The sauna is a magnet for overweight people who want to lose weight. While sweating out buckets of water causes a temporary drop in weight, saunas do nothing for long term weight prospects. A sauna, or any hot environment, forces the body to sweat for temperature control. You're not sweating out any excess body fat because that weight loss solution would be too simple. When has the simple way, with little or no work required, ever produced long term, quality results? (Hint: never).
Temporary vs. Permanent Weight Change
Body weight is made up of many parts, but the factors we're most concerned with for health and physical appearance are: muscle mass and fat stores. There are other factors that temporarily influence weight but have no lasting effects on health or physical appearance, specifically, water weight and meal size.
The body stores fat when you consume too much relative to the amount you burn. This excess body fat is what makes you unhealthy and unsightly. Excess body fat increases the risk of many obesity related diseases such as as heart disease, strokes, type 2 diabetes and cancer. Excess body fat also obscures muscle definition making you look out of shape and overweight.
Excess body fat is the enemy. Those temporary factors, especially water weight, don't cause health problems and don't negatively impact physical appearance in the same way excess body fat does. If body fat is the enemy, it makes little sense to temporarily get rid of water.
The sauna is a hot environment causing sweat. Over the period of 15-20 minutes, you can probably sweat out a good deal of weight. When you get out of the sauna and weigh yourself, you'll be pleasantly surprised to see a sizable decrease in weight. Unfortunately, this decrease is short lived.
Those temporary weight change factors are just that, temporary. You may be able to lose five pounds in 15-20 minutes, but the body is pretty good at regulating water weight. When you sweat, the body begins to hold onto more water. Over the next few hours, the body will compensate for all the "weight" you "lost" by holding onto an equal amount of water. A sauna has no permanent effect on body weight.
Sweating: Cause and Effect
Some are under the mistaken belief that sweating burns calories. Exercise burns calories and causes weight loss. In addition to burning calories, exercise also makes you sweat. If exercise makes you sweat and burns calories, sitting in a sauna and sweating should do the same, right?
This is mixing up cause and effect. Sweating is an effect of an increase in body temperature while moving burns calories. Sweating does not burn calories if all you're doing is sitting in a hot environment. Exercise causes sweating because when you move, body temperature increases. There are ways to cause sweating without moving; those methods are ineffective at burning energy and causing true weight loss. An increase in body temperature without exercise (sitting in a hot room) causes sweating without burning energy.
Shortcuts Don't Work
Sitting in a hot room trying to sweat your way to health is a shortcut, and a very ineffective one. Sitting around makes you fat. It makes no difference whether you sit in a cold, comfortable or hot room; the effects of physical inactivity combined with an excessive calorie intake are bad. The 15-20 minutes you spend (or waste) in the sauna each day are much better spent on the treadmill, lifting weights, climbing stairs, counting calories or eating a salad.
The Bottom Line
When in life does the option allowing you to sit in a sauna and do no hard work, lead to beneficial results? Great results come from hard work, pushing yourself to new limits and sacrificing the comforts that got you into trouble to begin with.