryopreservation type approaches for that. This is, anyway, we risk going into the esoteric now, so I'll steer us back to our discussion about your study.

But so if I do the math, these subjects are in the cold. Let's say they're doing three rounds of cold for one to two minutes, two or three times a week. What were the thresholds that you discovered were important for getting these positive changes such as reduced blood sugar or clearance of blood sugar being more efficient, reduced insulin, improved brown fat distribution and density?

How much cold exposure do people need? How much heat exposure do people need in order to extract these benefits? - Yeah, so when we then calculated the numbers together, we could see that this ended up being 11 minutes in total per week. So not in one session, of course, but they had two to three visits to the water and the sauna per week.

So when we divide that out, it corresponds to being in cold water one to two minutes at a time, but also in the sauna 10 to 15 minutes at a time. And I think this is very like also similar to what we see in other studies when we look, for example, to the observational studies from the Finnish cohort study from Lauken et al.

For example, they published this very amazing paper in 2015. Some results from this long course study where they show that up to 30 minutes in the sauna was healthy and you lower your risk of cardiovascular disease and that's like the threshold. And if you go further than that, then there is not more healthy benefits to gain from that.

So, and before that, it's like 19 minutes, then you will have this dose response relationship up to 19 minutes. That's really in decreasing your risk of cardiovascular diseases. And I think we really- - That's per week, 90 minutes per week. - 90 minutes per session now. - Per session, okay.

- Yeah, per session. If we then compare that with my study, which was 10 to 15 minutes per session, then I think it fits very well with what we call the hermetic stress or healthy stress that you expose the cells to this kind of like potent, very stressful situation where they increase heat shock proteins in the cells and that will repair the cells.

But if you then overdo it and you go beyond the maybe 30 minutes in the sauna, this observational study from Finland with more than up to 2,000 sauna bathers where they have followed these for 20 years, they see that 30 minutes per session is like enough. And then if you go above that, you don't get more health benefits out of it.

So I think there's a window where we can say the healthy stress corresponds to like 10 minutes and I think it's like- - Per session. - Per session and it's not much actually. So you don't need to, it shows that you don't have to expose yourself very much to the heat or very much actually to the cold to get this healthy benefits from going into cold, going to heat and have healthy benefits on your cardiovascular system.

So I think this is very important also message to get out that you don't have to go extreme, you don't have to swim for a half an hour in the cold water. You can go in the water for one to two minutes per session, but go up to 11 minutes per week in total.

And for the sauna, my study showed 57 minutes in total per week. And if we also then divide it out on these two to three days and two sessions each day correspond to 10 to 15 minutes. So it's a low threshold, but I think it's good to have that too.

Maybe we can aim for that if people need to have something to aim for. And I think it's really good to have that because then you don't overdo it. And if you overdo it, you exhaust the cells and that will increase your risk of cardiovascular disease also. - Well, I get a lot of questions about this and I did solicit for questions for this podcast on Twitter.

And one of the questions that I got was as one becomes more cold adapted, do the benefits start to wear off or can people do too much cold exposure? Of course, the answer to that is yes, you can become hypothermic, but I'm sensing a different answer now, which is if I understand correctly, the threshold is 11 minutes total per week of deliberate cold exposure divided into two or three sessions of maybe one to three minutes, depending on how long somebody stays in.

And then 57 minutes, I want to be careful not to round up to an hour, but divided into maybe three 20 minute sessions or so. So one doesn't have to be perfect as long as you get beyond that threshold. But I wonder something which is, is it the case that if somebody said, oh, I'm just going to do one 11 minute session per week, that might actually not be as beneficial as dividing it up because what you told us earlier is that the hormetic response depends on having that cold shock.

You actually don't want to become too cold adapted. I mean, once the blood pressure response drops down, so in minute four, five, and six, you're getting very cold and you're shivering, but one is not getting the autonomic stimulus that they want. I guess I could liken this to if exercise worked in a way where it was only the first few minutes of exercise that really triggered the adaptation.

Of course, this is not how it works, but in fact, probably quite the opposite. But if that were the case, then it's not simply the total amount of exercise, but dividing up the sessions into little bouts where every single time it acts as a stimulus, that seems to be the key here.

This is very important because having watched the landscape of this on social media, but also in books and generally, I think you're the first person to really touch on this, that the goal is not to get so cold adapted that you can sit in for the full 11 minutes in one session, where the goal isn't to be able to do an hour of very hot sauna.

If you want to, I suppose people could do it for other reasons, but if the goal is to improve these health metrics, then the idea is to keep the stimulus a stimulus. Short, exactly, yeah. - Great, well, this also, I think there's practical feasibility, as you pointed out, 'cause getting into a cold shower or cold immersion or natural body water for a couple of minutes is far less challenging to most people than finding a full morning to go spend there.

But I've never really heard it articulated that the longer sessions might not be beneficial and might actually be detrimental. That's very important. Were there any other observations that you made that did not make it into the paper or that were kind of in the margin notes, in terms of psychological benefits or anything of that sort?

