Fasting Does Not Increase Endurance

Fasting before a big event will not improve endurance.

Fasting became popular because of a study that showed rats can run further after fasting than after eating. But rats are different from people. In rats, fasting increased the rate that a rat's muscles use fat, to preserve stored muscle sugar. In humans, fasting does not cause muscles to burn more fat. After fasting, human muscles continue to burn primarily their own sugar. Fasting for 24 hours uses up the same amount of muscle sugar as running for 90 minutes.

How long you can exercise a muscle depends on how much sugar, called glycogen, you can store and how long you can keep glycogen in that muscle. When a muscle runs out of its stored glycogen, it hurts and you will have difficulty coordinating it. Every time that you move a muscle, some of the stored glycogen is used up. Every time that you eat, some of the food can be stored as glycogen in that muscle. When you go for a long time without eating, you use up glycogen without replacing it. If you fast before a race, you will start that race with reduced stores of glycogen in your muscles and you will not be able to compete at your best.

It is nonsensical to claim that fasting increases endurance by causing muscles to burn more fat and less glycogen so that muscles can retain their stored glycogen longer. When you start with less glycogen, you still use it up faster and run out of fuel earlier. You can increase your endurance by markedly cutting back on exercise in the days before your competition and eating as much or more than usual. If the competition lasts more than two hours, you should also eat and drink during the event.

Checked 9/29/08

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