Does the sauna help with weight loss?
The sauna makes you sweat, not slim: the weight you lose is fluid and returns as soon as you drink. Whether regular sauna use actually causes lasting fat loss has not yet been well demonstrated, and for now there is more evidence that it does not work than that it does.
Every sauna session causes weight loss, but this is almost always fluid loss through sweating. After two sessions of ten minutes each, you already lose a noticeable amount of weight, and the higher your body weight, the more fluid you shed. As soon as you drink again, it comes right back. That does not count as losing weight.
Real fat burning is a different matter. In a small study of 23 healthy young men who completed twelve sauna sessions at 100°C, fat mass did not decrease at all. There was actually a slight increase in muscle mass and bone density. For that group, the sauna therefore did not help to lose fat.
There are, however, small indications that long-term, regular sauna use may do something to body fat and blood sugar in people with significant excess weight or type 2 diabetes. A review article describes improvements in fasting blood sugar and fat mass with an approach of three times a week, fifteen minutes at a time, over three months. A separate small study in obese patients also found a decrease in weight and body fat after two weeks of repeated sauna use, without a rise in the hunger hormone. However, this research is still too small and too short in duration to draw firm conclusions from it.
For athletes, an additional warning applies. Sauna is sometimes used to lose weight quickly before a competition. Athletes lost an average of 1.4 to 1.8 percent of their body weight after the sauna, but this was not fully recovered after one hour of rehydration. In women, jump performance declined measurably. This makes the approach risky.
Light exercise in the sauna appears to mobilise slightly more fat than sitting still, but the measured amount was so small that it is of little practical significance. In addition, concerning changes in heart rate and blood pressure during sauna use have been measured in the heaviest participants. If you have severe excess weight or cardiovascular complaints, it is wise to discuss this with your doctor first.
This answer is based on several small studies in humans, a review article, and additional animal research. None of the studies was large enough or long enough to properly measure the effect on fat over the long term. The studies on fat mass loss are small (sometimes fewer than 25 participants), short in duration, and measure a variety of outcomes. The evidence on weight loss through sweating is clearer and also causal; the evidence on lasting fat loss is thin and contradictory.
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