Essentials: Cold and heat exposure for metabolic health with Dr. Susanna Søberg
Andrew Huberman interviews Dr. Susanna Søberg, a researcher specializing in cold and heat exposure and metabolic health.
Summary
Andrew Huberman speaks with Dr. Susanna Søberg about the science of deliberate cold and heat exposure and their effects on metabolism, brown fat activation, insulin sensitivity, and inflammation. Dr. Søberg explains how cold receptors in the skin signal the hypothalamus and directly activate brown adipose tissue (brown fat), which acts as the body's primary thermogenic organ. She presents findings from her own research on winter swimmers, showing that as little as 11 minutes of cold exposure and 57 minutes of sauna per week — spread across two to three sessions — produced measurable improvements in insulin sensitivity and blood glucose clearance. She also introduces what Huberman calls the "Søberg Principle": ending a cold/heat session on cold, which forces the body to generate its own heat for hours afterward, extending the metabolic benefit well beyond the session itself.
Key Takeaways
FULL TRANSCRIPT
What constitutes effective cold exposure
Andrew Huberman: My first question is: is there any way that we can start to gauge what is the best way to approach a deliberate cold exposure protocol? Should it feel uncomfortable?
Dr. Susanna Søberg: If the temperature is very cold and you feel very cold, then you should stay in the water a little bit longer. I think you should get uncomfortably cold. As long as you get uncomfortably cold, it's cold enough and you get what we call the cold shock. The cold shock is activation of your sympathetic nervous system and the activation of the catecholamines.
Andrew Huberman: Does the shock mean that I'm having trouble controlling my breathing? Is that a good gauge?
Dr. Susanna Søberg: You can say so, because that's kind of how we define it. You hyperventilate — you have a faster breathing rate. 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 and adaptation. You can train this cold exposure and get adapted to it so you don't have this hyperventilating response every time you go into cold water. This is building up your resilience, building up your adaptation. It's going to make the shock subside a bit. It's always harder in the beginning. But you should do hard things. You shouldn't think about cold water immersion as something that is comfortable. It should be hard, because that's the point of it. If you enjoy it, then I'm thinking something is wrong. You should not enjoy it.
Comparing different forms of cold exposure
Andrew Huberman: What are the differences in terms of impact — short-term and perhaps even long-term — between cold showers, cold plunge to the neck, immersion with dunking one's head, and simply being outside on a cold day in shorts and a t-shirt?
Dr. Susanna Søberg: There are different outcomes because they are very different exposures of cold to your cold receptors in your skin. The more you cover your body in the cold — which you do in cold water, because the molecules are closer to your skin — the more potent the activation of all your cold receptors in the skin. That will definitely activate your autonomic nervous system more rapidly compared to going out in a t-shirt in cold wind for a walk. But that is also something that's going to activate your sympathetic nervous system, meaning you will have an increase in norepinephrine and you will activate something called brown fat. This is a healthy kind of fat tissue that we have in our body, and when you activate it, it increases your metabolism. It is kind of like our first responder in the body to keep our temperature up.
We have two tissues which can increase our thermogenesis. The brown fat, which is always temperature-regulating our body, and then we have the muscles, which will secondarily start to shiver and that's going to increase our temperature in the body. As soon as you go into a cold shower you will activate your brown fat immediately. So it could be good also for increasing metabolism in theory — because we haven't really any studies showing how much it actually activates the brown fat. So if someone out there wants to do a study on that, that would be great.
How cold receptors signal brown fat
Andrew Huberman: It sounds like any form of cold to the skin that people register as a cold shock — jarring, uncomfortable — activates the brown fat. Do we know what the pathway is from cold receptors on the skin to the brown fat? How does the brown fat know that we're cold?
Dr. Susanna Søberg: The cold receptors will send a signal to our temperature-regulating center in the brain — the hypothalamus — and that's going to take in this message. We have so many cold receptors in the skin, so if you immerse the body into cold water, this is going to be very rapid. It will have a rapid increase in neurotransmitters in the brain — noradrenaline, adrenaline, and cortisol, which is not that much but it's still there. So you have this increase in noradrenaline which will then immediately activate the brown fat.
