Cold-­plunge partisans claim, for instance, that cold exposure activates “brown fat,” a special type of fat tissue that burns energy to generate heat. Activating this fat is said to convey almost-magical health benefits, reducing the risk of diabetes and other chronic diseases. Casey Means, President Trump’s pick to be surgeon general, pointed to brown fat when explaining to her followers why she’s come to “LOVE cold plunges” in a 2024 Instagram post. Unfortunately, most adults typically have only a few grams of brown fat, so any beneficial effect from activating it is likely quite small. Even a study of Wim Hof, the Dutch health guru nicknamed “The Iceman” who helped popularize ice bathing, proved disappointing: Using fMRI and other imaging techniques, researchers found that his brown-fat activation after a session of his Wim Hof Method (breathing exercises plus extended cold plunging) was “unremarkable.” (In an email to The Atlantic, Hof acknowledged that brown fat is not primarily responsible for warming the body in cold environments, but said that his breathing techniques support muscular activity that functions as a “physiological radiator.” He did not elaborate on the health effects of said muscular activity or cold plunging more generally.)