The Scandinavian winter swimming culture combines brief dips in cold water with hot sauna sessions – and now, a study of young men who participate regularly in these polar plunges finds that winter swimming may allow the body to adapt to extreme temperatures.
The findings, publishing October 11 in the journal Cell Reports Medicine, suggest that routinely alternating swims or dips in chilly water with sauna sessions might affect how brown fat, also known as brown adipose tissue (BAT), burns energy and produces heat.
“Our data underscore that BAT in adult humans is part of the collective body temperature regulation system in collaboration with skeletal muscle and blood flow,” says senior study author Camilla Scheele of the University of Copenhagen.
“Regular winter swimming combining cold dips with hot sauna might be a strategy to increase energy expenditure, which could result in weight loss if compensatory increase in food intake can be avoided.”
In the Denmark-based study, Scheele and her collaborators examined whether the Scandinavian practice of winter swimming is associated with changes in body temperature, resulting in acclimation to both cold and hot challenges. They also looked for differences in brown fat tissue, given its role in producing heat in response to exposure to cold environments.
To explore these ideas, first author Susanna Søberg of the University of Copenhagen recruited eight young male winter swimmers who had alternated several swims or dips in cold water with hot sauna sessions every week for at least two years.
For the purposes of this study, winter swimming was loosely defined as swimming or sitting in open water and wearing only swim trunks or nothing. By contrast, the eight control participants did not use any cold or heat therapies during the study and had no history of winter swimming.
“We expected winter swimmers to have more brown fat than the control subjects, but it turned out that they instead had better thermoregulation,” Søberg says. In preliminary tests, the participants submerged one hand in cold water for three minutes. While both groups responded to the cold exposure, the swimmers displayed signs of cold tolerance, with a lower increase in pulse and blood pressure.
They also had higher skin temperature, pointing to greater heat loss as a potential adaptation to frequent sauna exposure. In another preliminary test, the researchers used an adjustable system consisting of two water-perfused blankets to control and lower the participants’ body temperature.
Using positron emission tomography, the researchers next measured activation of brown fat tissue in the participants as they were exposed to a comfortable temperature. Unlike the swimmers, the control subjects showed signs of activated brown fat tissue, as indicated by an uptake of glucose.
“The findings support the notion that brown fat tissue fine tunes body temperature to a comfortable state in young adults,” Scheele says. “It was, however, a surprising finding that the winter swimmers had no activity at all when exposed to comfortable temperatures.”
Upon cold exposure, the activity of brown fat tissue increased in both groups. But the swimmers showed much higher heat production, or energy expenditure, in response to cool temperatures. “Winter swimmers burned more calories than control subjects during cooling, possibly in part due to higher heat production,” Scheele says.
The researchers also looked at thermoregulation for both groups over the course of a full day at a comfortable temperature. They found that swimmers reached a lower core body temperature—potentially a sign of heat acclimation due to regular sauna visits.
Their skin temperature in areas close to BAT showed a distinct peak between 4:30 am and 5:30 am and revealed signs of a 24-hour rhythm in brown fat tissue activity and heat production, at least during rest at a comfortable temperature. “The difference between groups is possibly explained by increased maturation and cold adaptation of BAT in the winter swimmer group,” Scheele says.
The study’s small sample size, the absence of female participants, and the inability to draw causal conclusions about the direct effect of winter swimming on temperature regulation or brown fat tissue are all potential limitations to the findings.
“We compared experienced winter swimmers with control subjects, which allows for the possibility that other lifestyle factors or genetic factors not measured in the current study also could impact the differences between the groups,” Søberg adds.
Nevertheless, the findings may have important health implications, given that brown fat tissue activity is associated with a lower risk of metabolic diseases. In future studies, the researchers plan to assess the potential effects of winter swimming on metabolic health in overweight participants.
They would also like to examine the molecular mechanisms underlying brown fat activation, and how brown fat communicates with the brain to regulate feeding behavior. “Our results point to winter swimming as an activity that could increase energy expenditure, thus proposing a new lifestyle activity that might contribute to weight loss or weight control,” Scheele says.
Cold water swimming – also known as winter swimming or ice swimming – describes swimming outdoors (lake, river, sea, swimming pool, etc.) mainly during the winter or in the colder and polar regions . This special form of endurance sport is becoming increasingly popular.
Cold water swimming can be used as a general umbrella term for swimming in cold to ice-cold water. Winter swimming specifically implies that it must be winter. In colder countries, it can be synonymous with ice swimming when the water is frozen over because ice swimming explicitly requires the ice to break.
In recent years, ice swimming (in water below 5 °C) has evolved into an all year-round sport , with many swimmers participating and competing regularly in both local and international events.
Several studies have suggested that cold water swimming has a wide variety of health benefits , including changes in hematological  and endocrine function [5,6], fewer upper respiratory tract infections , amelioration of mood disorders  and general well-being .
