The truth about cold exposure with Dr. Susanna Søberg

Most of us avoid the cold. We crank up the heating, bundle up in layers, and curse every icy gust of wind. But what if freezing — or even sweating — could be the key to better health?

Dr. Susanna Søberg, the researcher who coined the “Søberg Principle,” has spent years studying the effects of cold plunges and saunas on the human body. Her findings? Just minutes of exposure a few times a week could improve insulin sensitivity, activate brown fat, and lower stress.

She’s joined by Prof. Tim Spector - professor of genetic epidemiology at King’s College London, one of the world’s top 100 most cited scientists, and scientific co-founder at ZOE - who explains how these temperature shocks may even impact your gut microbiome.

This episode will change how you think about discomfort, explain the science behind extreme temperatures, and might just inspire your healthiest new habit.

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Transcript

Jonathan Wolf: Susanna, thank you so much for joining us today. 

Dr. Susanna Søberg: Thank you so much for inviting me. 

Jonathan Wolf: It's a pleasure. And Tim, thanks for being here. 

Tim Spector: Very excited with this one. 

Jonathan Wolf: Susanna, we'd like to kick off our show here at ZOE with a rapid-fire Q&A with questions from our listeners. Are you willing to give it a go?

Dr. Susanna Søberg: Of course. 

Jonathan Wolf: We have some very strict rules. You can say yes or no, or if you have to, you can have a sentence. Ready to go?

Dr. Susanna Søberg: Ready. 

Jonathan Wolf: Can a hot sauna mimic the effects of mild exercise? 

Dr. Susanna Søberg: Yes. 

Jonathan Wolf: Can shivering boost my metabolism? 

Dr. Susanna Søberg: Yes.

Jonathan Wolf: Tim, could extreme temperatures change my gut microbiome?  

Tim Spector: Possibly. 

Jonathan Wolf: Tim, is my metabolism mainly about my weight?

Tim Spector: No. 

Jonathan Wolf: Susanna, are there rules that we should follow before braving extreme hot or cold? 

Dr. Susanna Søberg: Yes. 

Jonathan Wolf: That was great. Now you get a whole sentence. What's the biggest misconception about extreme temperature exposure? 

Dr. Susanna Søberg: The biggest misconception around extreme temperatures is that women shouldn't cold plunge. 

Jonathan Wolf: I'm really excited by this episode. It is one of the ones we have had the most questions from members about over the last six months, I would say. I think that's because saunas and cold plunges are popping up everywhere right now. 

On the other hand, I'm going to be honest, Susanna, my experience of stepping into freezing water is it's absolutely awful. I hate it, I can't imagine why anyone would want to do it. 

I want to understand basically why everyone's got so excited about this, and whether it is just the latest fad or there's actually any reason to believe that making oneself miserable can possibly be good for oneself. 

Why don't we start with a sauna? What's happening in a sauna, and why is it that it sort of makes me feel sleepy and makes me want to slow down when I go in one? 

Dr. Susanna Søberg: The thing about sauna is that you actually both activate your sympathetic nervous system, so that is your fight and flight system in your body, but you also activate your rest and digest part of your system, and that is what is called the parasympathetic nervous system.

So when you go into the sauna and you sit there for a while, you'll heat up from the inside and out and you'll start to sweat. If you can sit there long enough for your heart rate to go up, you will have a sweating that will correspond to having a mild workout. 

So the cardiovascular, you can say, activity corresponds to doing this mild kind of workout, and that is stressing your system just a bit.

When you do that, you actually also increase cortisol levels, which people also misunderstand that that is so dangerous. But it is not, if you just do it acutely and then you will have that decrease in your stress response afterwards. 

That is why maybe you get a little bit sleepy because you actually get an increase in your temperature in the body, and that is what you do when you actually want to go to sleep. So if you get a little bit sleepy afterwards, it's kind of natural. 

So, taking a sauna, maybe at a certain time of the day, might be a good thing. So you can use it as a tool. 

Jonathan Wolf: You are saying just sitting in the sauna actually makes my body go through a sort of workout because it's making my heart rate go up.

Could you explain that for a minute? 

Dr. Susanna Søberg: Yeah. And even without moving a muscle, and that's kind of amazing.

There are these cohort studies performed in Finland, so researchers there have followed more than 2000 sauna bathers and saw that after 15 to 20 years, they could see that they actually had better cardiovascular outcomes based on how many times they had done saunas per week.

So the more they did, the better cardiovascular outcomes they actually had, and lower risk of developing cardiovascular diseases, and even had a lower mortality risk when they measured this over 20 years. 

When they then looked at the comparison between how many times a week you go to the sauna, and then also if you did exercise on top of that, they did see this additional or additive effect when you combine the two.

So you can see sauna as a cardiovascular workout that is not attributed to your muscles, but you still get that workout for your heart, and that is really good for your cardiovascular system. 

Jonathan Wolf: Why does my heart have to do any work in the sauna? I can see I need to sweat a lot so I don't die. That I can understand.

But what's going on? Why does it make my heart rate go up? 

Dr. Susanna Søberg: When you get warm on your skin, the temperature sensors are going to send a signal to your temperature-regulating sensor in the brain. There, the sensors say, well, are you getting warmer or colder than you were before? And depending on that, they will send out signals in the body saying, Whoa, no, you're getting too warm and send out noradrenaline. These are our stress hormones. So noradrenaline is released in your body, which will make your heart rate go up. 

