“A sauna session burns hundreds of calories.”

The research found this

The cleanest measurement comes from a randomized crossover study of 14 men. Each spent an hour sitting in 40°C water and, on another day, an hour cycling at a moderate pace. The researchers matched the two sessions for the rise in core temperature.1Source 1 · Crossover trialThe effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?Faulkner SH, Jackson S, Fatania G, et al.. Temperature, 2017.Read the paper ↗PubMed ↗ They measured energy use from the men's breathing with a method called indirect calorimetry. The hot water raised it by 61 kcal an hour over rest (a 79.5% increase), and the cycling added 556 kcal an hour.

The hour of heating burned an extra 63.4 kcal in lean men and 58.5 kcal in overweight men, so body size made little difference.1Source 1 · Crossover trialThe effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?Faulkner SH, Jackson S, Fatania G, et al.. Temperature, 2017.Read the paper ↗PubMed ↗ The dry-sauna evidence is a 1986 study of 10 men who sat in a Finnish sauna at 80°C for an hour twice a day. It reported metabolic rate up 25–33% after the first day, and its abstract doesn't say how or when that was measured.2Source 2 · Clinical trialSome cardiovascular and metabolic effects of repeated sauna bathingLeppäluoto J, Tuominen M, Väänänen A, et al.. Acta Physiologica Scandinavica, 1986.Read the paper ↗PubMed ↗

When 60 inactive young men took a 10-minute sauna at 90°C, heart-rate monitors estimated 66.80 kcal in the normal-weight group and 92.37 kcal in men with class 1 obesity.3Source 3 · Clinical trialPhysiological Response to Thermal Stress in Obese vs. Non-Obese Physically Inactive MenPodstawski R, Borysławski K, Pomianowski A, et al.. Biology, 2022.Read the paper ↗PubMed ↗ The monitors' algorithms assume heart rate tracks oxygen use. In the heat, much of the rise in heart rate goes to cooling the body, and the authors listed the monitors as a limitation.

Calorimetry counts the fuel the body burns, and the heat that sweat carries off is a different number. By the Institute of Medicine's figure of 0.58 kcal per gram, evaporating 500 g of sweat takes about 290 kcal of heat with it.4Source 4 · BookDietary Reference Intakes for Water, Potassium, Sodium, Chloride, and SulfateInstitute of Medicine. National Academies Press, 2005.Read the paper ↗ In a sauna most of that heat came from the room in the first place, and sweat that drips off never evaporates.