The Allure of the Ice-Cold Plunge

Two swimmers in a frozen lake.
Reuters / Alamy

Here are scientific reasons why you should consider a polar bear swim.

“This is the stupidest thing I have ever done.” Standing on the shoreline that separates Oregon and Washington state, I shiver underneath an oversized fleece robe, dreading what comes next. With the wind whipping around my shoulders, I unbutton the cloak, pull off my hat, and steel my bikini-clad self for 41° F water. Friends splash in next to me, some silently, others swearing. I freeze a few inches of skin with each slow step forward, shrieking to vent some of my anxiety. My brain is screaming: Stop. Don’t do this. Get out

There is an easy answer to why people swim in cold water. Once you wade in, breathe a few times, and suffer through the initial shock, the skin numbs, the breathing slows, and for a few minutes, it’s just you and the water. Also on the plus side, beaches tend to be clear, and parking lots are usually empty. 

Depending on your exact physiology, it could take two to three minutes before your skin numbs and you feel comfortable, if not cozy. But then the swim is unlike any other, and the mind focuses in ways that it normally cannot. Swimmers describe experiencing a lingering calm, feelings of relief from anxiety and depression, fewer colds and flus, and an acclimation to their polar bear ritual that grows, carrying over from season to season.

Sometimes, strange things happen. It’s not hard to mistake one’s own numb foot for a fish. As a swim progresses, there are signs it’s time to get out. Paradoxically, euphoria (“I could swim all day!”) or suddenly feeling warm are signals that it’s time to head for shore, according to Jaimie Monahan, a record-setting ice swimmer and the first person to complete the vaunted Ice Sevens challenge by clocking one ice mile on each continent, each swum in water at or below 41° F (5° C). As a bonus, one of the miles must be swum under 34° F (1° C).

What cold does to the body

Cold swimmers vary in their habits, but many simply wear a bathing suit, swim cap and goggles. Some add neoprene gloves and booties, ear plugs to prevent dizziness, or even a wool cap. Some cold swimmers go about their sport with meditative focus, while others float along and chat. Others like Monahan, travel the globe seeking colder waters and athletic challenges.

After the swim come the shivers, numb hands and feet, and a flood of well-being not unlike a runner’s high, or the dizzy relief of exiting a rollercoaster. Occasionally, it’s worse; staying in too long can cause excruciating hand and foot pain upon rewarming, confusion, loss of balance and coordination and, more rarely, hypothermia. Experienced cold swimmers stay safe by swimming in groups. They monitor weather conditions, water temperatures and their own sensations; rewarming by removing wet bathing suits quickly before reaching for dry clothes, robes, hats, hot water bottles, and snacks.

I can tell you from personal experience that cold swimming makes you hungry. And while each swimmer’s physiology is unique, over several seasons of winter swims near my home in Portland, Oregon, I’ve rarely seen anyone turn down a post-swim hunk of cake, even early in the morning. 

Sugary snacks aside, cold swimming might actually help people manage type 2 diabetes and metabolic syndrome, which is why it’s a subject of interest in several metabolics research laboratories around the world. Studies suggest that regular cold exposure, including activities like ice swimming, can increase insulin sensitivity, helping people’s bodies learn to efficiently regulate glucose production throughout the day.

In people without diabetes, glucagon and insulin are secreted by the pancreas to keep glucose levels within a normal range. Eating results in glucose entering the bloodstream, triggering the release of insulin, which signals the liver to stop producing glucose and ensures the body properly clears and stores excess glucose in muscle, fat and liver cells. During a meal, blood sugar levels rise, insulin does its job, and the levels fall to baseline.  

“The problem is we keep insisting that people have to diet and exercise and we don’t give them any alternative options.”

For many people with type 2 diabetes, pre-diabetes, or certain types of liver disease, the body becomes insulin-resistant. When glucose levels rise during a meal, insulin levels also rise, but the typical signals are crossed, and the liver keeps producing glucose anyway, causing high blood sugar levels. The body responds by releasing more and more insulin. If this persists over time, the pancreas’ capacity to produce insulin declines. This makes it difficult for energy from food to be properly stored. New research shows that cold exposure could help.

“With cold exposure, we tend to see an improvement in insulin sensitivity, which translates to an improved ability for the muscles to clear glucose, improved ability for our liver to respond to glucose, and also an improved ability of our white adipose tissue to respond to insulin,” says Denis Blondin, an integrative physiologist at the University of Sherbrooke in Quebec, Canada. These improvements, he says, can help regulate how our body uses and releases energy. 

