Shifting Between Warm and Cold Matters More Than Ice Water
Even a T-shirt outdoors, cool wind, or brief dips in mild water may activate calorie-burning brown fat.
Andrew Huberman · Essentials: Cold & Heat Exposure for Metabolic Health | Dr. Susanna SøbergDr. Susanna Søberg, a researcher who studies how temperature affects metabolism, was asked about fitting cold exposure into just two days a week.
Her answer turned the usual ice bath logic on its head. She said getting out and getting back in may be more useful than staying in.
“The changes in temperature is what is strengthening your cells in the body,” she said. In her view, the body learns from switching between warm and cold, not simply from being cold.
That switch, she argues, challenges cells to adapt to changing temperatures. It also wakes up brown fat, a type of fat that burns sugar and fat to make heat.
Even brief cooling of the skin can activate it, she said, citing studies in which putting a hand in cold water triggered a response. The options can therefore be modest.
She pointed to going outdoors in a T-shirt, feeling cold wind on the skin, or wearing a cooling vest, with studies finding brown fat grew after ten days of vest use.
Precise water temperature, she said, may matter less than the up and down change. Water as mild as 12 degrees Celsius, or air around 19 degrees, can activate brown fat, she said, so ice cold water is unnecessary.
Exact doses for every temperature are still not settled, she added. What she stressed was the up and down change in temperature, not how cold it gets.
Questions people ask
How cold is cold enough?
She pointed to water around 12 degrees Celsius or cool air around 19 degrees as enough to activate brown fat. The point, in her account, is that the skin senses a temperature change.
How often do I need to do it to see a benefit?
When asked about one to two minute dips, with getting out and back in, she agreed that short exposures can challenge cells to adapt to changing temperatures. Studies of cooling vests she cited found more brown fat after about ten days. She suggested repeated short exposures rather than chasing extremes.
Why end a hot shower or sauna with cold?
Ending on cold leaves the body working to warm up, which she thinks keeps brown fat and muscle active. That continued work to stay warm is her explanation for why the switch matters.