In February 2017, Sara Barnes had a grisly-sounding operation called a bilateral high tibia osteotomy. A keen road cyclist and trail runner, Barnes had been left in agony from osteoarthritis, scarcely able to walk. The procedure would, in effect, break both her legs below the knee and insert a bone graft in each. She would be in a wheelchair for six weeks, then spend another two months on crutches.

“It was incredibly tough, both physically and mentally,” recalls Barnes, who is 56. “I saw the world as a wheelchair user. I had to completely trust the surgeon that I’d walk again, because he basically chopped my legs off. I’m a single parent – my son was 13 at the time – I’m self-employed, work from home. So I was isolated for six weeks.”

The day before her operation, Barnes promised a friend, who was a keen swimmer, that they’d go for a dip together as soon as she was up on crutches. So in mid-April, Barnes made her way to Crummock Water, a lake where the pristine water mirrors the steep slate fellsides, not far from where she lives in the Lake District. “I went down through the woods on crutches, got my wetsuit on and then went on crutches into the water,” she says. “And that was it, really.

“It was really cold, the water was about 10C,” she says. “But it was a beautiful day, I do remember that, absolutely stunning. The water felt cool, but the effect on the pain was that it completely numbed my legs. And I could move. I hadn’t been able to move properly for so long and, to lose that, it was soul-destroying, really. That first time I got into the water, I just floated and looked at the mountains and the sky...” Almost two years on, Barnes is still almost overcome with emotion: “Yes, it was euphoric really.”

Facebook Twitter Pinterest ‘The water felt cool, but the effect on the pain was that it completely numbed my legs’: Sara Barnes at the Blue Lagoon, Wasdale. Photograph: Emily Cornthwaite

Barnes swims every week now – even at this time of year. And that first post-op dip was the last occasion on which she wore a wetsuit. “It was just so much fuss,” she says. In fact, this year she is one of 400 people worldwide attempting to join the Polar Bear Club, which requires members to swim 200m twice a month between November and March, wearing only a swimsuit, goggles and rubber hat.

“After the operation, I was very scared and felt very lost,” says Barnes. “And I still have a lot of time where I feel quite lonely and afraid of the future. But swimming has given me back my self-confidence. When I go, it brings me back to myself. I think: ‘Right, come on Sara, you can do this, if you can get into that lake you can carry on, keep going.’ Swimming gives me a community, it gives me friends. It’s taken nothing and given me loads.”

If you spend any time at a lido, or meet a group of wild or sea swimmers – especially in winter – you will hear many similar stories. Plunging into the cold water is as much a mental as a physical endeavour and there are clearly many people who believe the activity can be beneficial for a range of health problems, especially depression. This idea received some endorsement from a case report published in the British Medical Journal in September. The paper tracked a 24-year-old woman named Sarah who had been taking antidepressants from the age of 17 to treat anxiety and “symptoms of major depressive disorder”. But, after the birth of her daughter, she wanted to be medication-free and began – in consultation with doctors – a programme of outdoor swimming in 15C water. Her mood improved immediately, she gradually weaned herself off antidepressants and, after a year, had not gone back.

Ella Foote, a 34-year-old writer who lives in Berkshire, not far from the Thames, is at the extreme end of outdoor swimming. For the past three years, Foote has done Dip a Day December, which requires her to swim every day this month in a river, lake, pond or sea. The genesis of the project was to provide relief – or at least a distraction – from a period of the year that she finds far from festive.

“Generally, I’d gone through a wobbly patch in my early 30s: a long relationship had ended, and all my friends were getting married and having babies,” says Foote after completing this year’s day four, a swim next to the Flower Pot pub in Henley-on-Thames. “Work was all over the place, so it was a few little things chipping away. And in winter people are more likely to get home, have dinner and sit in front of the telly rather than go out. They don’t ever think to pick up the phone and invite you over. I’m quite a bright, bubbly, strong character, so I’d hate them to think I was vulnerable, that I was struggling.”

Swimming outdoors reminds you that risks are worth taking

Foote was diagnosed with depression in 2014, but initially decided not to take the prescribed medication. That changed after a freak incident in 2015 when she was swimming in the Thames and a woman tried to commit suicide by driving her Fiat 500 into the river. Foote swam over and with the help of some fishermen managed to wedge the car into the riverbank. And while the woman was stuck and they waited for the emergency services to arrive, Foote spoke to her. Through slurred words – she’d drunk a bottle of whisky and two bottles of wine – she told Foote she was supposed to be taking antidepressants but never had.

“Honestly you couldn’t write this stuff, it’s crazy,” says Foote. “But that was a moment where I was like, ‘Oh, maybe I should be taking medication because I definitely don’t want to end up driving my car into the Thames.’ So, I decided then to get my prescription. I got a very mild antidepressant, and a very small dose, but it did lift me.”

Foote has since reduced her dose even further, and often forgets to take her pills – “Which says a lot, really,” she notes. Various elements of her life have improved: she has a new partner, she’s happier in work (which includes shifts at the Outdoor Swimming Society). But Foote is convinced that swimming has played its part and when she meets other enthusiasts, she is often told: “It saved me.”

Barnes, too, hears the same thing. “I think we’ve all been brought to the water by something,” she says. “It’s very hard to describe and it’s not the same if you go swimming in an indoor pool, definitely not. There’s something about connecting with nature again; something magical.”

