This is particularly important on humid days when sweat doesn't evaporate as readily or cool the body as efficiently. Men always sweat profusely, Kenney says. Women are more efficient'' sweaters, he says, especially when it's hot and humid. On any given day, a human being will absorb through food and drink the same amount of fluid he loses through sweating and urination. Under stress from heat or exertion, the balance changes somewhat.
Much more fluid is lost through sweat than through urination.
Sometimes key salts are lost as well. Klesges found that the players were sweating out precious ounces of calcium. In a 1996 issue of the Journal of the American Medical Association , he urged that all elite athletes begin taking calcium supplements. To keep the sweat flowing, the system needs a constant supply of blood, its source of cooling water. On a very hot day, one quarter to one third of the body's blood supply may be pumped through the skin, research has shown.