The energy you have stored in your muscles is called glycogen. Glycogen stores usually last about 1 1/2 to 2 1/2 hours for moderate intensity exercise. Once your glycogen stores are low your muscles rely on blood sugar for energy. The blood sugar needed would come from carbohydrates eaten before and during exercise. If glycogen stores are empty and the person does not refuel with carbohydrates, extreme muscle and mental fatigue will set in. A marathoner may describe it as "hitting the wall."

When choosing a sports drink, pick one that has 6 to 8 grams of carbohydrates per 100 mL of the drink. You could also make your own carbohydrate drink by diluting one part fruit juice with one part water. When drinking during exercise, juice must be diluted because it is too concentrated to be absorbed quickly on its own.

Generally, most exercisers do not need to worry about sodium or potassium replenishment during exercise. Any electrolytes you lose through sweat can be sufficiently replaced with your post workout meal.
Athletes who exercise for more than four hours in extreme heat are at risk of developing medical problems from losing too much sodium (by this we mean long distance cyclists, soccer or tennis players in tournaments, ultra-endurance runners). They should eat salty foods before, during and after their workout. During exercise, they would need 250 to 500 mg of sodium per hour, the amount in 20 to 40 oz. of Gatorade. These athletes should not rely on juice for sodium replenishment, as it is low in sodium. Salty foods that help with maintaining sodium balance include pretzels, tomato juice, soup, and salted crackers.

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