- 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 was 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 cohort 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- - That's per week, 90 minutes per week.

- 90 minutes per session now. - Per session. - 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 hormetic stress or healthy stress. So 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 2000 sauna bathers where they followed these for 20 years, they see that 30 minutes per session is like enough. And 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 to maybe we can aim for that if people needs 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, so. - 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. (upbeat music) (upbeat music)