When you exercise, your body loses water through sweating and evaporation. Sweat is your body's coolant. During an intense workout, your muscles generate heat, which is carried by your blood through capillaries near the surface of your skin. Your sweat glands release perspiration that evaporates, cooling the skin and the blood underneath.

Sweating is therefore an essential mechanism for regulating body temperature. But when your body loses water, it limits the capacity of your blood to carry nutrients (such as glucose, fatty acids, and oxygen) to working muscles. Even mild dehydration (as little as 2 percent of your body weight) can impair athletic performance. The capacity of the blood to remove the by-products of metabolism, including carbon dioxide and lactic acid, is compromised as well.

How to Measure Your Body's Need for Fluids

Athletes must drink fluids to combat the sweat loss that naturally accompanies exercise. Check the color and quantity of your urine (it should be clear and copious). Weigh yourself before and after exercise. For every pound of of weight lost, drink two cups of water.

It's hard to over-hydrate before and during exercise. Drink one to two cups of water 5 to 15 minutes before your workout, and a minimum of three ounces every 20 minutes of exercise. During hot and humid conditions and during prolonged and/or intense exercise, even more water is required (drink up to 8 to 10 ounces every 20 minutes). After exercise, continue to drink when thirsty, plus more.

For exercise lasting less than 90 minutes, pure water is sufficient. For exercise lasting longer than 90 minutes, diluted juice or a sports drink will add beneficial electrolytes and help maintain blood sugar balance.

Training and competition in hot weather can cause problems for athletes, and everyone needs to be aware of the risks and preventative measures. Be aware of the importance of drinking fluids before and during exercise to prevent dehydration and heat- related illnesses which include muscle cramps, heat exhaustion, and heat stroke.

An athlete's temperature, normally about 98.6 degrees, may increase to 104 degrees or more during intense exercise. The circulatory system transports the heat generated by muscles to the skin to be dissipated. While a certain percentage of blood is used to regulate body temperature, large quantities of blood are still required to meet the energy and metabolic needs of working muscles. These demands may overtax the circulatory system, resulting in an inadequate removal of body heat and a corresponding rise in the athlete's body temperature.

Even in mild weather, you can run the risk of overheating. The threat becomes more severe when weather conditions are hot and humid. Sweat doesn't evaporate well in this sort of climate because the surrounding air is already saturated with water. If you continue to exercise in this state, you will increase your chances of suffering from heat exhaustion.

If you're preparing for competition, it's wise to drink extra fluids in the few days before you compete to ensure maximum hydration at the start of an event. You should also drink fluids before and frequently during a long event to keep your body temperature at safe levels.

Additionally, use common-sense strategies like progressively acclimating your body to warmer conditions, avoiding exercise in the hottest part of the day, and wearing skin and eye protection.

The carbohydrates we eat are stored as glycogen in the muscles and liver. This is the fuel which provides the energy our muscles need to exercise.

You will get better results from your competitions and workouts if you are properly fueled. The best fuel source is "complex" or "low glycemic" carbohydrates which include oatmeal, sweet potatoes, beans, brown rice, lentils, whole wheat bread and pasta, and vegetables.

You can greatly reduce your chances of "hitting the wall" if you follow these suggestions on what to eat:

For Exercise Lasting Less than 90 Minutes

Eat a small meal about 1-2 hours before exercising. This meal should include a serving of complex carbs and a large glass of water.

For Exercise Longer than 90 Minutes

The body's glycogen stores are sufficient to supply energy for competitions lasting up to 90 minutes. But for events lasting longer than this, extra consideration should be given to fueling the body. In the days leading up to the competition, meals should include foods rich in carbohydrates (it takes 48 hours for glycogen stores to be built up). You should also consume extra water during this time to hydrate your body.

About 1-2 hours before your competition, eat a small meal. This meal should include a serving of complex carbs and a large glass of water. Then during your event, you should drink diluted juice or a sports drink to add beneficial electrolytes and help maintain blood sugar balance.

After your workout or sports competition, you can help your body recover and rebuild with a meal consisting of the following:

Lean Protein
Examples include lean cuts of chicken or fish or turkey (grilled or baked without skin), canned tuna (packed in water), beans, lentils, veggie burgers, tofu, tempeh (soy cakes), low-fat cottage cheese and yogurt, skim milk, and egg whites.

"Sugary" or "High Glycemic" Carbohydrate
Examples include fresh fruit, fruit juice, yogurt, soy milk.

Water
Drink 1 large glass (and more after intense exercise or in hot weather).