There was this recent study on soldiers that talked about weight loss. It's sort of a controversial study for a lot of reasons. But one of the things they remarked in the paper was that there were a lot of psychological changes, improved buffering against anxiety. They even, the men and women in that study reported, one of the significant effects was significantly improved sexual satisfaction.

Of course, they didn't tell us what that meant for these subjects, so we won't go there. But a number of subjective improvements. Was there anything that you observed or took note of in your study that perhaps didn't make the main abstract but that we should be aware of? - Yeah, there were some.

And today, I regret that I didn't measure on sleep, for example. I frankly didn't really think about that when I designed the study. So we were very much occupied with the metabolism and kind of had the thought, maybe this could impact sleep quality. And I wish I had the thought that why don't you just ask them in a questionnaire?

But I asked them every morning, or every morning, it was not many mornings, just two mornings, actually, we measured on. But the winter swimmers told us before I wrote them that they had a really good sleep quality. The control group also had that. But they told me on the day where we measured the brown fat on a day and a night, so actually two days and two nights, they told me that they had a good night's sleep, but they also woke up.

So it's just telling me that they also had a quick wake up and then they fell asleep again. And the winter swimmers told they have a really good sleep. So it's like, in general, they also say we sleep very well, I sleep very well. So it's anecdotally, in general, it corresponds to what I heard in my study, but nothing that I measured on, which could be fun to do in the future, but we didn't measure on sleep quality.

That would have been a really good idea to do. They also told me that they were very comfortable when they were cold. They don't mind, winter swimmers, they don't mind going out, for example, in the cold with a T-shirt. They were also less scared of showing their skin. That was also one observation.

- Interesting. - Yeah. - So kind of a reduced social anxiety. - Yeah, they were just so comfortable in the lab. As you just mentioned before, coats on and everybody's joining around. It was very busy and all the other scientists out in the hallway. And also my supervisor had her office down the hallway and one of the winter swimmers one day just got out of bed after, had been in the study for eight hours.

It was a long day, right? He jumped out of the bed and had his clothes in the bathroom and he went out completely naked. He didn't care. He just went out. It was like, oh. - So that's a side effect perhaps of getting too comfortable with the cold. We're not recommending that.

Although in your book, you dedicated some, let me start that again. Although in your book, you dedicated some pages to naked winter swimming, or I should say naked cold water exposure as opposed to with bathing suit. Are there any data on this? I'm sorry, chuckling. But I think in most places in the United States, skinny dipping is not legal, most public beaches.

There are a few. In fact, my laboratory before moving to Stanford was in San Diego and I, at the Salk Institute for Biological Studies, beautiful building, incredible science is done there. The beach right below that is called Black's Beach. And it's a no-nude beach. And so whenever tourists were heading down the stairway there, I would sort of let them know, especially if they had kids, I'd let them know.

And it's a nude beach of a particular genre. So I'd give them a little warning about what they could expect down below. In any event, those beaches are quite rare in the United States, maybe compared to Europe. I don't know, but- - Yeah, maybe. - Yeah, so is there anything special about clothesless versus closed exposure?

- Yeah, I think in that sense, we are a bit more free with this kind of like, but remember we also had this winter swimming culture for hundreds of years in Denmark. And the oldest winter swimming clubs that we have, especially the one we have in Copenhagen, where I did my next study, which we haven't talked about, and it's also not published yet, but in that winter swimming club, it's the oldest one we have and it's huge.

And they swim naked at this facility. - Men and women. - Men and women, and they have sauna where they can go in together. And they also have separate saunas, but it's very much a Danish thing. And I think it's good if people want that. And I had it in my book because people want to know if they have to swim with their bathing suit on, or if they can take it off, or what's the difference?

Is there any difference in this? And if you ask me, there is no difference. If you have your little skinny bikini on, it's not gonna do any difference to your cold exposure or your adaptation, it's not gonna do any difference for your benefits, of course. But I think that it has something else.

It has something to do with how you also observe yourself, how you observe your surroundings. And it's some sense of freedom in skinny dipping. So I think people in Denmark who does this, they do the winter swimming because they feel free when they do it. They come home from work, they go to this club, and they skinny dip, and they feel in touch with nature.

And they have maybe done this their whole life. So this is an old tradition in Denmark in some of the clubs. But the newer clubs that are coming, they don't do a skinny dip. So everyone has bathing suit. I never skinny dip because there are people around, people with phones and taking pictures all the time.

So this- - It's different nowadays. Everything's recorded. - Yeah. And also this old tradition is also fading away because of that. - Yeah, I use sauna and cold at home, but when I travel, there's a banya. So Russian banya has hot sauna and cold plunge. There's one in San Francisco called Archimedes banya.

And that one is clothing optional. So some people are clothed, such as myself, and then other people are not. And it's co-ed most of the time. I think they have male, female separated evenings or something like that. And then the other banya is Spa 88, which is on Wall Street in New York, is an amazing banya as well.

And these are starting to crop up in different cities, or maybe they've been there for a long time. And as deliberate cold exposure and sauna gets more popular, more people are using them. The one in New York that Spa 88 is always clothed. And it's interesting because people here are naked or skinny dipping and they might get certain ideas in mind.