It will also activate your brown fat to make you heat up so you can sweat better. Because the only thing the body wants to do now is survive this heat, because the body doesn't know when you will ever get out again.

Because you are in control, you know yourself that you can sit there for a while, but the body doesn't know that, so it wants to get rid of all that heat. And to do that, we need to increase the heart rate. 

You also dilate your blood vessels, so the blood needs to get pumped around in a longer way. So the body has to get more blood flowing around, and for that, you would need to increase your heart rate.

Jonathan Wolf: And Tim, as you're listening to this, are there any reasons to be concerned about that activity, or is this all great, and healthy, and natural? 

Tim Spector: It is putting a strain on the cardiovascular system, so if you are unwell, it's probably not a great idea to do it in a proper hot sauna. So I think we have to take that in mind.

And there are probably individual differences. Some people do faint more easily when their circulatory system is changing. It might be on blood pressure medications, which means when it goes up or down, it can drop more than others. 

So, I think certainly the first few times people are doing it, do it for short times. Be rather careful, underdo it rather than overdo it. 

But as long as you are sensible, when you're starting, you sort of build it up gradually, the risks are pretty minimal, and most people are going to get benefit rather than any risk. 

Jonathan Wolf: I think the first time I ever went in a sauna was when I went to Finland. My grandmother thought this was completely mad, Susanna, and was like, Why would you choose to get put in the oven as she put it. 

So I think it's fascinating that you're explaining that there's these things that we understand, there's stress, these changes, and you're saying they can be beneficial.

Before we unpack, I guess why those stresses could be beneficial, could you talk about the opposite of this? So one is going into the sauna, but I know you're a big believer also in the cold plunge. 

What happens when I'm thinking about my own personal experience of going into the sea in England, which I've done once because it was way too cold, it's very unpleasant, what's going on in your body as you do that? 

Dr. Susanna Søberg: A lot of things are going on in your body when you go into the cold water. It's very different from a cold shower, also, but we can get back to that. 

So, immersing yourself into cold water will mean that you will put this very, very big stressor on your body because the body is in a natural temperature, you don't want to get too cold.

But this is so potent because it surrounds your body immediately with no air, of course. So the potent of activating your cold receptors in the skin is 100%, and that is like a huge stressor, sending a rapid signal to your brain that now you are definitely in a situation where you are in danger, actually.

So the body acts as if you are in danger, even though you do it deliberately, and sends a signal to the brain to regulate your body so that you can better survive this. So it sends out noradrenaline to activate your brown fat. 

Jonathan Wolf: And Susanna, can you explain exactly what is brown fat? 

Dr. Susanna Søberg: We have two kinds of fat tissue also in the body. One of them is the white fat, which we know. It's on our belly, it's on our thighs, and we want to get rid of that because it can grow. It's so difficult to activate it and get rid of those fat pearls, you can say, because it's all fat pearls just stuck together. So that stores our energy in the body.

Quite opposite to that, we have the brown fat. The brown fat actually increases our metabolism. So the brown fat can burn the white fat. So it's kind of like a good fat. 

We could say that the white fat is a bad fat, but we also need some of that white fat. But the brown fat, we want to increase so that we can have a higher metabolism, more energy expenditure, both when we are purposely activating it, but also when we are just sitting here, or when we are asleep.

The more we have of the brown fat, the better. You can compare it to the muscles. So we all agree and know today that it's good to have a lot of muscle mass because there's a lot of mitochondria in the muscle fibers. And the more we have of those, the better it's functioning, and it increases our insulin sensitivity.

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It's the same with the brown fat cells. The more we have of brown fat cells, the more mitochondria we have, the better insulin sensitivity we have. 

Also, it can burn more fat in our body, not only when we are activated on purpose, but also when we are just sitting here. So our basic metabolic rate will actually go up if we have more brown fat.

Just like the muscles, it can grow if we use it, you can say, and if we don't use it, we will lose it. So it's something that we can grow, and it can also shrink. 

Jonathan Wolf: Firstly, where is my brown fat? Because I didn't realize I had any, so I'm now really curious. And secondly, you mentioned the brown fat, because it's linked to this discussion around the sauna and the cold plunge.

Could you just help me again to understand that link, and our listeners too. 

Dr. Susanna Søberg: The brown fat is located in six different places in the body. The biggest you can say, depot, is located under your clavicular bones. So down your neck and out to your shoulders.

All the time, feeling what temperature are you in, making sure that you have blood to your brain and warm blood to your brain, and warm blood to your heart, and your liver, and your kidneys, so your vital organs.

So it's centered around your central nervous system, which is kind of smart from nature, right? Because then, when you get a little bit cold from walking outside, in the wind, there were actually studies on this showing that if you walk outside in just a t-shirt, you will activate your brown fat. 

So it's so sensitive, making sure that we are at the temperature balance in our body so we don't get too cold or too warm. By doing so, actually it uses fuel from the body, which is sugar, so glucose, and fat, which is the white fat, to increase this heat in the body. 

It's like an engine where it takes the fuel of sugar and fat from the bloodstream, and that is where we see that if you activate the brown fat, it can actually increase our insulin sensitivity, and it can also clear the blood sugar from our bloodstream. 

