When you are training or competing, your muscles need energy to perform. One source of energy for working muscles is glycogen which is made from carbohydrates and stored in your muscles. Every time you work out, you use some of your glycogen. If you don't consume enough carbohydrates, your glycogen stores become depleted, which can result in fatigue. Both sugars and starches are effective in replenishing glycogen stores.
Performance depends largely on the foods consumed during the days and weeks leading up to an event.
If you regularly eat a varied, carbohydrate-rich diet you are in good standing and probably have ample glycogen stores to fuel activity. The purpose of the pre-competition meal is to prevent hunger and to provide the water and additional energy the athlete will need during competition. Most athletes eat 2 to 4 hours before their event. However, some athletes perform their best if they eat a small amount 30 minutes before competing, while others eat nothing for 6 hours beforehand. For many athletes, carbohydrate-rich foods serve as the basis of the meal.