There's also a direct pathway from the cold receptors in the skin to the brown fat, which really shows that because of these different pathways, this tissue to keep us warm may have developed in our evolution as humans to keep us warm and to save us whenever the temperature on our skin varies even 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 warmer in our skin. So it may also be a temperature regulator in our body, but the pathway is different.
I think there's also a third pathway directly from the muscles. So the brown fat is also activated even when the muscles start to shiver. 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.
The after drop and the role of shivering
Andrew Huberman: 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?
Dr. Susanna Søberg: You shouldn't be so afraid of shivering, I think, because as long as you don't get too hypothermic — if you don't sit in the cold water for too long — 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. As soon as you get into the cold water, all your blood vessels constrict because you need to keep your blood in your core and keep your vital organs warm. As soon as you get up, those blood vessels will open again and the warm blood will flow out, get colder, and then flow back into the core, and that's going to decrease the temperature in your core.
Andrew Huberman: Years ago I heard Wim Hof 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. 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.
Dr. Susanna Søberg: Exactly.
Andrew Huberman: And then am I right in thinking that the shiver activates brown fat, which then warms me up again?
Dr. Susanna Søberg: Yes.
That's why you should end on the cold. It's training — training for all your cells, training for your muscles. It's training up your metabolism and that's going to increase your insulin sensitivity. So if you can, in your mind, get used to the thought that shivering is just like when you go exercising in the gym and get that feeling of "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 called healthy stress — hormesis in the cells. And the more you expose your muscle cells or your brown fat cells to these kinds of healthy stresses — exercise, cold and heat exposure — it's going to make them better at activating and also at keeping you healthy.
As long as the cells get exposed to this, it's going to keep them on their toes, because they become more robust — increasing heat shock proteins and cold shock proteins in the cells to make you more robust for the next time. That is also what happens when you go to the gym. I keep drawing that parallel because people today know more about exercise and what it does to your muscle cells. But the same kind of training is also what you do when you go into cold water and submerge, because that is just your cold training center. And also your heat training center going into the sauna, because the cells are getting stronger with hormetic stress. It's the same process, just different practice.
Long-term metabolic effects of cold exposure
Andrew Huberman: What are some of the other things that happen at the level of metabolism and brown fat in the hours and day after deliberate cold exposure?
Dr. Susanna Søberg: What happens is that you get adapted a little bit every time you go. Like exercise, you get a little bit stronger. Every time you go into the cold water, for every time you will feel more comfortable in the cold. You're going to build your adaptation, which happens on a metabolic level — which is the brown fat. You will have more activation of your brown fat. The mitochondria in the brown fat cells are going to be more numerous and more efficient at heating you up, because the body expects you to do this again, so you are prepared. The capillaries in your skin will also become better at constricting, so you will have a better shield of your body to prepare you for the next time. Your stress response will also subside a bit. You will have a less pronounced increase of your catecholamines.
Because of this activation of your brown fat or your muscles, you will have an increase in metabolism which will then make your insulin sensitivity better. This is shown in studies. For example, there was this interesting study I found just before I started my PhD — from Gibas and Stormer at the University of Lund from 2016 — where they measured insulin sensitivity in middle-aged men and women during one winter swimming season. During those four or five months of winter swimming, they saw that the participants had lower blood pressure after the season, a lower heart rate, and better insulin sensitivity. I think that is very interesting because if you can have better insulin sensitivity, you can prevent lifestyle diseases. Lower blood pressure is also a very strong outcome for telling how much inflammation you have in the body.
Because they didn't measure brown fat, I figured that could be the missing link — one of the explanations for why we see this less inflammation in the body. So the longer-term outcomes could be that you lower your blood pressure, have a lower heart rate, better insulin sensitivity, and better glucose balance. That is shown in my study.
Brown fat activation through everyday temperature variation
Andrew Huberman: Is it the case that as we're just moving around — I've heard of NEAT, non-exercise-induced thermogenesis — that we are activating brown fat? Or does there need to be this stressor, this shiver and a cold stimulus or a heat stimulus, to activate the brown fat?
Dr. Susanna Søberg: Cold is the most potent stressor and activator of our brown fat because it's our temperature-regulating organ in our body — first responder to that. Studies have shown that if you just put your hand in cold water — not that you're going to do that all day or every day, 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.
