“Sauna helps you lose weight.”

The research found this

A sauna takes weight off as sweat. In 674 first-time users, two 10-minute rounds at 90°C removed a mean 0.37 kg in women and 0.50 kg in men. That came to 0.49–0.90% of body mass depending on BMI group.1Source 1 · Clinical trialSauna-Induced Body Mass Loss in Young Sedentary Women and MenPodstawski R, Boraczyński T, Boraczyński M, et al.. The Scientific World Journal, 2014.Read the paper ↗PubMed ↗ An infrared cabin run at 65°C for 45 minutes took off 1.0 kg, and 2 of 12 people passed 2% dehydration.2Source 2 · Clinical trialFar‐infrared sauna exposure at 65°C elevates core temperatureJenkins EJ, Killick JA, Grimm SR, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

That water comes back. Seven mixed martial arts athletes lost 5.6 kg before a weigh-in and regained 7.4 kg in the 32 hours before they fought, and 43% of them had used a sauna to cut.3Source 3 · Case seriesExtreme Rapid Weight Loss and Rapid Weight Gain Observed in UK Mixed Martial Arts Athletes Preparing for CompetitionMatthews JJ, Nicholas C. International Journal of Sport Nutrition and Exercise Metabolism, 2017.Read the paper ↗PubMed ↗ In another study of fighters, a two-hour hot-bath protocol removed 2.17–2.24 kg, and after 24–26 hours of eating and drinking their performance tests were unaffected.4Source 4 · Clinical trialEffect of rapid weight loss incorporating hot salt water immersion on changes in body mass, blood markers, and indices of performance in male mixed martial arts athletesConnor J, Germaine M, Gibson C, et al.. European Journal of Applied Physiology, 2022.Read the paper ↗PubMed ↗

The randomized trial that measured fat found nothing extra from the heat. Forty-seven sedentary adults with at least one cardiovascular risk factor were randomized to three arms for eight weeks, and most of them were women.5Source 5 · Randomized trialEffects of regular sauna bathing in conjunction with exercise on cardiovascular function: a multi-arm, randomized controlled trialLee E, Kolunsarka I, Kostensalo J, et al.. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 2022.Read the paper ↗PubMed ↗ Exercise cut fat mass by 1.3 kg against the controls. Adding a 15-minute sauna after exercise changed fat mass by a further −0.6 kg (95% CI −1.9 to 0.7), which is no detectable difference.

Two other trials used much hotter rooms. When 23 young men took 12 sessions of five 10-minute rounds at 100°C over four weeks, fat mass in the sauna group went from 9.6 kg to 9.8 kg.6Source 6 · Clinical trialEffects of Twelve Sessions of High-Temperature Sauna Baths on Body Composition in Healthy Young MenToro V, Siquier-Coll J, Bartolomé I, et al.. International Journal of Environmental Research and Public Health, 2021.Read the paper ↗PubMed ↗ A trial in 36 semi-professional football players reported that weight and fat fell within the sauna group after nine sessions at 100°C, and its abstract reports no comparison with the control group's change.7Source 7 · Randomized trial3-Week passive acclimation to extreme environmental heat (100± 3 °C) in dry sauna increases physical and physiological performance among young semi-professional football playersBartolomé I, Siquier-Coll J, Pérez-Quintero M, et al.. Journal of Thermal Biology, 2021.Read the paper ↗PubMed ↗

Sweating off weight has killed athletes. In 1997 three previously healthy US college wrestlers died between November 7 and December 9 while trying to shed weight by exercising hard in hot conditions in vapor-impermeable suits, and one had a rectal temperature of 42°C at death.8Source 8 · Case seriesHyperthermia and dehydration-related deaths associated with intentional rapid weight loss in three collegiate wrestlers--North Carolina, Wisconsin, and Michigan, November-December 1997Centers for Disease Control and Prevention (CDC). MMWR. Morbidity and mortality weekly report, 1998.Read the paper ↗PubMed ↗ The NCAA then banned saunas, steam rooms, hot rooms above 26°C and vapor-impermeable suits for making weight.