“Sauna helps you lose weight.”

The research found this

The weight a sauna takes off is sweat. Among 674 first-time users, two 10-minute rounds at 90°C removed an average of 0.37 kg in women and 0.50 kg in men. Depending on BMI group, that came to 0.49–0.90% of body mass.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 the 12 people became more than 2% dehydrated.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 ↗

Fighters count on that water coming back. Seven mixed martial arts athletes lost 5.6 kg before a weigh-in, and 43% of them had used a sauna to cut. In the 32 hours before they fought, they regained 7.4 kg.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 ↗ Another study of fighters used a two-hour hot-bath routine that took off 2.17–2.24 kg. 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. Researchers randomly split 47 sedentary adults into three groups for eight weeks. Most were women, and each had at least one cardiovascular risk factor.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, and the plausible range (95% CI −1.9 to 0.7) included no change.

Two other trials used much hotter rooms. In one, 23 young men took 12 sessions over four weeks, and each session was five 10-minute rounds at 100°C. 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 ↗ The other put 36 semi-professional football players through nine sessions at 100°C. Weight and fat fell within the sauna group, and the 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. They were exercising hard in hot conditions in vapor-impermeable suits, which stop sweat from evaporating, 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.