People who work outside or move a lot, or go in and out of changing temperatures all the time, will have more brown fat, and activating that is going to keep your metabolism higher and your insulin sensitivity better. Studies have also shown this. So the brown fat can be activated as soon as you just change the temperature in the skin. Going outside in a t-shirt, wearing cooling vests — studies have shown this for 10 days — is also going to grow your brown fat. 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. Just exposing yourself to cold wind has also been shown to activate your brown fat.
Or if you don't want to be in this awake state, you can also just sleep in the cold and you won't notice it that much. But studies have shown that if you sleep at 19°C, you will activate your brown fat and grow your brown fat so you have more of it. There's a very nice study from Hansen et al. from 2017 that showed a group of subjects who slept in a room at 24°C — they made PET CT scans to see how much brown fat they had at baseline — then measured again after a month of sleeping at 19°C. And I think it's remarkable: just one month at 19°C sleeping — they had a duvet on and were still clothed — they had increased insulin sensitivity.
The next month they slept at 24°C. They measured again and had actually decreased a little bit. Then they slept at 27°C — quite a warm room — for the fourth month, and they saw even less activation of the brown fat and also less insulin sensitivity. So it seems that you can expose yourself and pretty rapidly the brown fat will respond, because it's so sensitive to noradrenaline. If you keep exposing yourself to a little bit of cold, you also get a little bit adapted to it. That's because the brown fat has grown more mitochondria in the cells — these small energy factories that activate the cells and take up glucose and fatty acids from the bloodstream to keep the thermogenesis up, clearing some sugar and some fat from the bloodstream.
So the brown fat can in that way decrease our unhealthy fat, which is the white fat. The white fat is what we don't want too much of, but we still need some — it's our energy storage, so it's very important that it's there. We just don't need a lot of it. It's located on our thighs and also around our inner organs. So if we can have activation of the brown fat just by going out in the cold, by sleeping in a cold room, or if you have the courage for it, by going into a cold plunge — cold showers will also do the trick — you can do different variations of this. Just exposing yourself to various temperatures is going to activate the brown fat because it evolved to keep us in a perfect homeostatic balance regarding temperature, to keep us alive.
Andrew Huberman: I want to get a clarification around this 19°C room that they're sleeping in. They're under a comforter, a duvet, and you mentioned they have clothes on. The room is 19°C, but the temperature underneath their blanket might not be 19°C. So presumably it's the cold on their face that's activating the increase in brown fat. Is that a reasonable expectation?
Dr. Susanna Søberg: I think so. You have so many cold receptors in your face. It's actually 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. I did this experiment myself in my studies just to see how well they responded to cold water. It was four degrees Celsius cold water for four minutes, and then I just measured blood pressure and heart rate to see if they had an activation. I actually also measured the brown fat during this cold exposure for four minutes with an infrared thermography camera.
Usually you cannot really see activation of your brown fat because it's located centrally, around your central nervous system. The biggest depot is up here under the clavicular bones, 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. I could see that after a few minutes, the activation was there — an increase in temperature arose from that activation, just from four minutes. So it's very rapid. I also measured in my study how deep the brown fat was under the skin. 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.
Dr. Søberg's winter swimming study
Andrew Huberman: I'd like to talk about your study. It 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. If you could tell us about your study — what you did and what you discovered.
Dr. Susanna Søberg: When I started this research in 2016, I wanted to see if we could do some kind of activity so we can have people move or go and do something together. And the cold made us think about winter swimming. We were just thinking, okay, winter swimmers must in theory activate the brown fat — but we kind of didn't really know if this kind of stressor was too much, too little, or what would actually happen.
I read the literature and understood in theory what happens when you go into cold water, but I completely understood it when I first tried it. It was really uncomfortable just doing it, but afterwards, as soon as I got up, I felt fantastic. We went into the sauna and I did three rounds because I just loved the feeling afterwards — you have all these neurotransmitters going in your brain and you feel more positive, more invigorated. I had so much energy. I could totally see why people would do this to get energy throughout the day, because I definitely had that.
