Many ultra-endurance events such as the Western States 100-Mile Run require that participants weigh in several times throughout the event to ensure they haven't lost too much weight from sweating. Although 100-mile runners are usually a skinny lot, this is due to the exercise, not their massive sweat losses. Experts agree that simply sweating isn't effective for long-term weight loss.
Function of Sweat
Sweat's primary function is to cool the body. As skin temperature increases, your pores begin to excrete sweat. As the water in sweat evaporates, it cools the skin. Sweat rates increase as temperature and exertion increase. Men can lose up to 5.25 pounds and women 2.95 pounds in an hour due to sweat losses. However, because that weight is all water, you will gain it right back once you hit the (water) bottle again. In fact, water is necessary to burn fat, so those interested in sustained weight loss should stay well hydrated.
Saunas
Some sources say saunas and sauna suits help burn fat through excessive sweating. Because heat raises your heart rate, they claim you can burn as many calories per hour sitting in a sauna as going for a walk. Some people believe that heavy metals stored in fat also impede weight loss, and that these toxins are released through sweating. However, as Dr. Donald Smith, a professor of environmental toxicology at the University of California-Santa Cruz explained to the L.A. Times, 99 percent of toxic metals are excreted through urine and feces, not sweat. If someone is claiming that saunas or rubber suits help lose weight, they may be selling saunas or rubber suits.
Making Weight
Many athletes such as jockeys, boxers and wrestlers use sweating as a way to "make weight" before a competition. Pre-competition weigh-ins usually take place the day before the event, giving athletes time to replace fluid losses before competition. Bodybuilders also shed excess water before competitions to enhance muscle definition. Excess sweating gives bodybuilders a performance trifecta: It helps cut weight, sheds excess sodium that causes water retention and sheds the subcutaneous water that makes skin supple, but masks muscle definition.
Athletic Performance
Losing just 2 percent of your body weight from dehydration can decrease endurance performance by up to 20 percent. At 3 to 5 percent weight loss comes grogginess, headaches, nausea and reduced reaction time. Once an athlete loses 10 percent of her body weight due to dehydration, muscles spasm, vision blurs and delirium sets in. Usually, 15 percent weight loss from dehydration is fatal.
Warnings
Water is necessary for all of the body's functions, including digestion and nerve function. As fluid is lost through sweating, blood becomes thicker and harder for the heart to pump, increasing the risk of cardiovascular emergencies. As the severity of dehydration increases, sweating ceases, causing the body to overheat (heatstroke). Extreme electrolyte depletion through sweat losses can lead to kidney damage or failure. Complications from dehydration can cause coma and death.
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