A study in Denmark shows that winter swimming combined with sauna visits acclimatises the body to extreme temperatures. The experienced winter swimmers demonstrated a more distinct on-and-off pattern in temperature-dependent brown fat recruitment. They also spent more energy when defending their body temperature against the cold challenge.
Winter swimming has become extremely popular in Denmark and the rest of Scandinavia – especially during the COVID-19 pandemic, when most people had to spend more time at home and find diversion in nature instead of visiting cafés and bars.
Many of the people who try winter swimming continue this activity because it improves their well-being and provides greater physical stamina in everyday life with more energy for their daily tasks.
Now a new study supports this, showing that winter swimmers who combine the cold dips with hot sauna sessions improve their ability to adapt to extreme temperatures and burn more calories. This benefit is largely triggered by altered thermogenesis of brown fat, which burns fat as fuel to produce heat.
Plays a key role
The study, published in Cell Reports Medicine, was led by Camilla Schéele, Associate Professor, Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen. Susanna Søberg, a PhD student of Camilla Scheele, organised and carried out the trials.
“Since cold activates brown fat, winter swimming has been assumed to increase the energy burned by causing the body to produce more brown fat,” explains Camilla Schéele, referring to several previous studies suggesting that repeated and sudden cooling of the body from room temperature to nearly freezing over several months forces the body to produce more brown fat:
“We now question this hypothesis since our study suggests that winter swimmers have higher cold-induced energy expenditure without showing more activation of their brown fat than the controls. Instead, we found that the amplitude between thermal comfort and cold was enlarged in the winter swimmers since, at thermal comfort, their brown fat was completely silenced.”
The hypothalamus in the brain regulates brown fat activity according to signals from the rest of the body. This means that brown fat only turns on when increased heat production is needed to maintain body temperature.
“Our data emphasize that brown fat is regulated in adult human physiology and that it, together with the skeletal muscles and blood flow, plays a key role in adapting the body to extreme temperatures and in maintaining body temperature homeostasis,” says Camilla Schéele.
PET reveals active brown fat
The researchers recruited and followed eight young healthy men in their early twenties who had been regular winter swimmers for at least 2 years by swimming or sitting in open water, wearing at most swimming trunks, and using a sauna. In addition, the trial recruited eight young healthy men who had never been winter swimmers or had otherwise used cold or heat immersion.
To ascertain how both extreme and normal temperatures affected brown fat, the participants were position emission tomography (PET) scanned at both room temperature and after cooling with a special type of blanket with channels of water flowing through the blanket that is connected to a device that can precisely regulate the temperature of the water.
The participants were cooled for 1 hour before a glucose tracer was injected, and after an additional 1 hour the participants were scanned while continuing to be cooled. PET scanning detects a radioactive glucose tracer absorbed by the active brown fat during its active periods. This enables researchers to take a snapshot of the brown fat activity during the time when the glucose tracer has been in the body. The participants were also scanned on a separate day when they were kept at a comfortable temperature instead of being cooled.
The PET images showed that the brown fat acted very differently in the groups of participants: the winter swimmers’ brown fat was completely silenced at comfortable temperatures, whereas the control group demonstrated some activity. In contrast, during cooling, both groups activated their brown fat to a similar extent.
“A problem with this method is that we do not know how active the brown fat was during the first hour of cooling and whether they simply improved their ability to adapt to lower temperatures,” says Camilla Schéele.
Adapt to the sauna heat
However, Camilla Scheele and colleagues also found that the winter swimmers have higher cold-induced energy expenditure, which must be related to higher heat production among these participants. In a separate experiment, the researchers observed that winter swimmers emitted more heat from the skin and generally had a lower core temperature and a characteristic diurnal variation in core temperature.
“The winter swimmers had a lower core temperature at thermal comfort. In addition, they also had a higher skin temperature, indicating higher heat dissipation. Linked with the absence of brown fat activity at room temperature, this indicates that winter swimming combined with sauna visits causes the body to acclimatise to enable it alternately to tolerate very low and very high temperatures. This adaptation involves the brown fat but probably also by muscle activity,” explains Camilla Schéele.
She adds that other researchers have previously documented that people who regularly use a sauna have lower core temperatures. Since the winter swimmer participants also used a sauna, she and Susanna Søberg think that their findings result from winter swimmers adapting to both the hot sauna and the cold water.
“We think that the winter swimmers adapt to the sauna heat by emitting more heat from the skin. However, since they also regularly immerse themselves in very cold water during the same visit, they have to use more energy than the controls to compensate for the extra heat loss from the skin and thereby use more energy than the control group,” says Camilla Schéele.
Possible weight-loss strategy
Camilla Schéele thinks these results add perspectives to the research on obesity, diabetes and other conditions since active brown fat is associated with a lower risk of developing metabolic diseases.
“Brown fat has been described as being healthy in terms of increasing metabolic rate and insulin sensitivity. A large retrospective study of brown fat activity measured by PET scanning showed that people with more brown fat activity had better cardiovascular health. Due to the fat-burning capacity of brown fat, many studies are examining whether accumulating brown fat could counteract obesity. We are very interested in further understanding the relationship between temperature regulation and metabolism among humans,” explains Camilla Schéele.
“This study examined young healthy men who still have brown fat. We show that similar people may be able to expend more energy by taking a winter swim and using a sauna – but we cannot yet say that this strategy actually counteracts obesity. Further research is needed to determine this based on other types of participants. We are going to pursue this ourselves, since we have just completed a study with overweight people who have been winter swimmers for one season. We have not yet analysed the data and firm conclusions are premature. But we hope that the study will enable us to conclude whether winter swimming can lead to improved health, including weight loss and improved glucose tolerance,” says Camilla Schéele, who emphasises that the current study calls for much more research since it included few participants.
“The small number of participants, the absence of women and the inability of the study design to directly show the relationships between winter swimming, temperature regulation and brown fat are all limitations we have had to bear in mind in interpreting our data. For example, we cannot conclude anything about the extent to which genetics and lifestyle factors influenced the results,” concludes Camilla Schéele.