I wanted to study the lowest dose — the lowest amount that we can get away with but still see health benefits. So I recruited winter swimmers who had been swimming for two to three seasons and they stayed only in the water for one to two minutes. The cold shock subsides very quickly and you would get this activation of your rest-and-digest system, which is your parasympathetic nervous system — the other branch of your autonomic nervous system. You get that activation because when you submerge into cold water you have an activation of your diving response, and you will slow down your oxygen consumption in your body. That's because the body tries to preserve oxygen so you will not get hypothermic too fast. It's kind of like a survival system in your body.
Because of that, after maybe one minute or so — and I can't be precise on that because it may also vary a bit in humans — after one to two minutes you will have full activation of the sympathetic nervous system but also the parasympathetic nervous system. The participants did this two to three times per week, and for each visit they took three rounds — three dips and two sauna sessions. They started in the cold and they ended in the cold water.
Andrew Huberman: What was the temperature of the sauna?
Dr. Susanna Søberg: About 80°C.
Andrew Huberman: And how long were they in the sauna?
Dr. Susanna Søberg: They stayed for 10 to 15 minutes, depending on whether they went two times per week or three times per week.
Andrew Huberman: We will talk about why it's important to end on cold — the so-called Søberg Principle. How cold was the water in this particular study? Give an average, because I realize it's outdoor winter swimming so it's going to vary.
Dr. Susanna Søberg: It's a very uncontrolled environment to do this kind of study, but I wanted to do something that was also very close to what people could do for free — going out in nature and using that, which also has a very healthy impact on us and lowers our stress level as well. It was within the spectrum of very cold water. I would say from around 15°C and down, but it was actually not colder than about 2 to 4°C on average when it was the coldest. So it doesn't have to be that cold to be good enough and to activate our metabolism.
Andrew Huberman: So what did you discover in terms of changes in brown fat, insulin resistance — or insulin sensitivity rather — and metabolism?
Dr. Susanna Søberg: What we saw was — we had different measures to try and unravel 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 increased insulin sensitivity. They produced less insulin on all the experimental days. Besides just cooling them and measuring the brown fat on each of these cooling days — there were two cooling days and one thermoneutral day — I wanted to measure insulin when they were fasting, meaning they hadn't eaten in eight hours before the study day, and were completely lying still, not moving, just in a bed. We measured insulin during the experimental day just to see what level they were at, and we could see that the winter swimmers had lower production of insulin.
And when they had a glucose drink — which we give them to test before we enroll subjects in studies to see if they have diabetes, for example — we could see that the winter swimmers had a faster glucose clearance in the bloodstream. 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 had better blood glucose clearance, which is really what you want. Excessive insulin is bad. Insulin being more or less a chaperone for blood glucose.
Minimum effective dose: the 11-minute and 57-minute thresholds
Andrew Huberman: How much cold exposure do people need? How much heat exposure do people need in order to extract these benefits?
Dr. Susanna Søberg: When we calculated the numbers together, we could see that this ended up being 11 minutes in total per week — not in one session of course, but they had two to three visits to the water and the sauna per week. When we divide that out, it corresponds to being in cold water one to two minutes at a time, and in the sauna 10 to 15 minutes at a time.
I think this fits very well with what we call hormetic stress, or healthy stress. You expose the cells to this potent, very stressful situation where they increase heat shock proteins in the cells and that will repair the cells. It shows that you don't have to expose yourself very much to the heat or very much to the cold to get these healthy benefits — healthy benefits on your cardiovascular system. I think this is a very important message to get out: you don't have to go extreme. You don't have to swim for half an hour in 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. If we divide that out on two to three days and two sessions each day, that corresponds to 10 to 15 minutes. So it's a low threshold, but I think it's good to have that to aim for.
And I think it's really good to have that because then you don't overdo it. If you overdo it, you exhaust the cells, and that will actually increase your risk of cardiovascular disease.
Inflammation, immune markers, and broader health implications
Andrew Huberman: 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 reveal additional benefits — say for immune system function or for any function for that matter?
Dr. Susanna Søberg: We looked at inflammation of course — we measured blood pressure and so on — but we also measured IL-6 in the study just to see an inflammatory and anti-inflammatory marker. IL-6 went up and it also follows with IL-10. That is very well known in the literature. I think it's very important to think about cold exposure and heat exposure as something that lowers the inflammation in the body. If we can do that, we will have an open door for preventing lifestyle diseases — for type 2 diabetes, but actually also for some mental diseases as well, such as depression and anxiety, and also Alzheimer's disease, which are all associated in research — newer research showing that inflammation increases the risk of depression, anxiety, and neurological diseases.
