“A sauna after training boosts your endurance.”

The research found this

The claim traces to a 2007 study of six competitive male distance runners. For three weeks they added a humid sauna at 89.9°C after training, and they went 12.7 times on average for about 31 minutes each. For another three weeks they trained without it.1Source 1 · Crossover trialEffect of post-exercise sauna bathing on the endurance performance of competitive male runnersScoon GS, Hopkins WG, Mayhew S, et al.. Journal of Science and Medicine in Sport, 2007.Read the paper ↗PubMed ↗ After the sauna weeks they ran 32% longer to exhaustion at their 5-km race pace, and plasma volume was 7.1% higher. The authors equated the gain to about 1.9% in a time trial.

Other small studies found the same blood and heat changes. Seven well-trained cyclists who sat 30 minutes at 87°C after training expanded plasma volume by 17.8% after four sessions.2Source 2 · Clinical trialEffect of sauna-based heat acclimation on plasma volume and heart rate variabilityStanley J, Halliday A, D’Auria S, et al.. European Journal of Applied Physiology, 2015.Read the paper ↗PubMed ↗ That study had no control group. Twenty middle-distance runners who picked their own groups did three weeks of post-exercise saunas at 101–108°C, and the sauna group handled a heat test better and gained 0.27 L/min more in VO2max than the control group.3Source 3 · Clinical trialIntermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runnersKirby NV, Lucas SJE, Armstrong OJ, et al.. European Journal of Applied Physiology, 2021.Read the paper ↗PubMed ↗

The randomized crossover that measured hemoglobin mass found nothing extra. Thirteen men cycled three times a week for two four-week blocks. In one block each ride ended with 30 minutes at 89°C and no fluids.4Source 4 · Crossover trialHematological Adaptations to Post-Exercise Sauna Bathing With No Fluid Intake: A Randomized Cross-Over StudySitkowski D, Cisoń T, Szygula Z, et al.. Research Quarterly for Exercise and Sport, 2022.Read the paper ↗PubMed ↗ Hemoglobin mass and peak oxygen uptake rose in both blocks, and the sauna added nothing measurable.

Sauna does acclimate you to heat. In a military study, five days of post-exercise sauna at 70°C or hot baths at 40°C lowered exercising core temperature by 0.2°C and heart rate by 8 bpm. Performance in the heat rose 14%, and the confidence interval for that gain included no change.5Source 5 · Crossover trialPost-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military contextAshworth E, Cotter J, Kilding A. Ergonomics, 2023.Read the paper ↗PubMed ↗

In sedentary adults, eight weeks of exercise plus a 15-minute sauna raised estimated VO2max 2.7 mL/kg/min more than exercise alone.6Source 6 · 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 ↗ That trial estimated fitness from heart rate during a submaximal test, and heat acclimation lowers exercise heart rate by itself.5Source 5 · Crossover trialPost-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military contextAshworth E, Cotter J, Kilding A. Ergonomics, 2023.Read the paper ↗PubMed ↗