This is shown both in mice studies, but it's also shown in human studies as well. So the cold is shut out in a way to your best, you can say you are only human, you are not made of metal. So that means that very quickly you will cool your inner tissue.

So that is why I'm very, very much a big speaker for only short-term cold exposure, because the long-term cold exposure has no benefits. There is no research showing that sitting in a cold bath for a very long time has any benefits. But the brown fat is keeping you warm.

If you cannot keep yourself warm enough with just brown fat activation, you will also start to use your muscles to shiver. And the brown fat and the muscles are the only two thermogenic tissues in our body, and they will signal to each other, so kind of talk to each other, actually, and make sure that it's activated.

So the muscles are sending signals to the brown fat and the other way around, to make sure that both thermogenic tissue is keeping you warm.

Jonathan Wolf: And Susanna, when you say thermogenic tissue, what does that mean? 

Dr. Susanna Søberg: Thermogenic tissue is tissue in your body which will make sure that you are in the right temperature for your cells in your body to have the right activation and function. 

Jonathan Wolf: It's funny, I'm thinking of my grandmother again, who was always like, Well, it's cold, you have to wear a scarf. 

Now I'm thinking about my scarf, is that literally going around my brown fat, because that's somehow I'm more sensitive. Or is this entirely by chance? 

Dr. Susanna Søberg: No, it could be by chance. But if you are a cold sissy, which I'm not afraid to say that I used to be a cold sissy, but maybe you are a cold sissy because you never expose yourself to the cold. 

So you get colder by staying, you can say, too warm, too comfortable. When you are comfortable, then you get colder. So it's difficult for you to temperature-regulate yourself.

The purpose of using your brown fat, then you'll get better at temperature-regulating your body. And for that, it needs fuel. That is why the brown fat is so closely connected to your metabolism. 

So if you go out and expose yourself to the cold like I did years ago, now it's 10 years ago, and I started doing that, it was horrible, really. I used to sit with blankets and layers and layers. Today I don't have to do that. I don't have cold hands and feet anymore, and I don't pack up in layers anymore like I used to do. 

I can actually do cold water swimming today, which was so difficult in the beginning because I actually don't think that my brown fat was really activated, or I didn't use it that much because I kept packing myself in warm clothing, and maybe I was just very comfortable. Maybe I just resembled a lot of the modern people today, we are so scared of getting cold. 

So my personal journey definitely helped me understand this tissue much better and how temperature-regulating ourself actually something that we can contribute to, and how our metabolism works is definitely something that has everything to do with our choices in our everyday life.

Tim Spector: Can you explain to me, Suzanne, a lot of the science was done on mice and rats that have a lot of brown fat, and it's easy to study them, and clearly it works for these rodents. It's a really crucial part of their survival mechanisms. 

But for humans, I don’t know what the latest data on it says, but it was something like only half a percent or something, of our fat is brown fat compared to our white fat. 

It's a tiny amount that we are expecting the same results from mice, and rats, in humans. Is it really that important in humans, or do you think we've overestimated it from these animal studies, or is that still the best explanation that we have of how humans control their temperature?

Or do you think there could be other mechanisms we don't know about? 

Dr. Susanna Søberg: I think there's definitely a lot we don't know yet, but brown fat has been one of these research areas for hundreds of years that we have known about. 

But it was not until the millennium that we discovered that a little bit of cold exposure could actually change our metabolism. So our glucose balance and our insulin sensitivity.

In humans, this is actually not about the size of the tissue. There was actually research in humans, and this is so fascinating. It's a study from 2014 where the researchers had people who had a bad insulin sensitivity, and some of them were obese, and some of them had type 2 diabetes. 

They had them sleep in a room which was 24 degrees Celsius for a month, and then they measured their brown fat. Then they slept at 19 degrees for a month, and then again measured their brown fat and insulin sensitivity. 

What they found was that sleeping in a room at night at 19 degrees Celsius actually increased their insulin sensitivity, and the glucose clearance got better. The measure of the brown fat, you can see that it had increased, and that was just from sleeping at 19 degrees in a cold room for a month. 

So it's a very potent tissue, which when activated, you can actually increase it, and when you're not using it, it shows also that it decreases. 

What they did after that month was having them sleep at 24 degrees again, measured the brown fat, and then at 27 degrees Celsius and measured that again. Then they could see it actually is shrinked when the temperature got warmer. 

So comfortable temperatures make the body tell this tissue that it's really not needed right now. So it's like the muscles, then they shrink. If you use the muscles and you go to the gym and you lift your weights, then you are telling your muscles we need you.

And the same thing you can do with your brown fat. Tell your brown fat that it's needed to help me keep a balance in my metabolism and keep my insulin sensitivity high.

Because that is exactly the pandemic that we have right now, that people's insulin sensitivity is just out of tune completely. 

Jonathan Wolf: Susanna, I would love to maybe take this point to talk about your own research. 

I think you've painted this wonderful picture about this brown fat that I had no idea I had. You emphasize a lot, I think the stress of both the cold plunge and the sauna. Why can stress be good for me? 

And maybe we could look at that through your own research, because I know when we were trying to figure out who to talk about this topic, very few people have really been studying this properly, scientifically, and you're one of them. 

Could you tell us about Scandinavian winter swimming?

Dr. Susanna Søberg: I would love to. Scandinavian winter swimming is definitely special. I feel it is. 

