The Copenhagen Winter-Swimmer Study

Use "about 11 minutes a week, split across a few short dips" as a reasonable starting target if you like cold water, not as a proven minimum. Nothing in this study tells you what it does for women, or for health outcomes.

What the study was

Researchers in Copenhagen compared eight young, healthy men who were habitual winter swimmers with matched controls who did neither winter swimming nor sauna. Each man was kept comfortable, then gradually cooled, while the team measured energy use, skin and core temperature, and brown-fat activity on scans.^[1]

Brown fat is a type of fat that burns fuel to make heat rather than storing it. Its activity was the study's main outcome.

What it found

• More heat production in the cold. At a comfortable temperature, the swimmers' resting energy use matched the controls'. During 30 minutes of cooling it rose by the equivalent of roughly 1,000 kcal a day in the swimmers, against about 550 in the controls.
• Brown fat did not explain it. Brown-fat activity rose no more in the swimmers than in the controls, and muscle activity did not differ either. The authors state plainly that brown fat cannot account for the extra heat, so where it came from is unidentified.
• Other differences. The swimmers had a warmer skin temperature over the collarbone (where brown fat sits) during cooling and a lower core temperature when comfortable. At a comfortable temperature their brown fat showed no measurable activity, and their skin was generally warmer — which the authors read as acclimation to the sauna's heat rather than a gain from cold.
• A daily rhythm. Skin temperature over the collarbone peaked in the small hours, around 4:30–5:30 a.m., in the swimmers only, hinting that habitual cold may set the daily rhythm of heat production there.

Repeated cold blunts the gasping, racing-heart shock response within a handful of dips.^[2] Yet these habitual swimmers produced more heat in the cold, not less. Getting used to cold seems to wear out the shock, not the heat-producing response.

Why it can't isolate the cold

• Eight men. It is a cross-sectional comparison of a very small group, not a trial.
• The sauna. Nearly all the swimmers also used the sauna regularly — about 57 minutes of sauna heat a week — while the controls did neither. Some of what looks like a cold adaptation may be a heat adaptation.
• Men only. There is no equivalent dose-finding data in women, and assuming a protocol designed in men transfers directly is a known weakness in this literature.

Where "11 minutes a week" comes from

The number describes the swimmers' habits: about three dips of 1–2 minutes each, on two or three days a week, adding up to roughly 11 minutes of cold water — alongside roughly 57 minutes of sauna. It was popularised afterwards in a trade book and podcast appearances by the study's first author.

The paper never tested 11 minutes as a threshold. Nobody has compared it with more or less cold, so it is a reasonable starting target, not a validated minimum effective dose.

"Always end on cold." The related advice — finish a sauna-and-cold session with the cold, so the body rewarms itself by producing heat — is plausible but has never been tested against ending on heat. File it as a reasonable rule of thumb, not a validated protocol.

Winter swimming carries risks a plunge tub doesn't

The study's men swam in open winter water, which adds hazards to the cold itself. Two thirds of drownings happen to competent swimmers, which heart researchers read as pointing to sudden heart-rhythm death rather than exhaustion.^[3] Fluid in the lungs (immersion pulmonary edema) ended about 1 in 230 swims at open-water events — 0.75% of swims in women against 0.09% in men, and 1.1% in people over 60 — though those were strenuous races.^[4] Never hold your breath under cold water, and get out and seek care if breathing becomes laboured.

What to do

1. If you enjoy cold water, about 11 minutes a week in short dips is a sensible place to start. It is a description of habitual swimmers, not a dose shown to change health.
2. Don't expect fat loss from a plunge. The extra energy use was measured during 30 minutes of cooling in a lab, not during a brief dip. See Cold exposure for why an ice bath isn't a fat burner.
3. Build up gradually and never swim in cold open water alone — the safety basics on Cold exposure apply whatever the dose.

Further reading

• Barwood MJ et al. Habituation of the cold shock response: a systematic review and meta-analysis. J Therm Biol 2024.^[5]
• Søberg S et al. Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Rep Med 2021.^[6]