Physical exertion requires a large amount of energy, which means that athletes need to consume more calories than the average person. The fundamental purpose of an athlete's diet does not consider weight loss, so athletes must consume as many calories as they burn. In fact, the athlete's intent might be to gain weight if muscle tissue needs to be added.
Calories are a measure of the energy within food. In technical terms, a calorie is the amount of energy needed to raise a single gram of water 1 degree Celsius. Energy is needed by the cells to sustain repeated muscle contractions and to facilitate athletic performance.
The amount of calories needed to sustain an athlete depends on the athlete's base metabolic rate and the sum total of activities completed in a single day. According to exercise physiologists William McArdle and Frank Katch, the average daily calorie expenditure for a man is between 2,700 and 2,900 calories; for a woman it's 2,000 to 2,100 calories. An athlete would need to add to this total all the additional calories expended from workouts. For example, a 160-lb. individual running at 8 mph would burn 986 calories in an hour. This amount needs to be matched in the diet. The body also requires 500 calories a day just to add muscles.
Each individual is different based on personal attributes and energy expenditure, so caloric intake should be highly personalized. An athlete, however, can easily consume at least 3,000 to 4,000 a day; sometimes it can be quite a bit more. Olympic gold medalist swimmer Michael Phelps, for example, is known to consume 12,000 calories a day. Though this total is 9,500 more than the Food and Drug Administration recommends for a young active male, it is needed to compensate for all of the calories burned during the 30 hours per week that Phelps spends in training.
In addition to total calories, it's important to consider calories derived from individual nutrients. Carbohydrates work best for athletes because they are the quickest fuel to burn. This is due to the fact that carbohydrates are easily used as energy by the cells. However, carbohydrates aren't stored very easily. The only form of carbohydrates that can be stored by the body are molecules known as glycogen which are made up of branched glucose units. Once glycogen and other carbohydrates are used up, the body turns to proteins and fats for energy. Both nutrients are slightly more inefficient due to the fact that it takes more time and energy to convert them into a usable form of energy.
Due to the speed at which they burn, carbohydrates should constitute most of your caloric intake, especially for endurance sports. A strategy known as carbohydrate loading increases the amount of carbohydrate to 70 percent of total calories three days prior to an event. Based on a 3,000-calorie diet, this is over 500 g a day of carbohydrates in that span. However, some degree of fat is always used to provide fuel for the body. For moderate exercise, about half the total energy expenditure is derived from free fatty acid metabolism. Once an event lasts more than an hour, the body may use mostly fats for energy. It's important to keep carbohydrate intake high so that it is metabolized for energy instead of fat.