If you are going to exercise for more than an hour, you can increase your endurance by eating every 15 to 20 minutes. A feeling of general tiredness during exercise is usually caused by low levels of stored sugar in your liver, while a feeling of muscle fatigue is usually caused by low levels of stored muscle sugar.

Your brain gets more than 98 percent of its energy from sugar flowing to it in the bloodstream, but there is only enough sugar in your bloodstream to last for three minutes. Your liver must constantly release sugar from its cells into your bloodstream. However, there is only enough energy in your liver to last about 12 hours without replenishment.

Your muscles get their energy from many sources: sugar and fat stored in them, and sugar, fat or protein in the bloodstream. When your muscles run out of their stored sugar supply, they can hurt and feel tired. You can increase endurance by eating frequently, and you can eat whatever you like: nuts, fresh or dried fruits, chicken, sandwiches or anything else. The amount of sugar you get in sports drinks is not adequate to support strenuous exercise for long periods of time.

You will also need to replace fluid and salt during prolonged exercise. Fruit juice and salted potato chips, soda and salted peanuts, or any other combination of salted foods and beverages you like can be used to meet your needs for fluid, salt and calories.