Exercisers spend millions of dollars on special sports drinks, even though none increase endurance more than the food from which they are made.

You become tired during exercise because you run low on fluids, salt and calories. As long as you replace these three components, you do not need to pay extra for a sport drink.

If you are going to exercise for more than 25 minutes, you can increase your endurance by drinking fluids. If you are going to exercise for more than 45 minutes, you can increase your endurance with sugared drinks, which provide a quick source of calories. Sugared drinks such as fruit juices, soft drinks and sports drinks can be absorbed just as rapidly as water.

The potassium listed as an ingredient in some sports drinks is irrelevant since you will not become potassium deficient from exercise, and you get plenty of potassium in virtually all foods. The only mmineral you need to replace during exercise is salt.

When you exercise, you lose water through sweat. Sweat contains much less salt than blood does, so you lose far more water than salt, which causes blood levels of salt to rise. You have to lose more than two pints of water for the salt concentration in your blood to rise high enough to make you feel thirsty. By the time you feel thirst, it is too late to catch up on your fluid loss and you will have to stop exercising. By then you are dehydrated and you may become nauseous, get muscle cramps, or feel dizzy. If you ignore the warning signs of dehydration, you can convulse and pass out.

Eating salt stimulates you to drink, and raises your blood salt level high enough to make you feel thirsty and able to retain fluid. Some sports drinks contain salt, but most people don't like the taste of a salty drink, so the salt content is usually too low to meet your needs for salt during heavy exercise.

Previous studies showing that temperature or carbonation affect absorption have been not been supported by more recent research. Your drink can be chilled or warm, as you prefer. If you prefer the taste of a sports drink over other beverages, use it. If your favorite beverage is a cola, iced tea, lemonade or plain water, that's what you should drink when you exercise. Research overwhelmingly shows that you will drink the most of the fluid you like best.

Since your drink won't supply enough salt to meet your needs when you exercise for several hours, you'll also need to eat salted peanuts, potato chips or anything else with salt that tastes good to you. For calories, it doesn't make much difference what you eat as long as it doesn't remain too long in your stomach and cause intestinal discomfort.

When you're not exercising, don't get in the habit of using sports drinks or any other sugared drinks to quench thirst. They'll add up to a lot of calories with little other nutritional value. Use plain water or other calorie-free beverages instead.

Dr. Gabe Mirkin has been a radio talk show host for 25 years and practicing physician for more than 40 years; he is board certified in four specialties, including sports medicine. Read or listen to hundreds of his fitness and health reports at http://www.DrMirkin.com

Free weekly newsletter on fitness, health and nutrition