Many Scandinavians undertake regular swimming during the cold winter months. Winter swimming, as it is known, is part of Scandinavian culture and has been enjoyed for generations. There is a feeling that immersing oneself in icy waters after enjoying a hot sauna brings benefits in terms of health and happiness.
In a new study, published today in the journal Cell Reports Medicine, scientists suggest that routinely swimming in cold water and alternating this with hot sauna sessions might benefit people in several different ways. The practice appears to affect the metabolism of brown adipose (fat) cells and the generation of body heat, two processes that help people adapt to conditions of extreme heat and cold.
“Our data underscore that BAT [brown adipose tissue] in adult humans is part of the collective body temperature regulation system, in collaboration with skeletal muscle and blood flow,” said 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.”
Brown adipose tissue is a special type of body fat that functions to generate heat when we need to maintain our body temperature at a constant level. It is activated when we get cold and contains many more cellular structures for generating energy and heat than are found in normal fat cells.
In order to test whether winter swimming had any effects on the body’s ability to acclimate in conditions of extreme cold and heat, the researchers used a sample of eight male winter swimmers who had all swum in cold water and combined this with a hot sauna session, several times a week for at least two years. The sample was matched for age and fitness levels. The control sample also consisted of eight men, matched for age and fitness, but they were not winter swimmers and did not use any form of cold or heat therapy during the study.
Initial tests of participants’ thermoregulatory responses to cold involved submerging one arm in icy water for three minutes and also lying between water-perfused blankets to lower body temperature. In both instances, the winter swimmers showed signs of cold tolerance and had lower increases in pulse rates and blood pressure. They also showed a higher increase in skin temperature in response to cooling, which may indicate that they were capable of greater heat loss as a potential adaptation to frequent sauna exposure.
“We expected winter swimmers to have more brown fat than the control subjects, but it turned out that they instead had better thermoregulation,” said study first author Susanna Søberg of the University of Copenhagen.
Next, the researchers used positron emission tomography, to measure the activity of brown fat tissue in the participants as they were exposed to a comfortable temperature. At this temperature, the control subjects showed signs of activated brown fat tissue, as indicated by an uptake of glucose, whereas the winter swimmers showed absolutely no activity in their brown fat tissue. These results indicate that brown fat cells were actively generating heat even at mesic temperatures in non-swimmers. The winter swimmers had lower core temperatures than the controls during these periods in thermally comfortable conditions.
“The findings support the notion that brown fat tissue fine tunes body temperature to a comfortable state in young adults,” said Scheele. “It was, however, a surprising finding that the winter swimmers had no activity at all when exposed to comfortable temperatures.”
When participants were exposed to cold, however, the activity of brown fat tissue increased in both groups. The winter swimmers showed much higher heat production, or energy expenditure, in response to cool temperatures. They were able to generate more heat than controls when faced with cold conditions. “Winter swimmers burned more calories than control subjects during cooling, possibly in part due to higher heat production,” Scheele says.
The researchers also tracked the daily pattern of changes in temperature for both groups over a full day at comfortable thermal conditions. They found that winter swimmers reached a lower core body temperature, possibly due to being acclimated to the high heat conditions in saunas. 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. “The difference between groups is possibly explained by increased maturation and cold adaptation of BAT in the winter-swimmer group,” Scheele says.
There are some limitations to the study, including the small sample size and the absence of female participants. In addition, the fact that it was a correlational study means that a causal relationship between winter swimming and brown fat tissue activation, or acclimation to extreme temperatures, cannot be assumed.
“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,” noted Søberg.
However, the link between winter swimming and the ability to generate more heat and burn more calories during exposure to cold may have important implications for overweight people or for those trying to control or maintain their body weight.
“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,” said Scheele.