The physiological benefits of cold water immersion
Dr Susanna Søberg used to be, by her own account, a coward about the cold. “I used to be a complete cold sissy,” she says. She liked to stay warm. She watched the winter swimmers from the dry end of the jetty and thought they were brave and a little mad.
Now the rest of the field quotes her. Søberg is a metabolic scientist, founder of the Soeberg Institute and author of Winter Swimming. The eleven-minute weekly cold protocol you’ve seen recycled across podcasts and plunge-tub adverts started with her research.
What you notice, talking to her, is how much of the conversation she spends pulling the subject back down to earth. She’ll give you the benefit and then the caveat: the blood-pressure data, and right behind it the reminder that the long-term studies aren’t done. In a field full of people selling certainty, she’s unusually comfortable saying we don’t know yet.
We spoke remotely for Unfiltered in the autumn of 2023, about what cold water actually does to the body, where the science is solid, where it’s thin, and why the people who dread the plunge most might be the ones who’d get the most from it. Her answers are below, condensed and lightly edited.
From cold sceptic to cold-water scientist
You came to cold research from cardiovascular medicine. What were you actually chasing?
I’d spent years in a hospital looking at the root causes of cardiovascular disease, and the question I kept returning to was: what can we say to help people lower their inflammation? Exercise is one answer. I went back to university to find others, and hormetic stress was one of them. Brown fat was the new thing in science. Since the year 2000 it’s been shown that if you activate brown fat, you lower the glucose in your bloodstream, and the most potent activator of brown fat is cold. I wanted applied research. I wanted to be able to tell people: do this, and you’ll lower your risk.
Cold water immersion has been swallowed by the biohacking crowd. Has the macho aura around ice baths helped or hurt?
Everything starts somewhere, and the macho thing may even be useful at the start. It’ll matter less in a few years. You do get confidence from it, and I don’t mind that people feel brave. But eventually people will realise it has nothing to do with showing off or competing. This is a journey you do for yourself, in company with others.
What cold water immersion does to the body
For someone who finds the whole idea horrifying, what’s the single biggest benefit of cold water immersion?
In time, I think it will be lower blood pressure. It sounds boring, but it’s a good thing, because you can follow your own journey with it. Even before my research, blood pressure was shown to fall after cold water swimming over three to six months. This short-term stressing up to stress down lowers inflammation, and when you reduce the plaque and inflammation in the bloodstream, the blood passes more easily to and from the heart. Blood pressure and resting heart rate are underrated, but they’re among the best measures of whether something is actually helping your health.
You describe a cellular stress response with three stages. Where’s the line between a healthy dose of cold and too much?
Acutely, the cells try to protect themselves. You get heat shock proteins, proteins being repaired. There are three stages. In phase two, the cell responds protectively, and that’s the healthy stress. But expose yourself too long and it tips into phase three, where the cell is exhausted and ages faster, rather than phase two, where we live longer. That’s why short-term exposure is the healthy kind. If you get too macho and sit too long because you want to compete, the cells are probably overexposed. You won’t feel it climbing out of a ten-minute ice bath. We might only see it in studies that follow people for twenty years. We already see it at the other extreme: the long Finnish sauna studies show that beyond a point, cardiovascular risk rises again. There’s a sweet spot.
Women and men don’t defend against the cold the same way, even though the payoff is identical. We unpacked the physiology in a separate piece on how women and men respond to cold differently.
The mental health benefits of cold water immersion
The mental health side seems the most exciting and the least proven. What does the evidence on cold water and mood actually show?
We need more long-term studies, and we don’t have many yet. But we have data on mood and on brain connectivity, both of which improve, along with a more positive way of thinking, compared with people who don’t do it. And we see why: dopamine and noradrenaline increase, which explains the better mood. What we don’t yet know is how it affects established depression or anxiety. There’s a UK case study of a depressed woman, with a randomised controlled trial coming that isn’t published. I think there are real implications for anxiety, depression, possibly addiction. But I never say use cold water instead of exercise, or instead of treatment. I see it as an add-on, and possibly as prevention, keeping a low mood more stable.
Why dreading the cold plunge may mean you need it
Some people dread the cold far more than others. Do the people who hate the plunge most have the most to gain?
The more you reject the cold, it’s probably because you reject the stress. Nobody really likes stress, even short-term stress. But you can get used to it, and you can widen the window for how much stress you can take, physically and mentally. It’s a deliberate, painful thing, but you learn you can manage the pain, breathe through it, calm your nervous system. There’s a self-belief in it. You do something very hard and overcome it, and then you know you can overcome other things.
How to start cold water immersion safely
What’s the easiest way for a complete beginner to start?
A cold shower, if an ice bath feels too big. End your hot shower with five seconds of cold and build to thirty. Cold water on the face works, and so do hand and foot baths, which activate brown fat and help prepare you. None of it substitutes for full immersion, but it’s a way in.
You teach people to end on cold, the Søberg Principle. Why does the ending matter, and what if someone finishes on a hot shower instead?
People don’t have to end on cold. But if you’re cold afterwards, you force your body to heat itself, and that raises your metabolism for hours, like an after-burn from a workout except you don’t have to do anything. The key is to keep moving afterwards. Don’t sit on the couch, or you get the after-drop and start shivering. It isn’t dangerous, just uncomfortable, and it can convince you the whole thing was a bad idea.
And the protocol itself, how much cold and heat per week?
What I advise is eleven minutes of cold per week, divided across two to three days, alternating with sauna, around fifty-seven minutes of sauna per week, also across two to three days. If you can’t alternate them, split them across different days. I can’t tell you exactly how that changes the outcome, because that study hasn’t been done.
If you train, the timing matters as much as the dose. We covered when to take an ice bath and when to take a sauna after a workout in a separate guide.
Winter Swimming: The Nordic Way Towards a Healthier and Happier Life, by Dr Susanna Søberg (£25, MacLehose Press) is out now.
Dr Søberg teaches the hows and whys of cold, heat and functional breathing in her Three-Week Thermalist Cure. Find out more.
Winter Swimming: The Nordic Way Towards a Healthier and Happier Life, by Dr Susanna Søberg (£25, MacLehose Press), is out now.
Ice bath or sauna after a workout? · Why women and men don’t freeze the same way
Dr Susanna Søberg also discussed her protocol on the Huberman Lab podcast. Visit the Soeberg Institute for her courses.
If you’re dealing with depression, anxiety or addiction, the research above describes cold exposure only as a possible complement to established treatment, not a replacement for it. If any of this is affecting you personally, a GP or a qualified mental health professional is the right first port of call.
