“Sauna improves your blood sugar control.”

The research found this

Only one trial has measured blood sugar after a sauna. Twelve people with type 2 diabetes sat for 40 minutes in an infrared cabin, once at 60°C and once at 21°C in random order, and then took a two-hour glucose test.1Source 1 · Crossover trialA Single Sauna Session Does Not Improve Postprandial Blood Glucose Handling in Individuals with Type 2 Diabetes Mellitus: A Cross-Over, Randomized, Controlled TrialSchenaarts L, Hendriks FK, Fuchs CJ, et al.. Experimental and Clinical Endocrinology & Diabetes, 2024.Read the paper ↗PubMed ↗ Glucose rose more after the heat (17.7 against 14.8 mmol/L over 120 minutes), and insulin sensitivity did not differ.

Hot water in the moment does something similar. Healthy men who took a glucose test while sitting in 40.3°C water had higher venous glucose than in 35.9°C water, and the authors read that as muscle taking up less glucose during heating.2Source 2 · Crossover trialHot water immersion acutely reduces peripheral glucose uptake in young healthy males: An exploratory crossover randomized controlled trialMaley MJ, Hunt AP, Stewart IB, et al.. Temperature, 2023.Read the paper ↗PubMed ↗ In people with diabetes, an hour in 40°C water before or during a glucose test left glucose unchanged.3Source 3 · Crossover trialTiming of acute passive heating on glucose tolerance and blood pressure in people with type 2 diabetes: a randomized, balanced crossover, control trialJames TJ, Corbett J, Cummings M, et al.. Journal of Applied Physiology, 2021.Read the paper ↗PubMed ↗ Holding core temperature at 38.5°C or more for an hour didn't shift insulin sensitivity either.4Source 4 · Crossover trialAcute effect of passive heat exposure on markers of cardiometabolic function in adults with type 2 diabetes mellitusBehzadi P, Ravanelli N, Gravel H, et al.. Journal of Applied Physiology, 2022.Read the paper ↗PubMed ↗

Repeated hot baths look better. A 1999 report without a control group described lower fasting glucose and HbA1c in people with type 2 diabetes after three weeks of daily hot baths.5Source 5 · Case seriesHot-Tub Therapy for Type 2 Diabetes MellitusHooper PL. New England Journal of Medicine, 1999.Read the paper ↗PubMed ↗6Source 6 · Clinical trialPassive exposure to heat improves glucose metabolism in overweight humansPallubinsky H, Phielix E, Dautzenberg B, et al.. Acta Physiologica, 2020.Read the paper ↗PubMed ↗ Obese women with polycystic ovary syndrome took 30 one-hour hot-tub sessions over 8 to 10 weeks. Their fasting glucose fell from 105 to 89 mg/dL with no change in BMI, and matched controls didn't change.7Source 7 · Clinical trialHeat therapy improves glucose tolerance and adipose tissue insulin signaling in polycystic ovary syndromeEly BR, Clayton ZS, McCurdy CE, et al.. American Journal of Physiology-Endocrinology and Metabolism, 2019.Read the paper ↗PubMed ↗ In overweight men, ten one-hour baths at 39°C over two weeks brought fasting glucose down from 4.44 to 3.98 mmol/L.8Source 8 · Crossover trialAcute and chronic effects of hot water immersion on inflammation and metabolism in sedentary, overweight adultsHoekstra SP, Bishop NC, Faulkner SH, et al.. Journal of Applied Physiology, 2018.Read the paper ↗PubMed ↗

In a heat chamber, insulin sensitivity improved less than it did with exercise. Six weeks of 40°C chamber sessions three times a week raised whole-body insulin sensitivity 15% in sedentary men, against 36% with cycling.9Source 9 · Randomized trialPassive heat therapy in sedentary humans increases skeletal muscle capillarization and eNOS content but not mitochondrial density or GLUT4 contentHesketh K, Shepherd SO, Strauss JA, et al.. American Journal of Physiology-Heart and Circulatory Physiology, 2019.Read the paper ↗PubMed ↗ A 2021 pooled analysis of five studies reported falls in HbA1c and fasting glucose that were not statistically significant, although its title announced a benefit.10Source 10 · Meta-analysisHeat therapy shows benefit in patients with type 2 diabetes mellitus: a systematic review and meta-analysisSebők J, Édel Z, Váncsa S, et al.. International Journal of Hyperthermia, 2021.Read the paper ↗PubMed ↗ In 2025, a pooled analysis of 20 randomized trials of heating without exercise found no effect on either measure.11Source 11 · 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 ↗