It’s hard to listen to these testimonies and not want to jump into the closest near-freezing body of water. But you might want to hang on to your towel for a second. Scientific evidence of the benefits of cold-water immersion is limited, to say the least. The recent report in the BMJ focused on one individual: the results were fascinating certainly, but – because of the sample size – clinically irrelevant.

This point is accepted by the authors of the study. “It’s a bit of a weird one for us because there’s no science behind the case study we produced,” admits Dr Heather Massey, a senior lecturer at the Extreme Environments Laboratory at the University of Portsmouth. “We have one case study, a couple of questionnaires that we’ve done on people who have found cold-water swimming to be useful for them for whatever medical purpose, but other than that there is no empirical evidence to suggest this works. It’s not that it doesn’t work, it’s just that there’s no science there.”

Since the report was published – Sarah also featured in a BBC documentary called The Doctor Who Gave Up Drugs, presented by Dr Chris van Tulleken – Massey, van Tulleken and the other authors of the study have been swamped with interest. They put out an appeal on the Outdoor Swimming Society website for potential case studies, expecting 30 to 40 people to respond. More than 600 people made contact wanting to share their experiences of managing everything from anxiety and depression to migraines and arthritis. “What we need now is to prove it,” says Massey. “And we need hard, scientific evidence with properly formulated empirical studies in order to do that.”

Facebook Twitter Pinterest ‘In terms of keeping my sanity, it was invaluable’: Alexandra Heminsley. Photograph: Chris Floyd

Until that happens – and the team needs to secure funding for further research – there will be no shortage of evangelists convinced that cold-water swimming has changed their life. Such unblushing enthusiasm can be overwhelming when you first hear it, accepts Alexandra Heminsley, author of Leap In: a Woman, Some Waves and the Will to Swim. “When I was writing the book,” she says, “I had this awful moment of thinking: ‘Oh my God, maybe I’m mad and I’ve sort of joined a cult.’ Because you hear infinite anecdotal stuff, but you can also find infinite anecdotal stuff about anti-vaxxers or whatever.”

Leap In follows Heminsley’s progress from wedding-day dipper to year-round sea swimmer. But it also charts a period of life where, after failing to conceive, she underwent multiple rounds of IVF. Heminsley, who now has a one-year-old son, is adamant there is “no connection between my swimming and finally getting pregnant,” but she does credit the activity with providing solace during a gruelling period of her life. “Because swimming outdoors is this constant confrontation of danger and the unknown, it reminds you that those tiny risks are worth taking. So in terms of just keeping my sanity, it was invaluable.”

Heminsley describes wild swimming as “a hangover in reverse”: pain for moments followed by a prolonged, wholesome buzz. It felt like time to experience that for myself so, on a recent Tuesday morning, I went to Highgate Men’s Pond on Hampstead Heath in London for a dip with Patrick McLennan, the co-director (with Samuel Smith) of a new documentary called The Ponds, about the iconic bathing spot. McLennan is in no doubt that there is a healing aspect to swimming outdoors, one that he finds is multiplied when the temperature drops to single digits.

“Most of the people who come to the ponds are suffering from or in recovery from trauma of some kind, whether it’s emotional, psychological or physical,” says McLennan. “There’s this old guy in the film and he talks about how swimming here is the one thing that makes him feel how he used to feel when he was young. And I think a lot of outdoor swimmers will recognise that.”

A blackboard beside the jetty advises that the water temperature is 8C. Outdoor swimmers tend to divide into “divers” and “creepers”, McLennan explains, with the latter group easing themselves into the water more gradually. There are also “tea-baggers”: people who jump in and get straight out. McLennan’s a diver, and after a couple of elegant bounces on the board, he disappears into the deep, opaque green of the pond. I follow him, involuntary yelps coming out of my mouth as I return to the surface.

It’s true that the piercing pain doesn’t last for very long and, after a minute or two, your breathing settles and you accept you might not be having a heart attack after all. But the main benefits are felt an hour or two later: when you’ve finally warmed up and you feel virtuous, cleansed and even a little pleased with yourself.

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When McLennan and I climb out – our chests beetroot-red – we chat with another swimmer, 52-year-old Oliver Perritt. Perritt has been coming to the Highgate Men’s Pond pretty much every day for a decade. “When I say to people that I swim here every day, they always list a whole load of days,” he laughs. “Like, ‘What? Even Christmas?’ Yes. ‘What? Even New Year?’ Yes. ‘What? Even your birthday?’ Yes. You’d have thought ‘every day’ was quite straightforward.”

Perritt is candid that he’s in recovery from alcoholism – he’s 19 years sober – and he thinks that up to a third of people who swim in Highgate Men’s Pond might be dealing with addiction. For him, it silences “radio Oliver”, presses the reset button on his life every day. “Before I get in the water I have built up a load of crud in the past 24 hours,” he says. “When I get out I’m the person my dog thinks I am, which is better in every respect. It’s a daily commitment to the good. It is also a commitment to the unknown, to the fact that there might be short-term pain, but it’s generally going to be a good thing for you.”

Like most swimmers I spoke to, Perritt would be intrigued to see a scientific study on cold-water swimming, but it would be unlikely to change how he feels about it. “When I dive into this icy cold water, the result is that I feel brilliant – and I don’t have to understand why,” he says, steam rising from his shoulders. “When you talk about it you are, in essence, trying to articulate what is impossible to articulate. But, if you want to know what we’re talking about, do it and see whether we’re talking bollocks or not.”

For more information, go to outdoorswimmingsociety.com; for screening details of The Ponds, see thepondsfilm.com