All these places are very well lit and they all have a tone of kind of health that is about the kind of health and wellness. I guess the point being that there's no requirement to do one thing or the other. Although in the studies that you did, obviously people were clothed.

But I did pay attention to those pages in your book. I thought it was interesting that you put some dedicated passages in your book that related to this. - My publisher wanted that. - Oh, your publisher wanted that, interesting. - It was not me. It was like my publisher really wanted to have a little discussion about that.

So I was like, okay. - Well, I think it points to a larger theme, which is I think for a lot of people who already do these practices, there's no shock there. For people that do not do deliberate cold exposure or sauna, I think that there is this idea perhaps that, oh, these are traditions that are kind of fringe or that they're cut.

And I just, I want to cue that point because there's so many things that are happening right now in biomedical research and medicine. Serious quality peer-reviewed studies published in excellent journals like your paper on things like deliberate cold exposure, sauna, the use of particular supplements, natural herbs and supplements.

I mean, there's an entire branch of the National Institutes of Health in the United States dedicated just to the study of supplements and behavioral interventions for health, like meditation and breath work. Really incredible. - Fantastic. - It's really incredible. And psychedelics, of course, being something that for a long time was part of a certain community and feel, and now is being frankly adopted by mainstream medicine, even pharma.

So the times are changing. And so, yes, I think it's important to know that it's perfectly acceptable and encouraged to wear clothing. - Absolutely, absolutely. Yeah, yeah, yeah. And one other thing that I wanted to mention, going back to your questions around with there were any observations in the studies, which we really maybe haven't discussed yet, and maybe it's in the back of the paper and not mentioned that much was one of the winter swimmers didn't have any brown fat when we measured him.

- Zero. - Zero. And we do see this in previous studies as well, that some humans don't have any brown fat. - Did he carry a lot of white fat adipose tissue? Was he obese? - No, he wasn't. No, he was not obese because he would not have been in the study then.

- Oh, right, yes, you mentioned this earlier. Forgive me. - No, no, it's fine. But what I did observe before I knew that he didn't have any brown fat was that during the cooling experiment where I cooled him for two hours before they go into the PET CT scanner, he was not able to control his shivering like the winter swimmers could.

So he got very cold very easily compared to the others. So, and without, I didn't know what was different about him, but we could, all of me and the three others were working on the experiment. We were like, okay, what's going on? Because we turned down the temperature, but he started shivering and then we had to turn it up again and it was just all over the place, the temperature.

It wasn't that controlled like the others. It was pretty similar protocol. I could just do pretty much the same because they were same size and also same gender. So it was easier to foresee what was gonna happen and when will they start shivering. I quickly learned that. But with this subject, it was just, with this volunteer was just very much different.

And then when we scanned him and didn't find any brown fat, I didn't even think about it. So when we scanned him, we didn't see anything. I told the PET CT people to like, oh, you put up the wrong scanning. - Right, blame the technology. - Yeah, the technology was like, this scanning looked like the thermo-neutral day, the thermo-comfortable day where we also scanned them to see if they have any brown fat.

So you have made a mistake, I was pretty sure. And they reanalyzed this scanning and they just concluded, well, the scanning was fine. The experiment went well. It was just that he didn't have any brown fat. So he was like what we just in the paper called a brown fat negative.

So he didn't have any. And in my studies, it would be called a knockout. So he didn't have any brown fat. So what the observation with him, and I think that's interesting, is that he both shivered very early on and didn't regulate his temperature as well. He also told me that then he was like a five on the scale of how comfortable he felt with the cold.

So it was from one to 10 and five being thermo-comfortable and 10 being very cold and one very hot. So on this like scale up and down. And he was like more up and down on this scale than any of the others. It was an observation that I did.

But we did see in his blood samples also that his blood samples looked a bit more like the control group. Also his insulin levels were like the control group. So a little bit higher than the other winter swimmers. And he also had his blood glucose clearance was not as fast as the other winter swimmers.

So he was like an outlier, what we call it. And in the analysis, we also had to take him out of the analysis because he was an outlier. So the results showing that the brown fat is more efficiently activated in the winter swimmers is without him having him in that group.

- Got it. - But it didn't ruin the study if I tried to put him in as well. And it didn't ruin the results or anything. But just to keep it more clear, we took him out of the analysis. - Yeah, so he was a mutant, a knockout. Yeah, and I'm sure they're out there.

Very interesting, so if you shiver early, then perhaps you have less brown fat to begin with. Although it's hard to conclude from one person, that's sort of the implication there. - Oh, you haven't adapted to the code. So you should build that up, yeah. - So in addition to looking at regulation of blood sugar, brown fat, metabolism, and so on, were there any markers that you examined in the deliberate cold exposure group as compared to controls that revealed to you that deliberate cold exposure could have additional benefits, say for immune system function or for any function for that matter?

- Yeah, so we looked at inflammation. Of course, we measure the outcome of blood pressure and so on, but we also measured the IL-6 in the study just to see also an inflammatory, anti-inflammatory marker. So IL-6 went up and it also follows with the IL-10. So that is like also very known in the literature.

