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, I think that of course, in the future, we will know much more about these pathways. But what we do know is that the cold 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 cold 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 from noradrenaline, adrenaline cortisol, and which is not that much, but it's but it's still there.

So you have this increase in noradrenaline, which will then immediately activate the brown fat. And because the you can say the activator is the most potent one, cold and noradrenaline, that's going to activate the brown fat. But there's also a direct pathway from the cold receptors in the skin to the to the brown fat, which really shows that if because of these different pathways, it shows that that it could be that this tissue to keep us warm, or was developed in our involvement 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, because it seems that the brown fat is also activated when we get warm on our skin. So it's also maybe a temperature regulator in our 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, our temperature up. So muscles and brown fat are working together to to keep us warm. So we don't suffer too much in the in the cold water.

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 mean, I think it's important for people to know about because, as you said, so so eloquently, the when something is very important to our survival or end or evolution, the brain and body install multiple mechanisms for it, not just one.

And and so it sounds like it's cold skin, cold on the skin, triggers and 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 when 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, it was 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, if you don't sit in the cold water for too long, and 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 it 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 that those blood vessels will open again, and the warm blood would 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. 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. Let's talk about it. Yes. Ending on cold, 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.

Yes, but I don't think that people should avoid it that much. It's just like seeing shivering as a way of your body, 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 going to increase what's called the insulin sensitivity.

So if you can, in your mind, get used to the thought of shivering, it's 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 going to hurt because that's what shivering also does, but it's just a different way of training your cells in your body.

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

So as long as the cells get exposed to this, it's going to 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 drawing that parallel, because people today know more about, we know more about exercise and what that is going to do to your muscle cells. But the same kind of 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 hormetic stress. So it's the same process, just different practices.