It's something that we have had for hundreds of years here, but we've been doing it because it feels nice, it's a good community thing to do. But recently we also starting to look more into the health benefits of it. 

Can we actually prove that it has any benefits, which all the winter swimmers in the Nordic countries are saying?

So, one thing is, there's something that we can feel. You feel better, you get more clarity in your brain. You don't feel the brain fog anymore. You have more drive, and those things you can feel.

But there are also things which you cannot really feel and tell at once, and those were the things that I was interested in. So the metabolism. 

Can this extreme cold exposure or heat actually create what is called hermetic stress? So this is what we call the good stress. Acute stress can be a good thing for you because it activates your mitochondria and it tells your body that you need to waken up and you need to do something to protect yourself. 

That is a good thing because you activate your metabolism, which is the brown fat muscles, all your mitochondria, heat shock proteins in the cells, everything is activated. 

In my research, I wanted to figure out something else that, besides exercise, which I think we have established now that is healthy for us. Is there something else that we can go out and say here you have some more healthy stress you can do, every day or almost every day, and that would also help on your metabolism.

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I figured, Well, Denmark is super lucky, we have winter swimmers going into the cold water, and they might actually have more brown fat because the brown fat is activated by the most potent stressor, which is the cold.

What if their metabolism was actually faster or better, or in some sense we could see a difference between a group who were winter swimmers and a group who was not.

Jonathan Wolf: And so, Susanna, what did you discover? 

Dr. Susanna Søberg: What we found was that the winter swimmers who were adapted to the cold water swimming did short-term cold exposure, so very short dips, and they had better insulin sensitivity and a faster glucose clearance from the blood. 

So we tested that by giving them a glass of sugar water and they drink that fasted. Then we measured insulin and glucose over two hours in both the groups. And that showed that the winter swimmers really rapidly had this increase in glucose, but also cleared it very fast from the blood. 

When we looked at how much insulin they had in the blood, they had lower insulin levels compared to the control group.

Some might think, Why is this good for me? Well, it's shows that their body doesn't need to produce as much insulin to get the sugar into the cells. 

That is a good thing because when you are insulin resistant, which is a state that we want to avoid, then the body over-produces insulin because the cells cannot really feel the sugar.

Getting more insulin-sensitive means that you don't have to produce as much insulin, and that is what we saw in our winter swimmers. They had lower levels of insulin and a faster glucose clearance from the blood. 

They also had a faster activation of the brown fat and a higher activation of the brown fat. So they were actually in a better temperature balance all the time, and they were warmer peripherally. 

That was another find, because when I was done with the winter swimming study, I invited them to another study where we studied their circadian rhythms. So we had them sleep at our facility for a couple of days, and we found out that the winter swimmers were peripherally, actually warmer.

Jonathan Wolf: Susan, is that a good thing to be warmer? 

Dr. Susanna Søberg: It is a good thing to be warmer on your skin. It shows that you can actually get better blood circulation of your skin, and you really want that. 

Jonathan Wolf: So if I understand this rightly, having done all this study, you're saying you have these two groups of people who are matched on everything else, and the ones who were suffering the freezing cold Scandinavian sea were just basically healthier than the other group. 

And that was based on quite short periods of time, they weren't sort of going for a half-hour swim. How long did you have to be in the water to qualify in this group and get this benefit? 

Dr. Susanna Søberg: After a year following them, we saw that they did 11 minutes of cold water immersion per week, divided on two to three days. And also 57 minutes of sauna per week, also divided on these two to three days. 

On each of these days, they did three dips in the water and two sauna sessions. So if you divide this out, it corresponds to one to two minutes in the cold water and only 10 to 15 minutes in the sauna per sauna session.

Jonathan Wolf: So, just to play it back, I go in the sauna for 10 minutes, I get really, really hot, then I go into the freezing sea for about 60 seconds. I'm now really cold, I go back to the sauna, get myself warm, and I basically repeat that three times. 

These people were doing this twice a week. So it was about a sort of half an hour session between these two, twice a week. And that was enough that you already saw this difference between these two groups? 

Dr. Susanna Søberg: Yes. So they did it very briefly. You might ask yourself, how can something that is so short and so brief have any longer-term effects? 

But that just shows how potent the cold and the heat is. So, extreme temperature can be a very good tool to activate your metabolism.

That is what we want to do because it activates more mitochondria in our body, and we want to increase our mitochondria in the body because that is how we keep our cells younger and healthier. 

And how we activate something more nerdy called the heat shock proteins in our cells. These heat shock proteins can actually repair ourselves from the inside.

So it's a way of keeping our cells younger or healthier, and they will survive longer. That is your longevity, you can say. So the cold is very potent, the heat is very potent, but just be mindful of how we use it. It's not more is better. That is definitely also a misconception out there that I'm trying to help people with.

Less is more. 

Tim Spector: I've done some cold plunges, unlike Jonathan, 

Jonathan Wolf: That's because I'm a cold sissy. I've now established that. 

Dr. Susanna Søberg: It's okay. 

Tim Spector: So why is the first 30 seconds the worst, and what changes after that 30 seconds? Is it the brown fat, or is it just your brain saying, okay, I'm not going to die, this is fine, I can carry on. 

Why is there this general rule about three minutes in something that's less than eight degrees, or whatever the ice bath is, that seems to be optimum? 

And the other one I've heard is that after a cold plunge, you should naturally try and rewarm rather than jumping into a sauna or a hot shower.

