“Sauna improves your blood sugar control.”

The research found this

Only one trial has tested a sauna against a glucose outcome. Twelve people with type 2 diabetes sat in random order for 40 minutes in an infrared cabin at 60°C or at 21°C before 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 at 40°C before or during a glucose test left glucose unchanged, and an hour holding core temperature at 38.5°C or more did not change insulin sensitivity.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 ↗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 case series without a control group described lower fasting glucose and HbA1c after three weeks of daily hot baths in type 2 diabetes.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 who took 30 one-hour hot-tub sessions over 8 to 10 weeks saw fasting glucose fall from 105 to 89 mg/dL with no change in BMI, and matched controls did not 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 ↗ Overweight men who took ten one-hour baths at 39°C in two weeks lowered fasting glucose 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 meta-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 ↗ A 2025 pooled analysis of 20 randomized trials of passive heating 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 ↗