Some people think (incorrectly) that fasting before a race or competition will increase their endurance. Fasting weakens and tires you. How long you can exercise a muscle depends on how much sugar, called glycogen, you can store in that muscle and how long you can keep it there. When a muscle runs out of its stored glycogen, it slows down because it requires more oxygen to burn more fat. It also accumulates more lactic acid to become acidic which causes a burning pain, and it becomes more difficult to coordinate.
Every time you move a muscle, some of the stored glycogen is used up. Every time you eat, some of the food can be stored as glycogen in that muscle. When you go for more than an hour without eating, you use up glycogen without replacing it. If you don't eat before you compete, you start 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 cutting back on exercise four days before your competition and eating as much or more than usual. Eat one to three hours before competing. If your event lasts more than an hour, take fluid, sugar and protein (whole grain bars, sugared drinks, etc.) during your event.