Athletes sweat. And the hotter the day, the more intense and longer lasting the activity, the more they sweat. Sweat loss during strenuous exercise on a hot day can amount to as much as 2 L/h, and the total sweat loss during a 24 h endurance run can exceed 16 L, or approximately 35 lb.

The composition of sweat is highly variable, not only with the individual but also with the time during the exercise and the athlete`s overall conditioning. Typically, however, the Na+ ion concentration in sweat is about 30-40 mEq/L, and that of K+ ion is about 5-10 mEq/L. In addition, there are small amounts of other metal ions, such as Mg2+, and there are sufficient CP ions (35-50 mEq/L) to balance the positive charge of all these cations.

Obviously, all the water and dissolved electrolytes lost by an athlete through sweating must be replaced. Otherwise, dehydration, hyperthermia and heat stroke, dizziness, nausea, muscle cramps, impaired kidney function, and other difficulties ensue. As a rule of thumb, a sweat loss equal to 5% of body weight—about 3.5 L for a 150 lb person—is the maximum amount that can be safely allowed for a well-conditioned athlete.

Plain water works perfectly well to replace sweat lost during short bouts of activity up to a few hours in length, but
Endurance athletes avoid serious medical problems by drink--5 sports drinks to replace water and electrolytes.
a carbohydrate-electrolyte beverage, or "sports drink," is much superior for rehydrating during and after longer activity in which substantial amounts of electrolytes have been lost. Some of the better known sports drinks are little more than overpriced sugar-water solutions, but others are carefully formulated and highly effective for fluid replacement. Nutritional research has shown that a serious sports drink should meet the following criteria. There are several dry-powder mixes on the market to choose from.

•    The drink should contain 6-8% of soluble complex carbohydrates (about 15 g per 8 oz serving) and only a small amount of simple sugar for taste. The complex carbohydrates, which usually go by the name "maltodextrin," provide a slow release of glucose into the bloodstream. Not only does the glucose provide a steady source of energy, it also enhances the absorption of water from the stomach.

•    The drink should contain electrolytes to replenish those lost in sweat. Concentrations of approximately 20 mEq/L for Na+ ions, 10 mEq/L for K+ ion, and 4 mEq/L for Mg2+ ions are recommended. These amounts correspond to about 100 mg sodium, 100 mg potassium, and 25 mg magnesium per 8 oz serving.

•    The drink should be noncarbonated because carbona-tion can cause gastrointestinal upset during exercise, and it should not contain caffeine, which acts as a diuretic.

•    The drink should taste good so the athlete will want to drink it. Thirst is a poor indicator of fluid requirements, and most people will drink less than needed unless a beverage is flavored.

In addition to complex carbohydrates, electrolytes, and flavorings, some sports drinks also contain vitamin A (as beta-carotene), vitamin C (ascorbic acid), and selenium, which act as antioxidants to protect cells from damage. Some drinks also contain the amino acid glutamine, which appears to lessen lactic acid buildup in muscles and thus helps muscles bounce back more quickly after an intense workout.