We have to describe it for Jonathan, because you can only imagine, you see what it's like. But everything is screaming at you to get out, basically. Your whole body, your whole evolution, everything's saying you're in the wrong place, everything hurts. 

There's like a tight band around your chest, it's like you can't breathe, you know, get me out of here. 

Dr. Susanna Søberg: Exactly. 

Tim Spector: If you lost that, if you can stay there or someone's forcing you to stay in, then it suddenly gets easier. And I'm interested to see why we think that is. 

Dr. Susanna Søberg:  So when we activate the cold shock response, and we do that immediately as we go into the cold water, and if you're new to this, you will also activate your gasping reflex.

So that might be you, Jonathan. And I can tell you I've been there many times. You'll be like [gasping] and then it's okay. So just take the whole journey in and be okay with whatever comes out. 

The cold shock is where your brain goes, oh my God. It's a control-alt-delete button, what that is what I call it, because nothing in your body wants to do anything else, but just survive.

Even your brain is actually zooming in totally on the now, which is a good thing because all your worries, everything you just thought about before, just is totally deleted. It's gone, it's away. Your body and brain is in the now. 

After the 30 seconds of cold shock, where you have to breathe through it, your body goes into an activation of parasympathetic activation.

That's because you are activating your diving response as well, because you're submerged into cold water, right? And when you do that, you'll feel like you went from this awful, stressful situation, which you could almost not breathe yourself through, and then suddenly you feel like the world is stopping. Or you are standing still, and you cannot even understand how you were almost panicking 30 seconds ago or one minute ago.

I feel like this is the point where you have to build yourself up to. It took me at least five attempts or even more, I would say, before I was at that stage where I felt completely calm. 

Jonathan Wolf: You've talked quite a lot about metabolic health, about how it's helping me with my blood sugar and my insulin.

Are there any other benefits in your mind from doing this, in terms of my health? 

Dr. Susanna Søberg: I think so. Yeah. So there is also the mental health part of it, but why I talk so much about the metabolism is because your mental health is so connected to your metabolism. 

We know that if we can activate our metabolism, we can decrease inflammation. And we know that depression, anxiety, we know that Alzheimer's disease are connected as an inflammatory state also on the brain. 

So we do really want to activate the brown fat because that's going to affect our mental health as well. 

But what the cold water does immediately for your mental health is activate dopamine, which will give you that drive and that energy and noradrenaline, which is our stress response also will also give you that energy. 

So the clarity it will give you afterwards, that is something you will feel immediately. You cannot feel immediately that your inflammation is going down, right? But it's all connected. 

The immediate response of your mental health is there when you leave, and you can feel that, drive that energy, and you'll be definitely more focused. 

Jonathan Wolf: I'd love to tie this back now to our initial questions at the beginning. Because Tim, I asked you this question that seemed mad, and you said, well, actually maybe it's true, which is how this might impact our gut microbiome.

I think anybody who's been listening to a lot of our shows knows that the gut microbiome is really central to our health, and we understand that now. But it seems to me like if there's one bit that ought to be insulated from hot or cold, it ought to be my gut. 

So, how can this be having any impact on my microbiome, and is it, Tim?

Tim Spector: Well, your gut microbes are not going to feel the icy cold when you get in the bath, right? 

They are well insulated from that, and that's the reason they're there, is they like a nice, cozy home that keeps the thermostat very closely regulated. The acid level is just right. The mucus is nice and comfy.

But of course, your microbes pick up on the signals coming from the body, so they pick up on stress levels, and as the microbes are talking to your immune system and your nervous system, which are lining the gut, there's a two-way interaction. 

So they're picking up anything that the nerve system is sending out as chemicals, anything the immune system is sending out as chemicals. 

We've heard this low levels of stress, which can be beneficial, this so-called hormesis is something. We know that microbes will respond to stress if it's too much, they go in a bad way. 

But if the body is getting just the right levels of stress and perhaps reducing inflammation, which I think is perhaps the critical thing here, then I think we should see this translated into a happier microbiome that fits in with this.

So I don't think we have any good studies to really show this yet, but theoretically, this is how I think it could work. We know that the gut microbes are a bit of a barometer of what's going on in the body, and if they're in a bad way, they'll transmit anxiety, stress, inflammation, but also the other way around.

If the body is in a good state, its environment is a good state. Just like it gets signals from food, it will also get signals from the environment about exercise and temperature. 

As Suzanne is saying, it's a bit like your body's having a bit of a workout. And if that workout is just right, then it's getting the right signals and should improve it.

But it'd be lovely to do some really large-scale studies on this. Maybe with the ZOE dataset, as these practices get more common and more people are getting hot and cold therapies. 

Jonathan Wolf: Just very much on my mind as I'm listening to this, I have a friend who suffered from depression in the past, and he absolutely swears that his mood is dramatically lifted by a cold plunge.

This is a friend who has now bought one for his garden and is absolutely committed. It's clear it works for him, but is there any evidence more broadly about whether this affects mood, and particularly people who have real mood disorders? 

Dr. Susanna Søberg: Yeah, so there is a study, I think it was from 2024, that came out showing that people doing cold plunges changed the way that they have negative emotions.

So they have more positive thinking and positive emotions after they have done a cold plunge. It makes sense when you think about it, because when you go into the cold, you will have that increase in dopamine and noradrenaline, oxytocin, which is also part of our stress response. Oxytocin is our love hormone in the body. 