Although chronic exposure to colder water temperatures has been shown to be beneficial to one’s health, several studies have outlined the potential risks [10,11,12,13]. Therefore, the primary purpose of this review is to outline the potential benefits and risks of cold water swimming.
In certain northern countries, such as Finland, Poland, Russia, Norway, Sweden, Denmark, Estonia, Lithuania, the Czech Republic and Latvia, cold water swimming is practiced regularly in the sense of winter swimming. In Eastern Europe and Russia, winter swimming is part of the celebration of Epiphany .
Naturally, many field studies investigating the influence of cold water swimming on the body come from these northern countries on various topics such as adaptation to the cold , changes in lipid metabolism [23,24], adjustments to hematological values [25,26], effects on the immune system [27,28,29,30] and the hormones [5,31] or aspects of thermoregulation [32,33,34,35].
Events in which large numbers of people swim over a relatively short distance in cold water in winter can also be called classic winter swimming.
Cardiovascular and Endocrine System
Several studies have described a positive effect on the cardiovascular system and cardiovascular risk factors. Cold water swimming appears to have a positive impact on cardiovascular risk factors such as lipid profile [23,24,56] or blood pressure .
Various hormones such as catecholamines, insulin, Thyroid-stimulating hormone (TSH), Adrenocorticotropic hormone (ACTH), and cortisol also react to the cold stress [11,55,63]. As a form of endurance training, winter swimming – even if it is more strenuous to swim in cold water – can improve adaption to stress. In a field study with 34 middle-aged cold-water swimmers (48–68 years old), different values of lipid metabolism were determined at the beginning (October), in the middle (January) and after the season (April) of winter swimming .
There was a decrease in triglycerides between January and April, a lower concentration of homocysteine (high levels are linked to the early development of heart disease) between October and January and between October and April. The decrease in homocysteine was more pronounced in women than in men . These changes were most probably also due to the fact that these swimmers were active, not sedentary. Unfortunately, no control group was investigated.
Cold water swimming seems to have a positive effect on insulin metabolism, although here too, the effect appears to be sex-specific [3,56]. In a field study, 30 cold water swimmers were examined for six months with regard to body composition and insulin sensitivity .
The chilled water swimmers were overweight compared to a control group and had a higher percentage of body fat with differences between the sexes. For female and swimmers with lower body fat percentage, there was an increased insulin sensitivity as well as a reduction in insulin secretion and resistance .
Swimming in cold water also affects other hormones, such as ACTH and catecholamines [5,58]. As such, it was found that if swimmers participated in winter swimming three times a week at water temperatures of 0–3 °C for 12 weeks, there was an increase in ACTH and cortisol as well as norepinephrine . Water immersions were 20 s per week for 3 winter months in water of a temperature of 0–2 °C.
It is believed that the increase in norepinephrine may lead to reduced pain perception, such as with whole-body cold therapy or with ice swimming . In contrast, regular three-month winter swimming resulted in a decrease in the concentration of catecholamines when measured immediately after immersion. It was concluded that adaptation through habitual exposure to the cold of winter swimming weakened the physiological response and inhibited the rise of the catecholamines .
There is rising evidence that winter swimmers are more resistant to certain illnesses and infections, experiencing them less frequently and more mildly . The incidence of infectious diseases of the upper respiratory tract is 40% lower in winter swimmers compared to a control group .
Furthermore, it has been shown that swimming in cold water has an impact on immune-specific hematology [29,67]. Anecdotally, cold water swimmers state that they suffer fewer and milder infections from regular swimming in cold water . Improved immune response and function is biologically plausible primarily through the release of stress hormones [31,68] in response to cold exposure.
Dhabhar  argued that short-term physiological stress such as cold exposure prepares the immune system to fight infections. The study of the effects of cold water swimming on the function of the immune system (especially leukocytes and immunoglobulins) has led to contrasting results.
This is possibly due to the majority of studies examining individuals and study protocols of unfamiliar people who take a short bath in ice-cold water  longer static cold water swimming (stay in the cold water without moving)  and experienced long-distance swimmers who trained for 8 h (dynamic cold water swimming)  were very different.
If cold water swimming has a positive effect on the immune function, then there should be observable changes in the immune system markers and actual health should improve over the course of an acclimatization program. Ideally, studies should focus on chilled water swimmers who participate in regular cold water training and thus would yield the most robust values. However, there may be differences in response to static cold-water swimming since exercise and cold can both induce increased physiological stress, and their combined effects can exceed the individual effect of each state .
In a study by Jansky et al. , the immune system’s reactions to static cold-water swimming was investigated through study participants being initially immersed in cold water and then, followed by repeating cold water swimming three times a week over six weeks. The subjects underwent regular winter swimming at least once a week, for 2 to 10 min, at the natural water temperature (6.8 °C (October 1992) to 2.0 °C (January 1993)) in the southern Baltic Sea.
It was seen that the adjustment changes both the number of leukocytes at rest and their response to static cold-water swimming. However, these changes were minor and of uncertain importance, and repeated cold-water swimming did not change the response of the immunoglobulins . Furthermore, Brazaitis et al.  investigated the response to being immersed intermittently in cold water.