Insulin sensitivity can persist, but a single swim or cold shower isn’t enough. Blondin says there’s evidence that insulin sensitivity could last, at least as long as a person is regularly exposed to cold.

“How long it persists afterwards is probably very similar to exercise,” he adds. “If you followed an exercise program, you would see steady improvements in your insulin sensitivity, glucose regulation, and lipid regulation. But if you stop, within a week or so, it really starts to deteriorate.” In a healthy person, insulin sensitivity boosts last 24 to 48 hours, according to Blondin. In a person with diabetes, the benefit lasts 18 to 24 hours.

Several icy dunks per week may sound hard to achieve, but not if you listen to cold swimmers as they describe the sport’s addictive qualities. For Vicki Watson, who’s swum through ten winters, getting back into the water near her home in Nottingham, England, was a way of reclaiming sanity after COVID-19 lockdowns. “Just standing in the water calmed me down. …You’re home, this is calm. This is one place that will never change, whatever craziness is going on in the world.”

Watson’s mention of the pandemic is curious because it’s a reminder that there are many more studies on how to safely endure the cold, and explorations into how cold could ameliorate the impacts of diabetes, than there are investigations into the benefits most often described by cold swimmers: stronger immune systems, psychological boosts, resilience to stress, and social bonding.

A picture of a winter swimmer.
Cold water swimmer Vicki Watson.

Scientists like Mike Tipton at the University of Portsmouth in England say the reason for the dearth of research is because such studies are expensive. There’s little profit to be made from, say, showing that cold water improves mood. His research has focused on cold water immersion and survival, work that informs military efforts by suggesting ways to mitigate exposure for air crews of navy sailors who wind up in freezing waters of the North Atlantic or elsewhere. His work also helps courts compensate families of drowning victims by detailing the specific suffering of their loved ones’ last moments.

Overall, Tipton points to the complexities of individual physiology; it’s nearly impossible to predict an individual’s response to cold, unless they’re already acclimated. He also cautions against claims made by extreme athlete and self-described “Iceman” Wim Hof. Hof touts several as-yet-unproven benefits of cold exposure, particularly when coupled with his trademark breathing technique, claiming it can stave off infections, mitigate symptoms from a range of illnesses, and even extend lifespans by modifying DNA. “You can’t change the laws of physics,” Tipton says. “There’s lots of studies out there that show an improvement in the markers of immune function, particularly with short-term exposures. But that doesn’t necessarily translate into a smaller number of infections.”

Preparing for an ice mile

Despite what Blondin’s research might show, weekly cold swims are a staple for many winter swimmers. Monahan says her preparation for ice miles balances speed and cold water acclimatization. She swims with a master’s team two to three times a week, in addition to driving the hour to New York’s Brighton Beach from her home in Manhattan once a week, where she’ll swim for 10 to 20 minutes on a winter day.

“The danger increases exponentially with every degree of temperature drop, and then also with every minute that you’re in the water,” she says. She points to an ice mile in Siberia as among her most physically demanding due to weather conditions. “The air was so much colder than the water that there was actually steam coming off the water,” she says. It was one of her most frozen feats, with water hovering just above freezing, and a -24° F (-31° C) air temperature. Monahan swam the mile in a speedy 30 minutes and 20 seconds (cold tends to slow swim rates).

With nine ice miles certified with the International Ice Swimming Association, of which she is a board member, Monahan is one of just three people on the planet to have completed the Ice Sevens challenge. The soft-spoken New Yorker has gained fame among open water swimmers for her unflappable attitude, and she believes, somewhat controversially, that swimmers can safely attempt an ice mile in their first season with adequate preparation. She advises swimmers to regularly swim a mile while water is warm, then continue at that distance into colder months. (This method, she says, is easier than starting when the water is cold, and trying to gradually increase time in the water.)

McKinlee Hand tackled two challenges last winter, surviving her first nursing job in a Seattle emergency room while COVID-19 raged and venturing into Washington state’s waterways during the winter. She’d work a night shift, then meet friends for a swim nearly every morning at dawn. Hand had already hitchhiked across several countries and tackled other daring challenges solo, and she decided to attempt an ice mile. She persisted even when a 2021 snowstorm hit Seattle, forcing last-minute changes to her plan.

