“Cold plunges burn fat by switching on brown fat.”

The research found this

Adults do have brown fat that switches on in the cold. In a 2009 Dutch study, scans found active brown fat in 23 of 24 men in 16°C air and in none at 22°C.1Source 1 · Clinical trialCold-Activated Brown Adipose Tissue in Healthy Menvan Marken Lichtenbelt WD, Vanhommerig JW, Smulders NM, et al.. New England Journal of Medicine, 2009.Read the paper ↗PubMed ↗ Men who were overweight or obese had less of it.

The one controlled trial with a fat result came from Japan. Men with little or no detectable brown fat spent two hours a day at 17°C for six weeks, and the 12 in the cold group lost 0.70 kg of fat mass while the 10 controls gained 0.03 kg.2Source 2 · Randomized trialRecruited brown adipose tissue as an antiobesity agent in humansYoneshiro T, Aita S, Matsushita M, et al.. Journal of Clinical Investigation, 2013.Read the paper ↗PubMed ↗ Their cold-induced energy use rose from 108.4 to 289.0 kcal a day. The authors scaled those numbers up to a full day from a two-hour test, so a single session's extra burn came to about 9 kcal at the start and 24 kcal after six weeks.

Brown fat can grow without any change in body fat. Seventeen young adults spent up to six hours a day in a 15–16°C room for ten days, and their detectable brown fat volume rose from 665 to 913 cc.3Source 3 · Clinical trialCold acclimation recruits human brown fat and increases nonshivering thermogenesisvan der Lans AA, Hoeks J, Brans B, et al.. Journal of Clinical Investigation, 2013.Read the paper ↗PubMed ↗ Their body weight and body composition stayed the same, and so did their resting metabolic rate.

Water brings its own complications. An hour at 14°C raised metabolic rate 350%.4Source 4 · Clinical trialHuman physiological responses to immersion into water of different temperaturesŠrámek P, Šimečková M, Janský L, et al.. European Journal of Applied Physiology, 2000.Read the paper ↗PubMed ↗ Young athletes who repeated that hour three times a week for 4–6 weeks adapted by letting their body temperature drop further, so by the end they made about 20% less heat during an immersion.5Source 5 · Clinical trialChanges in thermal homeostasis in humans due to repeated cold water immersionsJanský L, Janáková H, Uličný B, et al.. Pflügers Archiv - European Journal of Physiology, 1996.Read the paper ↗PubMed ↗ Their body fat showed a trend toward a small increase.

People also ate more after cold water. In a crossover of 15 active adults, an all-you-can-eat pasta meal after 30 minutes in 16°C water came to 2,783 kJ against 1,817 kJ after 35°C water.6Source 6 · Crossover trialEffects of cold-water immersion on energy expenditure, ad-libitum energy intake and appetite in healthy adultsGrigg MJ, Thake CD, Allgrove JE, et al.. Physiology & Behavior, 2025.Read the paper ↗PubMed ↗ That is about 231 kcal more, and the participants' appetite ratings didn't differ.