So we measured that and I think it's very important to think about the cold exposure and the heat exposure as something that then lowers the inflammation in the body. And if we can do that, we will have an open door for preventing lifestyle diseases, right? So for type 2 diabetes, but actually also for some mental diseases as well.

So as known as depression and anxiety and also Alzheimer's disease, which are all associated in research, also newer research showing that inflammation increases the risk of depression, anxiety, and Alzheimer's, if it's neurological diseases. So if we can decrease inflammation in the body, we will decrease our modern lifestyle diseases, but also these increasing mental diseases that we see in these modern lifestyle times.

So I think that it's, I think it's very interesting that we can go out in nature and we can use these natural stressors. And I don't want to have it sound very romantic or anything. It's just, it's just exposure to temperature actually, just a cold or to heat that is gonna twerk our body into a natural state again, and reset it where the homeostasis, the balance is lost a bit.

So the body is gonna repair itself in that way. And I think it's beautiful that we can do that just by changing the temperature of our body. And although people are very scared of doing this, because in our times we have been away from cold, away from heat, temperature for some, for decades now, since we isolated our houses better and we are more sedentary, we also sit more indoor, we don't move as much.

So this very modern sedentary lifestyle has made us more temperature comfortable, just neutral. So no wonder, I mean, that obesity is increasing. We don't expose ourselves to the natural stresses that we did earlier on in our involvement, but up until maybe the '70s, the '60s, where we started having more comfortable lifestyles, right?

And obesity increases in the '80s, we can see that from statistics. So I think that if we can take in cold and heat, and you mentioned other things also before, but of course, exercise is very important here. And also a bit of fasting actually, because it all increases the hermetic stress in the body.

So it doesn't have to be other than natural stresses to the body, which then could keep us in that natural balance again. - Could we talk about what I refer to as the Soberg principle, which is to end on cold. And the reason I called it the Soberg principle is because in reviewing, oh, by the way, I wasn't a official reviewer of your paper, but I mean, in reading and reviewing your paper for its after published contents, I noticed that you had people end on cold.

And this has been a longstanding debate in the deliberate cold exposure community. Should you warm up with a warm shower afterwards or get back in the sauna? What should you end on, end on cold or end on heat? And the Soberg principle says, end on cold, as I understand it, in order to force your body to heat itself back up and thereby increase metabolism further still, is that right?

- Yes, so when you end on the cold, you force your body to heat up by itself. And that will require that you activate, you keep your brown fat activated and also your muscles, which is a good thing. It's a good collaboration to keep your thermogenesis up. And that's like an exercise, even when you go home.

So in that way, you don't have to think about your cold exposure dipping in your plunge, open sea or what it is, as just an exercise that you do for one to two minutes and then it's over. If you end on the cold, you have an exercise for your body going on for hours afterwards.

And that's not only on your metabolism, but it's also gonna keep your neurotransmitters activated as well and increase that because your body is still cold. So you need those neurotransmitters to activate the brown fat as well. So that's gonna make your brown fat cells more efficient and also your muscle cells more efficient, so increasing mitochondria in the cells, which will then generate heat very fast.

So if you have done this for a few times, so maybe three, four, five times, being new to this, but have tried it a few times, you will notice a switch where you feel that you get easily warmer and you can keep yourself warmer. And that is also what was shown in my study is that the winter swimmers were physically warmer on the skin compared to the control group.

- When they are out of the cold. - When they're out of the cold, just relaxing. And we tested this on the days where they were sleeping in the lab. So we could see that they had more activation of the brown fat, higher temperature. So probably because they also lose heat, they have a higher heat loss to the body compared to the control group because they have a more vascular skin because of the contrast of cold and heat.

So they lose heat faster from the body during that day but is that a bad thing? No, probably not because that's going to keep your brown fat and your muscles a little bit activated. So you will have to, it has to work to keep you warm. - And I would hypothesize that it also might lead to some of the subjectively reported improvements in sleep because in order to fall asleep, you need your core body temperature to drop by about one to three degrees.

So it's not just sufficient to be sleeping in a cold room and under the blanket, you also need your body temperature to drop. And so what you're saying, if I understand correctly, is that by ending on cold and forcing oneself to heat up naturally, that increases the brown fat stores, which I sort of see as kind of like the oil in the candle of the furnace that is thermogenesis.

And that in turn leads to increased heat loss, which people might think, "Oh, I don't want to lose heat from the body." But there are times when we want to lose heat from the body. Basically, it sounds like what we want is to be a very efficient heating and cooling system.

That it's not about being cold or being hot. It's really about keeping the system tuned well, keeping the oil in the candle, this brown fat functioning. Could I ask one question about fed or fasted? Is there any, or rather, are there any known benefits of doing deliberate cold exposure and or sauna fasted versus after a meal, say within the last hour or something of that sort?

I do my deliberate cold exposure first thing in the morning. So in general, I'm fasted 'cause I don't eat until a little bit later in the day. But what's known about that? And was that looked at in your study? I know you measured glucose, but that was as a separate test away from the cold.