Hand finished the ice mile, but at grave peril. She doesn’t remember the second half of it or her recovery. Nursing friends who helped her later reported that Hand became combative, refusing to take her wet bathing suit off, and berating helpers. She believed they’d turned on the air conditioning in her vehicle and were feeding her cold tea. In reality, friends wrapped Hand in blankets and bear hugged her in the back seat while she shivered with the heat on full blast. It took two weeks before she could tolerate a cold swim again.

Still, Hand says swimming keeps her physically and mentally healthy.

“It doesn’t matter if it’s pouring down rain, it doesn’t matter if it’s snowing, it doesn’t matter if it’s nighttime or daytime. We are there every single day pretty much. Nothing is really a deterrent to going swimming—mostly because it has such a mental health benefit.”

An aerial photograph of women swimming.
Zuma Press, Inc. / Alamy

Skip the walk—and take a dip?

Confounding variables have made it tough to test claims like mental health benefits or increased immunity. Exercise itself can boost mood and increase insulin sensitivity, so researchers must control for the benefits of exercise when sussing out the benefits of cold specifically. “Whether it’s cold itself or whether it’s the activation of metabolism from cold, or whether brown adipose tissue has any effect is not clear,” says endocrinologist André Carpentier, a colleague of Blondin’s in Quebec. 

The browning of white fat has hit headlines recently, igniting hopes that cold exposure could convert demonized white fat to mitochondria-laden brown fat, which the body burns for warmth. While it does appear that cold exposure can cause the “browning” of white fat, it’s an appearance-level change only, Blondin says. White fat does not—despite our hopes—appear to be convertible to a calorie-burning machine. 

The amount of brown fat on an average person’s body is around 50 grams for a healthy, lean person, Blondin says, an amount of tissue that, even if impacted, is unlikely to drive systemic metabolic changes. So where are the metabolic changes seen in cold-exposed people coming from?

A small-scale study with nine obese participants published in Nature Communications last March showed that mild cold acclimation, which did not cause shivering, did not impact insulin sensitivity in the same way, causing none of the metabolic changes associated with cold exposure that did induce shivering. Researchers hypothesized that shivering itself is part of the upregulation of genes that influence insulin sensitivity in skeletal muscles. In other words, shivering appears to be part of cold’s benefit in ways that go beyond calorie burn. 

“I’ve always felt the water gives you whatever you need.”

“This is something we’ve been yelling from the rooftops for ages now: It’s from skeletal muscle. Skeletal muscle is what’s driving these responses, shivering and contracting,” Blondin says. And in some ways, shivering is exercise. 

“The difference between cold exposure and exercise is that when you exercise, your contractions are very local. So if you’re running, it’s mostly your legs and some postural muscles. In cycling, it’s just your legs,” Blondin says. “In shivering, it’s everywhere; all of your muscles are recruited.” And if cold exposure can impact the large muscle mass of our skeletal systems, even a small change, Blondin says, can have a big impact. 

The idea that even a cold dip can have exercise-like health benefits isn’t news to many in the cold and ice swimming community. It frustrates pragmatists like Tipton, who points out that, instead of taking the time to prepare for a safe cold-water swim, one might just as easily burn more calories taking a walk. Why take the icy plunge?

“The problem is we keep insisting that people have to diet and exercise and we don’t give them any alternative options,” Blondin says, pointing to cold exposure as part of a toolbox approach that gives people more than one way to improve insulin sensitivity, particularly those who can’t tolerate exercise.

Whatever answers these debates reveal seem less important to winter swimmers than the basic beauty of overcoming the logical impulse to run back to a warm car and, instead, steady the breathing, steel the nerves and see how cold the water feels that day.

“I’ve always felt the water gives you whatever you need. You just have to listen to it and listen to your body and listen to yourself. I do a lot of training, but some days I’m like, ‘You know what, today is not a training day,’” Watson says. She’s taking a break from training to wade in regularly and bond with her daughter-in-law, who is 35 weeks pregnant. “Right now, that’s what’s important to us.”  

Some days, we swim for the company, other days for the endorphins. But in those first few moments in the river, the brain’s only task is be calm be calm be calm as every inch of skin sings with icy pain. As I start to swim, first head up, then face down, I breathe every stroke until my brain joins my body in realizing I’m not dying. And then I join friends, stroking into the current, waiting to see how long we can stave off the cold. Often, I’m not sure why I’m there until we reemerge, minds wiped clean, exuberantly smiling.

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