“Sauna makes your arteries more flexible.”

The research found this

The claim traces to a Kuopio experiment in which 102 adults with at least one heart risk factor sat in a 73°C sauna for 30 minutes. The researchers measured pulse wave velocity. It rises as the big arteries stiffen, so a lower number means less stiff vessels. That reading fell from 9.8 to 8.6 meters a second right after the sauna and stood at 9.0 after half an hour of rest.1Source 1 · Clinical trialSauna exposure leads to improved arterial compliance: Findings from a non-randomised experimental studyLee E, Laukkanen T, Kunutsor SK, et al.. European Journal of Preventive Cardiology, 2018.Read the paper ↗PubMed ↗ The experiment had no control condition.2Source 2 · Clinical trialAcute effects of sauna bathing on cardiovascular functionLaukkanen T, Kunutsor SK, Zaccardi F, et al.. Journal of Human Hypertension, 2018.Read the paper ↗PubMed ↗

Another measure of blood vessel health didn't budge. Flow-mediated dilation tests how well an arm artery widens when blood flow rises. In a randomized crossover study, 21 healthy adults in their sixties each tried both one and two 10-minute bouts at 80°C, and neither improved their flow-mediated dilation.3Source 3 · Crossover trialAcute effect of Finnish sauna bathing on brachial artery flow‐mediated dilation and reactive hyperemia in healthy middle‐aged and older adultsGravel H, Coombs GB, Behzadi P, et al.. Physiological Reports, 2019.Read the paper ↗PubMed ↗

The biggest reported change came from hot water in Oregon. Ten sedentary young adults soaked in 40.5°C water four or five times a week for eight weeks, and each time they held their core temperature at 38.5°C or higher for an hour. Pulse wave velocity in their aorta (the body's main artery) fell from 7.1 to 6.1 meters a second, and their flow-mediated dilation nearly doubled. A comparison group in 36°C water showed no change.4Source 4 · Clinical trialPassive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humansBrunt VE, Howard MJ, Francisco MA, et al.. The Journal of Physiology, 2016.Read the paper ↗PubMed ↗ There was no randomization. The investigators sorted people into the groups by matching them on sex, age, body mass index and time of year.

Randomized trials at more ordinary doses found no such gains. Forty-one adults with stable coronary artery disease used a 79°C sauna four times a week for eight weeks. Neither flow-mediated dilation nor pulse wave velocity changed compared with a control group, even though their bodies had adapted to the heat.5Source 5 · Randomized trialFinnish sauna bathing and vascular health of adults with coronary artery disease: a randomized controlled trialDebray A, Gravel H, Garceau L, et al.. Journal of Applied Physiology, 2023.Read the paper ↗PubMed ↗ When researchers pooled 20 randomized trials of heating people at rest, they found no effect on either measure. A separate analysis of 51 controlled studies found artery stiffness unchanged.6Source 6 · Meta-analysisNon-acute effects of passive heating interventions on cardiometabolic risk and vascular health: systematic review and meta-analysis of randomized controlled trialsHamaya R, Joyama Y, Miyata T, et al.. American Journal of Preventive Cardiology, 2025.Read the paper ↗PubMed ↗7Source 7 · Meta-analysisHeat thermotherapy to improve cardiovascular function and cardiometabolic health: A systematic review and meta‐analysisPrice BS, Lucas SJE, Akerman AP, et al.. Experimental Physiology, 2025.Read the paper ↗PubMed ↗

A few results point the other way. An earlier pooled analysis found a 1.95-point gain in flow-mediated dilation across five studies, and its authors rated the certainty low.8Source 8 · Meta-analysisThe effect of heat therapy on blood pressure and peripheral vascular function: A systematic review and meta‐analysisPizzey FK, Smith EC, Ruediger SL, et al.. Experimental Physiology, 2021.Read the paper ↗PubMed ↗ In Japan, people who took hot baths at least five times a week had less stiff arteries than people who bathed less. That was a survey, so it can't say which came first.9Source 9 · Cross-sectional studyHabitual hot water bathing protects cardiovascular function in middle-aged to elderly Japanese subjectsKohara K, Tabara Y, Ochi M, et al.. Scientific Reports, 2018.Read the paper ↗PubMed ↗