- Away from the cold, yeah. But I also tested glucose on the days on the cold. So we measured that as well on the cooling days. And specifically on fasting and fed, I don't know. I don't think that I have seen studies specifically on this. - Okay, more science needed.

A number of people asked about the use of deliberate cold exposure to offset some of the symptoms of various diseases. Now, here, we're not talking about curing disease. We're talking about offsetting symptoms. One question I've seen quite often is whether or not people with Raynaud's syndrome. - This is a syndrome, and my high school girlfriend had this syndrome, and I'll never forget.

We were at a school dance together, and this was when we first started dating, and she had Raynaud's, which leads to very poor blood flow to the extremities. And she was very cold, so she left to go to the bathroom and warm up her hands in the warm water.

And I was left standing there at the dance, and people came up to me and asked why I was there and who I was there with. And I kept telling them who I was with, and they didn't believe me 'cause they couldn't believe that she would be with me.

Made total sense if you knew me at the time. I was way out of my league with her at the time. I like to think eventually I caught up. But in any case, she was in the bathroom for about an hour. So at one point I did consider the possibility that she had just left, but indeed she hadn't.

She warmed her hands back up. But people with Raynaud's suffer from this thing of very, very cold extremities. Their fingertips will even turn blue. You know, as if they were starting to get frostbitten. It's quite dramatic. And that question gets asked whether or not there's any use of cold to try and increase the elasticity, the plasticity of the small capillaries and vessels.

By everything you've described up until now, it seems like that would be a logical thing to do. And in addition to that, whether or not people with autoimmune conditions, people with any other types of conditions are known to benefit from deliberate cold exposure. I'm not aware of any studies, but I get asked about this a lot.

And there were a lot of questions about this for you in the Twitter feed. - Yeah, thank you for those questions. And I get them as well on social media. And I have to say that I haven't seen any studies directed on this outcome on measuring Raynaud's syndrome. I do know that it's not that rare actually a problem.

And I know that many women or more women than men suffer from this. But logically it would help them if they expose their hands to cold and also heat to make it more vascular. And I have heard from people saying that it had helped them, but also heard from some others saying it didn't help them.

So studies are needed on this specific topic, I think. I hurt my hands when I go into the cold one. I don't have this syndrome at all, but I keep my hands above the water. - You do? - Yeah. I do that, often I take a little bit of a swim and then of course I have to have my hands in the water, but it helps me when I then get back to the daddy and then take my hands up because then I can stand there for a little bit and get my one to two minutes exposure.

And then I can go up because then otherwise that would stop me from being in the water enough time that I, as long as I would like to. So if people suffer from having this pain in the fingers and it can be very intense. So just take the hands up a bit from the water and that's going to help you.

Also boots, neoprene boots, it's going to help on the feet. Some people have the hurt and feel the pain in the feet and on the ankles and that's going to help them also a little bit. - Okay, so there is no problem with keeping hands out or feet in neoprene booties if people feel the need to do that.

If pain of the hands or feet is a barrier for people doing deliberate cold exposure, then it seems it would be okay to keep hands out or to keep your feet in the booties. - Yes, because then you do get the exposure, but of course hands and feet are very potent places in your body to get a fast activation of your nervous system, of course.

But you can also just dip them and then take them up. It's still going to activate that, but you have your full body is covered in a coke receptors. You'll have a full activation anyways, so yeah. - You are providing very reassuring information to people 'cause I know a number of people that do not like to put their hands in.

I find that the more of my body I get in, the more comfortable I am. I don't know if it's psychologically and or physiologically. I find that where there's an interface between the water and the cold, it's most uncomfortable. So I prefer to just get everything under. I keep my head out.

Although these days I've been dunking all the way in and then coming out and then dunking once more with my head under before I get out after the plunge. That raises a different question. Now we're getting into kind of the practicalities of deliberate cold exposure, which I think are important.

Sometimes I'll experience, and I hear from a lot of people that they'll get a kind of back of the head headache at the interface of the water when they're doing cold immersion to the neck. I assume this has to do with blood flow, that there's vasoconstriction right up until the neck and in the region surrounding it, but that maybe there's still blood flow to the head.

But do we know what the origin of these headaches is? And again, this doesn't happen for everybody, but some people do experience them. - Okay, yeah. I haven't really heard about that one specifically. But I would say that there are different reasons for maybe keeping your head out of the water.

But it seems like maybe for some, that could be a reason for just getting a quick head dunk. - Going all the way in. That's what I've started doing to eliminate. I wasn't getting headaches, but I noticed that interface. And I wasn't in the rest of the experience of it so much.

So I started dunking all the way in. I noticed in some of the photos that you've put out and in your book that you'll sometimes wear a cap while you go in. - Yeah. - Okay. - And well, it comes from different reasons. So let's talk about some of the physiological reasons.

So when you submerge in cold water up to the neck, studies have shown, and this is from Denmark, studies from BSPBI hospital, that when you submerge into cold water up to the neck at zero degrees, so zero degrees Celsius, very cold, you will have a decreased blood flow to the brain by around 30 to 40%.