When you increase those chemicals in the brain, it's got to be almost impossible to be in the same brain state as you were before. You will always have maybe the same problems, they don't disappear from cold water swimming, but you would have another way of thinking about them. 

We all have our problems and worries, but the way that you address them and think about them is how well you will handle them also.

I think this study shows that from these brain scans, that the cold actually changes the activation of those sensors of the brain where you think about negative emotions and you have the positive emotions. 

There are also some new studies. I think we have a pilot study, definitely also in Denmark, on people with depression going into cold water. They saw that they could also get rid of their medication, or maybe not, they could just go down in medication dose. 

So it seems that there are studies coming also where we get more proof on pathology. So on diseases, brain diseases, it could be a depression, it could be the anxiety, it could be psychosis, it could be many things, where they have used this cold water immersion. 

I think more studies are coming, but there seem to be some good case studies also published. One from the U.K., where a woman had been swimming in cold water for three months, and could actually lay off all her depression medication. And she was apparently heavily medicated.

Jonathan Wolf: Susanna, I'd love to switch to actionable advice now. I think our listeners have been very patient with me and are like, yeah, yeah. I want to actually understand what to do. 

So if someone's listening to this, they've never done it before. They didn't grow up in Scandinavia, so they weren't wild swimming and jumping into a sauna the other time.

How can they integrate this into their lives? I'm also very interested in how can they start if they might be a cold sissy like me? 

Dr. Susanna Søberg: Yeah. So if you are a cold sissy, like you or me, I think the best thing you can do is don't think too much about it, actually. If you want to try it out, then go and try it out, but don't have expectations like I have to sit there for a while.

This is not a competition. Cold exposure is an inner journey. So you use some kind of cold exposure. I would say a cold plunge is definitely a good place to go if you have that, the ocean, if you have that, or if you don't, then a cold shower is also fine to get started. 

So, 30 seconds cold shower. Use your nose to calm your nervous system to breathe in through your nose and out through your mouth if you need to. If you can switch totally to nasal breathing, that will help your nervous system to calm down as well. 

You can use that also with cold plunging. So breathe in through the nose, to calm the nervous system, and never do any hyperventilating breath work before or during your cold plunges. That is also, I think, important to note. 

Head dunking, please don't do that because that would just decrease the blood flow to the brain, and it actually also increases the risk of fainting, and we also don't want that. 

You have no extra benefits of putting your head down in the water. You can splash some water to the face, which will activate the vagus nerve, but dunking your forehead down doesn't do any health benefits for you.

So take it slow and feel what is good for you. Breathe through 10 seconds, and that is fine, but try to get over the cold shock. That is the goal. 

Tim Spector: So, try and get past the first 30 seconds. 

Dr. Susanna Søberg: Yeah. I think the cold shock might actually be a little bit longer than that for new people. It could be up to one minute and for some one and a half, depending on how cold-adapted you actually are in the first place. 

So, back to the question that Jonathan asked. How different is this from people to people? And it can be actually very different. 

If you are used to being much outside, for example, then you get very quickly adapted. But if you are not very much outside, an outdoors person, then you would take a little bit longer to get adapted to the cold. 

Be gentle to yourself and don't compare yourself to others. 

Tim Spector: I like to have a sauna first, but does it make much difference do you think? 

Dr. Susanna Søberg: You can, Tim, start with the sauna, and I mean, people should do it the way that they feel is best for them, but if you start in the cold water, you have that increase in oxytocin, noradrenaline, and dopamine would change how you sit in the sauna.

Because it's sort of a place where you meditate, also.

Jonathan Wolf: In order to get the health benefits, do I need to combine a cold plunge and a hot sauna together? 

Dr. Susanna Søberg: No, you don't have to combine them. The thing about alternating between one extreme temperature and the other, this cold plunge and the sauna, is that you push your cardiovascular system, and that is a workout for all your blood vessels.

That is one thing, but also going from one extreme to the other will push your cells to either activate to generate heat or to shut down. And that is also a workout for your body. 

So in that sense, it has benefits doing it together. But if you just do cold plunges on one day and sauna on other days, that is also getting the benefits. It's just more divided. 

Tim Spector: Susanna, can I ask, our U.S. listeners will know all about cryotherapy, and I was in Los Angeles recently, and every shopping mall has a cryotherapy center now, so you can just pop in and do your three-minute cryotherapy session and go back to work. 

So it's incredibly practical. You don't have to find a lake in the middle of a city and shiver. It's reversed because you go in, and the first 30 seconds are quite fun, then it gets progressively colder because it's minus a hundred degrees in this freezer. And you come out after three minutes.

The last 30 seconds are tough. I got a similar buzz after it, but I wondered is there any science behind the difference? Is it as impressive as the cold water stuff? 

Dr. Susanna Søberg: So there are studies showing that the cryotherapy also activates your cold receptors, which means it also activates the brown fat.

I think that three minutes with these extreme temperatures it would definitely help your metabolism. But of course, there are differences because when you submerge into cold water, you also have that hydrostatic pressure from the water. 

Tim Spector: Yeah, I didn't feel I was going to die either. That was the other thing. So I didn't get the gasping or the sort of life-threatening feel of it. 

