Contrary to popular belief, scientific research supports no direct weight-loss benefits either from dry heat saunas or steam baths. You cannot "melt off the fat" by sweating, but if used safely, saunas can help you lose weight in an indirect way: relieving the stress that makes healthy eating habits hard to maintain.
When you sweat, you don't lose fat: You lose fluid. In other words, you become dehydrated. The weight you lose from dehydration will come back as soon as you drink more fluids. Don't let your reluctance to put on weight keep you from rehydrating; dehydration isn't healthy. Excessive dehydration makes you feel drained and weak, and prolonged dehydration can be fatal. According to "Weight Issues In Wrestling" by Mike Viscardi, several college wrestlers died from attempts to lose weight via dehydration.
Sweating also carries a small amount of salt out of the body. Excess salt in the system promotes fluid retention, some of which ends up under your skin, obscuring the muscle definition that many consider attractive. Sloughing off some of that fluid can help you look more "cut," according to a CBS Moneywatch article by Matt Fitzgerald, but perspiration is not the best method to lower your sodium levels. In fact, sodium is one of the electrolytes your body needs to survive, and if you sweat out too much of it, your system goes into electrolyte imbalance. A better strategy is to lower the amount of sodium in your diet.
Provided that you are healthy, a short sauna stint can help your body relax. The heat helps to soothe and loosen sore, tight muscles, and a relaxed body promotes a relaxed mind. So, even though your excess fat won't melt away, your excess stress will.
Temporarily Lowered Blood Pressure
According to Columbia University's Health Q&A column "Go Ask Alice!," the high temperature of a sauna causes lowered pulse and blood pressure by dilating your blood vessels. If you don't suffer from abnormal blood pressure, heart disease or any other cardiac problems, this can be another safe option to relieve your stress.
The stress relief you experience in a sauna can help prevent your putting on excess weight. Stress is bad for maintaining healthy eating habits, since it can lead to "stress eating," the practice of eating in response to negative emotions. So while a sauna may not help you take off weight, it can cut the stress that would encourage you to put it on.
Drink water before and after your sauna session to prevent dehydration. Don't mix saunas with alcohol or other drugs, as these can interfere with your body's ability to handle periods of high heat and lowered blood pressure. Limit your time in a sauna to 20 minutes or less, at least for the first few times. Increase your time gradually over several sessions -- don't attempt a marathon sauna sit-in too suddenly.
If you are pregnant, suffer from heart problems, have high or very low blood pressure, or are over 65, you should limit your time in saunas or avoid saunas. Do not allow children to use saunas without supervision, as they are more susceptible to hyperthermia.
Allow your body to come back to a normal temperature slowly. Going straight from a sauna to a cold shower puts unnecessary stress on your heart.
Get out of the sauna the moment you notice feelings of nausea, dizziness, chills, headache or other uncomfortable symptoms. These signs are how your body warns you that you're overdoing it. Ignoring them can be fatal.