So if we can decrease inflammation in the body, we will decrease modern lifestyle diseases but also these increasing mental diseases that we see in modern lifestyle times. I think it's very interesting that we can go out in nature and use these natural stressors. I don't want it to sound very romantic or anything — it's just exposure to temperature, just to cold or to heat, that is going to tweak our body into a natural state again and reset it where the homeostasis, the balance, has been lost a bit. The body is going to repair itself in that way. And I think it's beautiful that we can do that just by changing the temperature of our body.
Although people are very scared of doing this, because in our times we have been away from cold, away from heat temperature, for decades now — since we insulated our houses better and we are more sedentary. We sit more indoors, we don't move as much. This very modern sedentary lifestyle has made us more temperature-comfortable, just neutral. No wonder that obesity is increasing. We don't expose ourselves to the natural stresses that we did earlier on in our evolution, but also up until maybe the 1970s and 1960s, where we started having more comfortable lifestyles. Obesity increases in the 1980s — we can see that from statistics. So I think if we can take in cold and heat, it all increases the hormetic stress in the body. It doesn't have to be other than natural stresses to the body, which then could keep us in that natural balance again.
The Søberg Principle: ending on cold
Andrew Huberman: Could we talk about what I refer to as the Søberg Principle — which is to end on cold in order to force your body to heat itself back up and thereby increase metabolism further still?
Dr. Susanna Søberg: When you end on the cold, you force your body to heat up by itself, and that will require that 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. In that way you don't have to think about your cold exposure — dipping in your plunge, or open sea, or whatever 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 — it's also going to keep your neurotransmitters activated as well and increase them, because your body is still cold. You need those neurotransmitters to activate the brown fat. That's going to make your brown fat cells more efficient and also your muscle cells more efficient — increasing mitochondria in the cells, which will then generate heat very fast.
If you have done this a few times — maybe three, four, five times — you will notice a switch where you feel that you get easily warmer and you can keep yourself warmer. That is also what was shown in my study: the winter swimmers were physically warmer on the skin compared to the control group.
Andrew Huberman: When they are out of the cold?
Dr. Susanna Søberg: When they're out of the cold, just relaxing. We tested this on the days where they were sleeping in the lab. We could see that they had more activation of the brown fat, a higher temperature. Probably because they also lose heat — they have a higher heat loss from the body compared to the control group, because they have more vascular skin because of the contrast of cold and heat. So they lose heat faster from the body during the 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. It has to work to keep you warm.
Practical protocols and the importance of temperature variation
Andrew Huberman: Let's say somebody only had two days a week to do deliberate cold exposure. Maybe they don't have access to a sauna. 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 four or five minutes — to try and get to that 11 minutes total per week?
Dr. Susanna Søberg: Definitely. I think the changes in temperature are 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 going to strengthen your cells because you are challenging them to adapt to changing temperatures.
Andrew Huberman: 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.
Dr. Susanna Søberg: I think if people can internalize that idea, 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.
All the time it's "how much and how cold" — and I mean, it's just like, well, because we also don't have studies showing exactly what happens if you just keep your water at 5°C and do that for a month. 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 logically, changing that temperature up and down, up and down — and you also do that in your water — it's not that important what temperature you have your water at. Just keep changing it, going up and down. It could be all the way up to 12°C — you're going to activate your brown fat anyway. I mean, 19°C cold air is enough to activate your brown fat. So maybe we don't have to go as cold as I think many people think. Putting ice in all the time — you don't have to. I don't think it's necessary to expose yourself to that cold a temperature all the time. Vary it a bit.
Andrew Huberman: I want to really thank you for coming here today to talk about your work and the incredible direction that it points to. No one study is definitive, but your study really stands as a landmark in the landscape of exploring deliberate cold exposure and heat. I just want to thank you for doing the work that you've done and for the work that you continue to do, which includes coming here today to share knowledge with me and our audience. Thank you ever so much.
Dr. Susanna Søberg: I'm very grateful for being here.