And it makes sense because you activate the sympathetic nervous system. And therefore you will have less blood flow to the brain. It makes you maybe a little bit dizzier. - Proof again that you need a heart more than a brain because when the sympathetic nervous system gets activated, blood flow is maintained to the heart to keep you alive, but obviously taken away from the brain to keep you from thinking.

That's why it's hard to think when you're stressed. - Yeah, well, the muscles and your vital organs need to, you have to be able to run away from that tiger, right? - The rationale makes total sense. And who am I to disagree with mother nature? - Well, but yeah, so one of the reasons being that you should keep your head out of the water is that you could increase that decrease in blood flow to the brain further if you dunk the head.

So there's just a very nice paper from a research group in Canada where they have collectively looked at different papers where they compared heat loss in a group, in the papers where they dunk the head and compared to heat loss, submerging up to the neck to see how much extra heat do we lose from our core when we dunk the head.

So, and I think it's very interesting that if you submerge up to the neck, you have a heat loss of 11% from the body core. And when you then also dunk the head, you will increase that heat loss rate by 36%. So that means, I'm not saying that, I'm not here to say what is right and what is wrong.

I just think that people should know the information so they can for themselves evaluate what is best for them. But if you increase your heat loss rate by 36% from your core, that's gonna increase your after drop, which we touched upon a little bit earlier, even further. So that's meaning that you are closer to hypothermia than you are if you just submerge up to the neck.

So you should really think about whether this is like something that you want to do, or if it's just better for you not to get that cold in your core. The beanie is also because I have a little bit of sensitive ears, so meaning that if there's wind, and because we swim in the open sea in Denmark, we have a lot of wind.

Our conditions are just very rainy, very windy, and when the temperature is also freezing, you could get this, what is that called? So very cold and lightheaded just from wind. So if you also submerge into cold water and you then get up, you will get a brain freeze immediately.

So it is enough to just go up to the neck, wear a beanie to just not get dizzy also because the heat loss is increased, of course, but also the blood flow to the brain has decreased. So the beanie will keep you a little bit warmer so you can stay for one to two minutes.

So it's just a way of getting around some of the conditions also. So people can choose that if they feel that, but it's quite normal to do in Scandinavia wear a beanie. - Love it. So for those of you afraid of doing a two-minute cold shower what Dr. Silberg just described, you see how she and others are capable of doing things far harder than that.

When the way you describe it with the cold wind in Scandinavia is quite striking. Along the lines of covering the head, there's this seemingly paradoxical thing of people going into hot saunas and wearing wool caps. You know, if you go to a banya or you go to a sauna and there are people who are, well, from Eastern Europe or typically are Finland or Russia or Ukraine or elsewhere, what you'll see is that many of them are wearing wool caps in the sauna, which many people think is to make it hotter.

That's actually not the case. It actually insulates you from the heat environment. The sense of urgency to get out of the hot sauna is a brain-driven mechanism. And so the reason that people wear wool hats in the sauna is it actually lets you stay in the sauna longer because it takes a lot of heat to the skin before you feel that you have to get out.

Whereas so when you insulate the brain, you don't get that signal. It's pretty interesting. I've tried this before just by putting a towel over my head in the sauna and you can stay in there much more easily and for much longer. You know, as we talk about these different stimuli for the hormetic response, the adaptation distress, you know, it occurs to me that the big ones in our evolutionary history have been light, right?

I mean, you were talking about seasonal changes. We know there, especially as you go up to Nordic countries, there are seasonal changes in the amount of light by time of year, dramatic ones, in fact, less so at the equator, of course. Light, temperature, food, movement. And it's sort of interesting.

And at the same time, perhaps it should have been obvious to us that there are stimuli that our bodies have evolved to adapt to in very powerful ways. And so the idea that temperature, heat, and cold could evoke these tremendous physiological changes that are beneficial for us probably shouldn't surprise us at all.

I mean, this is why, I mean, these are not esoteric mechanisms. They're actually the foundational mechanisms by which our body and the bodies of other animals adapt. So I do have a question about the different ways that people could approach deliberate cold exposure. So for instance, children. I've been to banyas where there are kids six or seven years old with their parents at the banya.

And so they're in hot sauna. I'm not suggesting people do this if they're not adapted to it. And talk to your parents' kids and talk to your kids' parents, talk to your doctors. But it is remarkable. I mean, children doing sauna from a young age or deliberate cold exposure.

Are there any data on this and is it safe, assuming that, you know, obviously that they can swim and or they're doing this in a tub or shower? And then I'd also like to ask you about, are there any additional male, female differences? I know your study focused on men, but other studies have focused on both.

And you of course are a woman and can attest to your own experience with this. So children, men, women, differences there in terms of protocols. Is there anything that people should build into the structure of their deliberate cold exposure that's unique to that? - So yeah, so this was on cold exposure.

So yeah, I think that, starting with the question about children, I think that it's important to think about, as children are smaller than adults, so we cannot really completely transfer all the information and the benefits and also protocols for how long and stuff like that to children. We cannot do that because they are just smaller in mass.