Cold stress was induced using intermittent immersion in bath water at 14 °C. It was observed that participants demonstrated different rates of core temperature cooling. Specifically, those who cooled slower showed signs of leukocytosis. However, responses to static cold-water swimming seem to be heavily influenced by the study protocol and participants.
The difference in leukocytosis between those who cooled down quickly or slowly could potentially be attributed to the fact that people who cool down more slowly were immersed for a total of 120 min, whereas people who cooled down faster were immersed for an average of 96 min. The use of alternating cold-water swimming and reheating could also have complicated the physiological response as well.
It also seems that the extent of leukocytosis could correspond to the strength and duration of the stress. Jansky et al.  found no increase in neutrophils after 60 min in water at 14 °C, while Brazaitis et al.  demonstrated an increase of 55% after a total of 120 min in 14 °C cold water with periodic reheating. Within ~1 min of leaving the bath, the volunteer was towel dried and temperatures were measured.
It is interesting to note that the ice swimmer only spends a few minutes immersed in cold water, however, the short exposure duration is still sufficient to illicit a measurable physiological response. For example, blood tests performed immediately before and after a 150 m winter swim at 6 °C showed that the leukocytes (neutrophil granulocytes, lymphocytes and monocytes) increased significantly in the blood due to the cold, so that protection against inflammation and respiratory infections can occur .
Another study also showed an increase in leukocytes and monocytes, which was seen as a sign of an improvement in the body’s response to stress . However, the clinical significance of these findings is still uncertain. Short-term leukocytosis is caused by leukocytes that leave organs such as the spleen in response to the increase in catecholamines and cortisol in order to be prepared for defense .
The most important part of this short-term reaction is a subsequent decrease in the number of leukocytes in the blood when they reach tissues such as the skin . This has not been explicitly investigated in the context of cold-water swimming, but Yeager et al.  found that monocytes and neutrophils migrated in response to a concentration of cortisol that is equivalent to that released during acute stress.
However, it is difficult to measure in vivo immune function adequately, therefore upper respiratory tract infection is often a useful measure as it is a very common infection that affects both congenital and acquired elements . Dugué and Leppänen  found that trained cold water swimmers had a higher concentration of certain leukocytes than those who were not cold acclimatized. The authors also examined the reactions of both groups to a brief immersion in ice-cold water. However, since this happened after a sauna use, it is impossible to separate the effects of the two temperature ranges.
Furthermore, this was the only study that specifically looked at the cold water immersion effects in men and women, separately. However, the small number of participants complicates the meaningfulness of these results. Interestingly, Kormanovski et al.  is the only study to have documented the incidence of actual illness. The authors examined 15 experienced long-distance swimmers over a period of six months. Seven swimmers in the group completed three long-distance swims, once at 6 h (in month 1) and twice at 8 h (in months 3 and 6), while the other swimmers rested and served as a control group.
Differences between the group of long-distance swimmers and the control group were found with regard to the reaction of the leukocytes and immunoglobulins, both over the entire investigation period and over the period of the long-distance swimming. The heavy training load may have led to a slight reduction in leukocytes in the long-distance swimmer group in the stress-free phase, but the load caused noticeable increases.
The number of granulocytes increased almost fourfold during the 8 h swim. The group of long-distance swimmers showed a significant decrease in the concentration of serum immunoglobulins and IgA (immunoglobulin A) in saliva (sIgA, secretory immunoglobulin A) during the training period, whereas this was not the case for control swimmers. SIgA decreased markedly during all three periods of long-distance swimming, but remained unchanged in the control swimmers, whereas the concentration of the serum immunoglobulins showed no clear pattern in any group.
This suggests that there is a connection between the extent of stress and the concentration of leukocytes [60,67]. Long-distance swimmers in the study by Kormanovski et al.  showed no significant change in neutrophils after 1 h, but after 2 h the numbers had increased by ~50%, with a quadrupling after 8 h.
The non-acclimatized swimmers in the study by Lombardi et al.  showed the fastest response with a 38% increase in the number of neutrophils after a race over 150 m. However, this was compared to the previous day, so part of the increase could be due to the psychological stress on race day.
All of the studies mentioned report higher leukocyte counts in cold water swimmers, but it is important to emphasize that it is not known whether these higher numbers reflect in the body or a redistribution between different tissues. Finally, it is crucial that the level of cold water acclimatization should be clearly defined as habitual exposure, as discussed previously, heavily influences the magnitude of physiological reaction.
The swimmers in the study of Kormanovski et al.  were very well trained, whereas those of Lombardi et al.  were not acclimatized.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7730683/
More information: Cell Reports Medicine, Søberg et al.: “Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young healthy winter swimming men.” www.cell.com/cell-reports-medi … 2666-3791(21)00266-4 , DOI: 10.1016/j.xcrm.2021.100408