Some people believe they can improve their performance if they fast before a competition. An article in the Journal of Applied Physiology explains why this is ridiculous. 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. So, 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 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 endurance by markedly cutting back on exercise four days before your competition and eating as much or more than usual. During the competition, eat and drink as often as is comfortable.
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