And one study that actually improves this is a study where they have compared heat loss in children, boys who were 12 years old compared to adults, men, and looked at heat loss of the core temperature and exposed them to one or two minutes cold exposure, so immersion up to the neck.

And what they saw was that the boys in this study could actually defend the core temperature in the same way as the adults could, but they had to use their muscles way faster. So it means that they couldn't stay for as long and they use more energy to defend their core temperature compared to the adults.

But for one minute, it seems that they could actually, but they will be colder when they then come out because they are smaller in their mass to their ratio. So it means that if the surface is so large on children and their mass and muscles being smaller to that ratio, it means that they can be in the water less time before they get hypothermic.

So just think about that. They are just smaller. They can't defend their temperature for a very long time. But in this study, they saw that for up to, I think it was a minute or so they could- - One minute. - One minute, yeah. - I'm glad you mentioned hypothermia in smaller bodied people, children.

I used to do some Pacific ocean swims in the morning without wetsuits and I adapted to it pretty quickly. And these are fairly long swims and we brought an excellent swimmer with us that was interning with me for a while. He's 16 years old at the time and very lean.

And he wasn't small for his age, but he was smaller than us, than it was all guys on the swim that day. Sometimes women join us. And he got hypothermic and he's an excellent swimmer. And he didn't report feeling overly cold, but fortunately we got him to shore and heated him up again.

So he lived. I don't think his mother is going to ever let him go swimming with us again. He's thriving in the world. He's a university student now and he recalls that swim. I mean, this is why you always want to ocean swim with a buddy, with people. Yeah, he became hypothermic.

His teeth turned yellow. He was kind of slurring his words. He wasn't making sense. We got him onto shore and he was kind of drooling and a little semi euphoric and then kind of, hypothermia is no joke. So I think, yeah, so I'm really glad that this is coming up because the cold is a powerful stimulus and kids are at a, and smaller bodied people are at a greater risk of hypothermia.

So a good reason to approach it with caution, maybe start with cold showers, get then cold immersion and still water, natural water and open bodies of water. Of course, they're always going to be more dangerous for other reasons, currents and things of that sort. Okay. - And drowning, yeah.

- So important note there. What about any additional male, female differences or similarities that we should be aware of? And this comes up all the time on social media. Anytime I post anything about a study, it's what about women? Because oftentimes there are differences. - Yeah, yeah. And we also just talked about the difference in temperature in men and women.

So it means that if we replicated my study in women, it could be that they would have enough, you can say cold exposure with just nine minutes per week. It could be because they apparently are also just colder and they have increased metabolism in their brown fat. It's just they have more brown fat.

It could be, but this is just something that I frankly don't know. But women also do cold exposure winter swimming with the 11 minutes protocol. I do it myself and feel good about it. So I would say that women also regarding activation of the brown fat, it should be the same in theory, but I don't know if women actually do need to have another protocol when it comes to this rapid cold exposure.

I think that it's another question if we are talking about ice swimming, when it comes to how far can you be in the cold water without getting hypothermic, then there will be differences in men and female. But if you do this cold exposure for a very brief amount of time, which is what I try to talk about, what we call also microstressing the body to increase the hematics stress, the healthy stress, then this is such a short amount of exposure that it's fairly the same.

I think women can look at this as a fairly good protocol for them as well. - I always say that if you really dread the cold and don't like the cold, then you are a perfect candidate for using deliberate cold exposure because the sympathetic, aka the stress response, will be greater and thereby the adaptation to that shorter one or two minutes is going to be much greater, right?

For people that are perfectly comfortable in the cold, it's harder to get an adaptation response, the same way that if somebody is very strong and they can lift a very heavy weight, that very heavy weight is unlikely to evoke the same kind of, or same degree of adaptive responses if somebody is not quite as strong.

So another reason to keep these exposures relatively short and more frequent than to do longer duration exposures frequently. However, let's say somebody only had two days a week to do deliberate cold exposure. Maybe they don't have access to a sauna, maybe they do. Would you suggest that they get in for one or two minutes, then get out, then get back in for another couple of minutes, then get out and call that for four or five minutes to try and get to that 11 minutes total per week, as opposed to getting in for a full five minutes and then getting out and coming back a second time that week.

I know this is getting down into the weeds, but these are the sorts of things that I think people really want to know because a lot of people either don't live close to a body of water or don't have a cold plunge that they can do this with, although cold shower apparently works too.

So most people live close to a shower. - Yeah, so definitely, I think the changes in temperature is what is strengthening your cells in the body. So if you can do the short amount of exposure and then get out and get back in, that is gonna, you can say, strengthen your cells because you are challenging them to adapt to changing temperatures.

So doing one session, you can change this, right? You can do it if you are able to go to cold water, but also a sauna, then you just do it that automatically and you will have a change in temperature. But you could also do it by variating the temperature in your cold plunge if you have a plunge or if you have the open sea or you have seasons even, we have that in Denmark, so we have four seasons and the temperature is gonna vary with that.

So we have nature who can just change this for us and we don't have to think about it. But if you have a cold plunge, well, then I would say that changing the temperature is what is gonna create this hermetic stress and also keep your cells on its toes, you can say, because the body will still be stressed to try to adapt to the new temperature as it's seen as something actually toxic to the body, right?