Dr. Susanna Søberg: It's not because the body needs to feel like it's dying in order for you to get the benefits. You do get some benefits, and I do think that the cryo actually has benefits. It's just another modality. It's just another dose that you get.

But you can say when you submerge into cold water, that is surrounding yourself with a hundred percent molecules around your body in cold, you cannot be packed more into cold. 

So that's of course, more potent to activate your metabolism and of course also all your chemistry in the brain. 

Jonathan Wolf: So we had another common question around this, which is, does a cold shower work at all, or does it need to be a full plunge into icy water?

Dr. Susanna Søberg: So cold showers work because you do get cold, of course, and you activate your cold receptors. 

There's a randomized control study from the Netherlands showing that if you do that for 30 seconds, end your showers on 30 seconds, cold showers, you'll have less sick days at work. 

It might be that it's not because you actually get less sick. It might actually be that you just get more energy to go to work. I'm not sure about what exactly is going on there, but it's definitely not a bad thing that you feel fresh enough to go to work.

So it will activate your receptors, your brown fat. It will also activate noradrenaline and dopamine, and you will get that increase in your sympathetic nervous system.

But it's probably not going to activate your parasympathetic part of your nervous system. And that is kind of what I would like people to experience, that they have this kind of stress up and also the stress down. 

Jonathan Wolf: And what about the flip side, which is the sauna?

If you don't have a sauna, is there any way you can get access to that? Can you have a really hot bath or something, or is there no alternative? 

Dr. Susanna Søberg: So hot baths are actually very good. There, you also activate your parasympathetic part of your nervous system. 

You don't have to sit that long in a warm bath to have the metabolic benefits of better insulin sensitivity and better glucose clearance. That is also shown in studies that hot baths are really good for your metabolism. 

If you cannot sleep, just do a hot bath before you go to bed or do a hot sauna, that will increase your core temperature and that will make your body go into the state of wanting to sleep.

So don't do your cold baths before you want to go to sleep because that increases your noradrenaline, and then you maybe want to go for a run instead, or you want to start working. It gives you too much energy. So I always advise people to do the cold plunges in the daytime. 

Jonathan Wolf: Now, if somebody's getting a little bit more committed and they're like, I'm going to figure out where I can do a cold plunge and a sauna, what is the minimum frequency?

How often a week do you need to do that, and do you need to do it three times backwards and forwards in the way you described in your study in order to get benefits? Is that what people should be looking to achieve? 

Dr. Susanna Søberg: I think that people should do what is possible. It's like exercise. So, if you can run two kilometers or five kilometers is definitely better than not doing it at all. 

So if you have time to do it once a week, then that is good. There's probably a dose relationship here. So you can do it up to two, three times per week. But I also think that after doing that, it might also be too much for you. 

So we don't want to exhaust the cells in our body with too much stress.

Jonathan Wolf: And Susanna, can I just ask a follow-up, just to understand that? Because I think with exercise, we're a bit used to this idea that more is somehow always better. 

But here it sounds like you're saying cold plunging every day might not be a good idea. 

Dr. Susanna Søberg: I don't know if every day is not a good idea, but we do see, from my studies at least, that doing two to three times a week is definitely enough to get the benefits.

Doing more, I don't know whether that is good for you, and if it actually becomes too much of a stressor for you. It is very potent. It is not exactly like exercise because exercise is… You can pause that and then you can lift your weights again, or you can pause when you run.

Here you are getting the full thing at once and doing that every day that it might not leave room for recovery in your body. You also don't want to exhaust your nervous system. 

So that is why I just always say, well, at least we have studies showing two to three times a week is enough to get the benefits.

Tim Spector: Makes sense as well, because exercise also [there is] generally suggested a rest day for your body to recover. So it sort of makes sense that you don't overdo this as well and go crazy. 

Dr. Susanna Søberg: About the cortisol level, because I think that all exercise, or cold plunge, sauna, all increases our cortisol level just a bit.

But what we do see, and I think this is one of the misconceptions that I really want to get out there, that cortisol is something that we have to increase because that is our stress system. 

We are stressing the body a bit, but acutely it's okay. But in the long term, we don't want cortisol levels to be high, but what we see from cold exposure and also from sauna, that cortisol level baseline goes down in the long term when we do cold plunges and we do sauna.

So there's a good reason for doing this on a regular basis and not only looking at, oh, when I do a cold plunge today, and right now I have a little bit of an increase in my stress, that is a good thing because it's just acutely. 

But in the long term, it will decrease your stress. 

Jonathan Wolf: Can I come back to your question at the very beginning, where you said the biggest misconception was that this wasn't good for women?

I didn't even realize that was a misconception. Do men and women respond differently? And is there any concern for women or for anybody else, actually where what you're talking about today, you would say, actually, that isn't safe? 

Dr. Susanna Søberg: So the misconception that women shouldn't cold plunge has come out there somehow.

I think it's important to say that there is a difference in women's and men's physiology, of course. Our metabolism acts differently when we go into cold water. 

Women have more brown fat than men, and men have more muscle mass than women. That evens out how we defend ourselves to the cold. 

When we go into cold water, women have an ability to activate more brown fat, but also vasoconstriction is better in women. So we can actually shut out the cold better than men can, but men have more muscle mass to generate heat, so it evens out. 

There's a study showing that men and women going into cold water activate their metabolism, meaning they defend the cold equally, but how they do it is just different. 

