Danish winter swimmers burned more energy in the cold than men who avoided it
The 2021 Cell Reports Medicine paper behind the "11 minutes a week" rule compared seven young winter swimmers with eight men who used neither cold water nor sauna.
The paper
Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men
Søberg S, Löfgren J, Philipsen FE, Jensen M, et al.. Cell Reports Medicine, 2021;2(10):100408.
See the study at a glance
- Design
- Cross-sectional study comparing two groups at one point in time
- People
- 7 winter swimmers and 8 controls
- Who they were
- Healthy young men in Denmark. The controls matched the swimmers on age, sex, BMI and aerobic fitness, and they used neither cold water nor sauna. The researchers recruited eight swimmers and dropped one whose scan showed no cold-activated brown fat.
- What they did
- Blankets with water running through them chilled each man in steps until he was on the verge of shivering. The lab days also included PET/MRI scans of brown fat, energy measurements, fat biopsies and a thermal comfort test. The only immersion was one hand in 4°C water for 3 minutes.
- How long
- Three lab days and a 24-hour sub-study
These numbers came out of it
11 min
The paper's Table 1 lists 11 minutes as the swimmers' weekly time in cold water. The study never tested it as a dose.
57 min
The same swimmers reported 57 minutes a week in sauna heat.
3,044
During cooling the swimmers burned energy at a rate of 3,044 kcal a day, against 2,560 for the controls. The paper scaled both figures up from 30-minute measurements.
12.0%
The swimmers' tissue fat came to 12.0%, against 18.2% in the controls.
The researchers did this
Researchers in Denmark compared young men who swam in cold water through the winter with men who used neither cold water nor sauna. The two groups were matched on age, sex, BMI and aerobic fitness. One swimmer's scan showed no brown fat switching on in the cold, so the team dropped him. That left seven swimmers and eight controls.Source 1 · Cross-sectional studyAltered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming menSøberg S, Löfgren J, Philipsen FE, et al.. Cell Reports Medicine, 2021.Read the paper ↗PubMed ↗
In the lab the cold came from two blankets with water running through them. After 45 minutes at a comfortable temperature, the researchers cooled the blanket water step by step, going by how cold each man felt, until he was on the verge of shivering. PET/MRI scans tracked how much glucose his brown fat took up.Source 1 · Cross-sectional studyAltered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming menSøberg S, Löfgren J, Philipsen FE, et al.. Cell Reports Medicine, 2021.Read the paper ↗PubMed ↗ That was the study's main measure, and it rose about equally in both groups during cooling. At a comfortable temperature most controls showed modest brown fat activity and the swimmers showed none.
The researchers measured energy use over 30 minutes of cooling and scaled it up to a daily rate. On that scale the swimmers burned 3,044 kcal a day and the controls 2,560. At a comfortable temperature the figures were 2,038 and 2,005, and that difference wasn't significant.Source 1 · Cross-sectional studyAltered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming menSøberg S, Löfgren J, Philipsen FE, et al.. Cell Reports Medicine, 2021.Read the paper ↗PubMed ↗ Scaled back down to a half hour, cooling added about 21 kcal to what the swimmers burned at a comfortable temperature.
A 24-hour sub-study of 5 swimmers and 6 controls found lower core temperatures in the swimmers. The skin above their collarbones hit its daily temperature peak between 4:30 and 5:30 in the morning.Source 1 · Cross-sectional studyAltered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming menSøberg S, Löfgren J, Philipsen FE, et al.. Cell Reports Medicine, 2021.Read the paper ↗PubMed ↗ The swimmers also had lower blood sugar two hours into a glucose tolerance test, and their blood pressure rose less when one hand went into 4°C water.
The 11 and 57 minutes come from Table 1, where the swimmers described their own habits. On average they swam 2.6 times a week and went in 2.6 times per visit, for 1.4 minutes each time. Each visit also included two sauna rounds of 11.1 minutes.Source 1 · Cross-sectional studyAltered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming menSøberg S, Löfgren J, Philipsen FE, et al.. Cell Reports Medicine, 2021.Read the paper ↗PubMed ↗ The full text never mentions ending a session on cold. It speculates that regular sauna use, by adapting the body to heat, might explain the swimmers' lower core temperature and their missing brown fat activity at a comfortable temperature.
The study can't tell you these things
- A comparison made at one point in time can't show that winter swimming caused any of the differences. The authors point out that men whose bodies handle cold in a particular way may be the ones who take up winter swimming.
- The groups were small and all male. Groups that size can produce false positives and miss real effects.
- The controls carried more body fat and had worse glucose tolerance. That could account for some of the metabolic differences.
- The scans estimated brown fat indirectly, and the lab cold came from blankets.
- Table 1 lists sauna use as 7 yes and 1 no in a column headed n = 7, so its numbers don't add up.
People quote it like this
The Huberman Lab newsletter of 1 May 2022 recommends about 11 minutes of deliberate cold a week and credits a recent study without naming it. It also names a rule of ending on cold after Susanna Søberg.Source 2 · PodcastThe Science & Use of Cold Exposure for Health & PerformanceHuberman Lab. Huberman Lab Neural Network newsletter, 2022.Read the page ↗ In the paper, 11 minutes describes what seven men said they did, and the study never tested whether heat or cold should come last.
From our sister magazine
Sauna Monthly covers how to pair a sauna with a cold plunge at home
We cited these sources
- 01Søberg S, Löfgren J, Philipsen FE, Jensen M, Hansen AE, Ahrens E, et al.. Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine. 2021;2(10):100408. doi:10.1016/j.xcrm.2021.100408 PMID 34755128
- 02Huberman Lab. The Science & Use of Cold Exposure for Health & Performance. Huberman Lab Neural Network newsletter. 2022. Accessed Sep 24, 2026.