It's a small piece of toxicity that you are exposing yourself to. You don't have to swallow it, but it's enough that you touch it actually. - Yeah, great way to frame it. That brings me back to this idea of circadian time. In your study, you didn't control for a specific time of day and now I'm realizing that may be a great asset to the whole thing.

So we know, for instance, that our bodies go through pretty dramatic shifts in temperature from the time we wake up, our body starts heating up as we wake up and continues to heat until the afternoon and then starts to drop in the later afternoon and then assuming all things are working correctly, that body temperature drops and we sleep.

So I could imagine that doing deliberate cold exposure at different times just by way of convenience or by way of intention could be very beneficial because my body temperature is going to be quite a bit warmer at one time of day versus another and in that way, keeping the system tuned.

And that's really what I keep hearing coming through as you explain these data and all these beautiful studies, yours and others, is that it's not really about getting cold. It's about going from warm to cold and from cold to warm. And I love this idea because, I probably said this a hundred times on my podcast and a million times in my life and I'll continue to, which is that biology is not an event, it's a process.

Like these metabolic and thermoregulatory processes are indeed like the turning of a knob. It's a verb as opposed to a noun. And so I think if people can internalize that idea, that they're going to have a lot more flexibility, a lot more fun and get a lot more benefit as opposed to thinking, okay, I need to get into X degrees of water for X amount of time on X number of days in a very rigid way.

- And I get this question all the time, how much and how cold. And I mean, it's just like, well, because we also don't have studies showing exactly if you just keep five degrees in your water and you do that for a month, then what happens? Maybe in the future, we will know much more about this and I'm sure it's going to come and I really hope so.

But I just think by logically changing that temperature up and down, up and down, and you also do that in your water, it doesn't really, it's not that important what temperature you will have your water then, then just keep changing it, going up and down. It could be all up to 12 degrees Celsius.

You're going to activate your brown fat anyways. I mean, 12, 19 degrees cold air is enough to activate your brown fats. So maybe we don't have to go as cold as I think many people think and putting ice even all the time, you don't have to. I don't think it's necessary to expose yourself to that cold temperature all the time, but vary it a bit.

- So keep the system off balance. - Yes, off balance. - It's the way to keep it tuned. - Yeah. - You mentioned a study that is more recent or an ongoing that's not published. If you're willing, could you share maybe some of the data from that findings from that study with of course the cue to everybody that these are not yet published data so that the conclusions could change, the data could change for that matter.

- Yeah, so we haven't analyzed all the data yet. And I know from the study that we did publish that we would need to look more at the data. So I don't really have any results yet that I can share because we are still in very preliminary analysis of this.

So I wouldn't know yet what to exactly say about it. But what we looked at was both men and women method. So that's coming. - Oh, that's fantastic. That answer is going to please a great number of people and intrigue everybody. Well, listen, I want to really thank you for coming here today to talk about your work and the incredible direction that it points to.

Because I think that no one study is definitive, but your study really, again, stands as a landmark in the landscape of exploring deliberate cold exposure and heat, how it can impact and potentially impact our health. Because frankly, there just haven't been that many high resolution, detailed modern studies of this.

There have been studies of sauna. There've been some studies of cold. There are a lot of groups in physiology that work on hypothermia and very cold exposure. But most of the temperatures used in those days just aren't practical. So first of all, I just want to thank you for doing the work that you've done and for the work that you continue to do.

I'm waiting with bated breath, as they say, to hear the results of this study that's ongoing on both men and women. So we'll have to have you back to inform us about that soon. And I want to thank you for the incredible public education efforts that you've been doing on social media and with respect to your book.

And we, of course, we'll put links to all of those things in the show note caption so people can learn from you and can continue to learn from you. We certainly need more scientists who are both experienced with doing hardcore research, as it's called, and who also do the practices.

I think that's a wonderful additional asset. You're not just behind a lab coat or bundled up in a down feather jacket as everyone else is getting into the cold. You do these things. And that you are so open and generous in the way that you share knowledge, which includes coming here today to share knowledge with me and our audience.

So thank you ever so much. - You're very welcome. I am so pleased to be here and thank you so much for inviting me. And I could explain my study and I can share some of my insights from doing that. So I'm very grateful for being here. Delighted, and we'll have to have you back again.

Thank you for joining me for today's discussion, all about deliberate cold and deliberate heat exposure science and protocols with Dr. Susanna Soberg. If you'd like to learn more about Dr. Soberg's research, or you would like to learn about the research of her institute, the Soberg Institute, please see the links in the show note caption.

Also in the show note caption, you can find a link to Dr. Soberg's excellent book, Winter Swimming. If you're learning from and/or enjoying this podcast, please subscribe to our YouTube channel. That's a terrific zero cost way to support us. In addition, please subscribe to the podcast on Spotify and Apple.

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Thank you once again for joining me for today's discussion with Dr. Susanna Soberg. And last, but certainly not least, thank you for your interest in science. (upbeat music)