That doesn't mean that women cannot do cold plunges, it just means that we act in a different way towards it. 

Then there is the thing about our cycle. Women are more stressed when we are in our luteal phase. So it's important that we think about when we do the plunge.

If we have our menstruation, for example, that is where we feel the most stressed.

Also, people who do cold plunges or winter swimming probably notice that there are times during their cycle where they feel a little bit more cold. They feel not able to conquer the stress in their everyday life as good as they did just three weeks ago.

That is because of our changes in hormones. So be gentle to yourself, women, when you are in that phase, and probably do cold plunges, but very briefly, or maybe just wait until you are over that phase, or maybe just take a cold shower.

So people will need to figure that out for themselves.

Jonathan Wolf: And is there anyone or any situation where you would tell people to be particularly cautious about this and be concerned about the safety implications? 

Dr. Susanna Søberg: Yes, so if you have heart diseases and if you have unregulated blood pressure, I don't recommend that you do these kind of stressors. If you are pregnant, also just skip it for these nine months.

I mean, if you are already doing cold and heat exposure, then the body will remember it, but you don't have to do it those nine months, because we actually don't know exactly what will happen to the fetus. And we want to, of course protect that. 

We cannot do studies where we randomize people, but that is my advice to that.

Jonathan Wolf: I've heard that there's a sort of hack with your face that can actually get you some access to a cold plunge without actually doing a cold plunge. 

Dr. Susanna Søberg: If you pour water to your face, so wash your face in cold water, you can activate your vagus nerve, and that is activating the rest and digest part of your nervous system. 

Which is a good thing because that will stress you down. Of course, you'll also feel energized by it because you will also activate your sympathetic part of your nervous system. But eventually it will actually stress you down.

So it's a good, quicker, like you say in the morning, if you don't have time for cold plunge, pour some cold water to the face and you will have also sort of like a quickie cold face plunge, you can say. Yeah. 

Jonathan Wolf: Suzanne, a final question. If you were going to say to somebody, this is the best way to start, for someone who's never done this, what would you advise them as the best way to get into all of this? 

Dr. Susanna Søberg: If you are about to get started with cold plunging and sauna, I think you will need to figure out when to do it firstly.

Don't be in a situation where you feel that you are about to get sick. You should feel well. You should also feel not too stressed, because it's going to stress your body the first few times mostly, and think about the time of the day. Do it in the daytime. Preferably, I would say do it when you were about to take a cup of coffee.

So if you don't drink coffee after five o'clock in the afternoon, then don't do your cold plunges after five o'clock in the afternoon. Always have someone with you because doing it alone is not safe. 

Use your breath to navigate your nervous system. It's your steering how to calm your nervous system down?

Jonathan Wolf: Amazing. Susanna, Tim, thank you so much. 

I would like to try and do a quick summary, and, please, both of you, correct me if I got any of this wrong. 

So the first thing that's still sticking in my mind is I actually do exercise by going to the sauna, even though I just sit there and do nothing, but apparently my heart is racing. 

If I do this regularly, it's sort of like doing exercise without doing exercise. I love that because I'm always looking for a way to not have to do any exercise. So, I'm sold on that already. 

My second big takeaway, Susanna, is I'm a cold, sissy. I knew that, but I didn't realize it was a technical term before. So I am a cold sissy. But the good news is, apparently, you once were as well. So it sounds like it is possible to start as one. 

It sounds like most of us probably do because it's normal. Because actually, the whole experience is that your body thinks it's going to die. We don't expect to want to do that.

So what you describe is the first time gasping and screaming is alright, you are stressing your body. 

The reason why you get these benefits is because you're creating this stress. What tends to happen is you have 3 seconds of this real shock. If you can get through that, then I think you described this wonderful meditative calm that you get out to on the other side.

But even then, interestingly, you aren't saying then go and stay in this cold water for a really long time. Everything you're talking about is actually quite a short period of time. One or two minutes in the cold, then coming back. 

The optimum that you saw in this research that was really delivering the results, was two to three times a week. Where people were actually getting a habit where they were combining sauna and cold plunge. Which I think was about, so 10 minutes sauna, still just like one or two minutes of the cold plunge three times.

There is an easier way to get into this. So you can just do a cold shower, and that is going to already create quite a few of those impacts. 

Make sure you're breathing through the nose, because apparently if you're a cold sissy like me, even that is going to be too much. 

Interestingly, a hot bath, on the other hand, which I've never felt as scared about, has proven benefits and helps with sleep.

Then I think the final thing I'm taking away is that cryotherapy might work, and I'll be honest, I've been a bit skeptical. I always feel that anything that appears in the mall, I don't know. I'm not sure I really believe it is a health impact. 

But you're saying it might work because there is a stress associated with it. But it's probably going to have less health impact than this ice plunge because, actually, you don't feel the full weight and pressure, and there is very little data yet. 

So you think it could well work, but there's not a lot of proof. The data is coming along more on the cold plunge, but even then, your research is key here, and it's still very early. So there's a lot more still to explore. 

Dr. Susanna Søberg: Beautiful resume. Yeah. Beautiful. 

Jonathan Wolf: Thank you so much. Tim, I want to hear more about the impact on the microbiome as we discover more about where that happens.

Susanna, I hope that as your research continues, we can get you back again in the future. 

Dr. Susanna Søberg: Of course. Thank you so much for having me.