As summer loosens its grip on the Northern Hemisphere, long days at the beach, eating ice cream or splashing in the ocean are long gone for most. But for some swimmers, the fun is just beginning.

“The best part is going to Brighton Beach in the fall. Each week the water is a little colder than the last. Before you know it, it’s 48 degrees!” Bonnie Schwartz Nolanmanagement, operations and finance consultant, swimming coach and successful Channel swimmer from New York, says Popular science. She’s been floundering in the frigid waters off Brooklyn for more than two decades.

To train for most marathon swim (swim over 6.2 miles or 10K)swimmers must get used to spending time in the cold, as swimmers often cannot wear a wetsuit or technical suit to keep warm and must instead rely on their own bodies.

“Your internal body temperature is 98 degrees, so even something like 80 will feel cold after a while,” Nolan explained. To even qualify to swim the English Channel, swimmers must pass a documented examination six hours under 60, or continuous swimming in water that is below 60 degrees Fahrenheit (about 15 degrees Celsius).

[Related: Swimming is the ultimate brain exercise. Here’s why.]

Open water swimming has even grown to include ice swimmingor swims in waters below 41 degrees Fahrenheit (5 Celsius). For me, it’s all about the challenge,” he says Elaine K. Hawleyjournalist and experienced swimmer, in an interview for PopSci. “It’s that uncertainty about ‘can I do it,’ in the same way as marathon swimming.” Hawley is an accomplished marathoner and ice swimmer who completed ice mile in 2012 and is currently training for his second.

some anecdotal “wellness” Claims including weight loss, better mental health and increased libido have been made by followers of regular cold water immersion, but what about concrete evidence?

A scientific review published today in the peer-reviewed journal International Journal of Circumpolar Health found that cold water diving can reduce white adipose tissue (WAT) in men and reduce the risk of diseases such as diabetes, but other benefits of ice swimming are inconclusive.

The authors analyzed 104 scientific studies and found an additional effect on brown adipose tissue (BAT). The difference between the two is that WAT stores energy instead of burning it the way BAT does. Repeated exposure to cold water or air increases BAT productionwhich is also located in blubber of marine mammals such as whales and seals to keep them warm.

BAT helps the body burn calories, keeps the body warm when exposed to cold temperatures, and also helps the body control blood sugar and insulin levels. It produces heat in the blood when it’s cold outside and is mostly found around the neck, kidneys, adrenal glands, heart and chest in adults. According to Cleveland Clinic, it’s brown because fat cells are full of mitochondria, which contain a lot of iron. The iron gives BAT the brown hue.

Exposure to cold water or air also appears to increase the production of a a protein called adiponectin from adipose tissue. It is a protein that plays a key role in the defense against insulin resistance, diabetes and other diseases. According to the data reviewed in these studies, repeated cold water immersions in winter significantly increased insulin sensitivity and decreased insulin concentrations. This happens to both inexperienced and experienced swimmers.

Swimming in cold water also has a great impact on the body and causes a shock reaction such as increased heart rate. Some of the studies reviewed show evidence that cardiovascular risk factors are actually improved in cold-acclimated swimmers. However, other studies have shown that the workload on the heart is still increased. Overall, the authors have not given a definitive conclusion on the overall health benefits of “the fastest growing extreme water sport.”

“It is clear from this review that there is growing scientific support that voluntary exposure to cold water may have some beneficial health effects,” said lead author James Mercer of UiT The Arctic University of Norway, in a press release.

According to the authors, many of the available studies on the health benefits of ice swimming involve small numbers of participants, often of the same gender, and do not account for differences in water temperature or whether the water is fresh or salt. It is also unclear whether winter swimmers are naturally healthier than the general population.

[Related: How to avoid (and treat) hypothermia.]

“Many studies have demonstrated significant effects of cold water immersion on various physiological and biochemical parameters. But the question of whether they are useful or not for health is difficult to judge. Based on the results of this review, many of the claimed health benefits of regular cold exposure may not be causal. Instead, they can be explained by other factors, including an active lifestyle, trained stress management, social interactions, and positive thinking. Mercer added.

The authors point out that the swimmers participating in these studies ranged from elite swimmers or established winter bathers to those with no previous ice swimming experience. Some were only ice bathers, but used cold water immersion as post-treatment exercise.

The review also identified the need for better education about the health risks that can come from immersion in ice water. These include hypothermia if the swimmer is in the water too long or jumps in without acclimatizing, as well as heart and lung problems related to cold shock. Jumping into cold water alone is very dangerous and it is best to start ice swimming slowly over a period of time.

If swimming in icy waters sounds like fun to you, Hawley and Nolan recommend increasingly longer dives in colder waters to acclimatize. Nolan also took cold showers, slept with open windows and a lighter blanket, and wore a vest instead of a coat outside to help her body adjust to the frigid temperatures.

Is swimming in ice water good for you?

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