Athletes need a varying amount of energy depending of their size, body composition, and the type of training being performed. A person who is very small may need 1,500 calories to maintain her body weight, whereas a large person with a great deal of muscle mass may need 4,000 calories to maintain his body weight.

Each individual should start with a rough estimate of what's appropriate for him or her, and calories can be reduced or added from there.

Anyone who exercises vigorously, especially for more than one hour per day on a regular basis, needs to consume moderate to high amounts of carbs. This is considered 50 percent or more of your daily calorie intake. This is why diets like the Zone, which recommends 40 percent of calories coming from carbohydrates, are not ideal for athletes or people who have intense training sessions.

Individuals who engage in sixty minutes of exercise per day may need as much as 6 to 10 grams of carbohydrates per kilogram of body weight per day. Athletes who are training for a marathon, triathlon, or those who perform multiple training sessions per day can eat up to 500 grams of carbohydrates per day. They need to consume this amount in order to prevent chronic fatigue and to load the muscles and liver with glycogen (which is the stored form of glucose).

Protein intake for the athlete is 1.2–1.4 grams of protein per kilogram of body weight. It is important that the energy needs are met, meaning the appropriate amount of carbohydrates need to be available. Otherwise, much of this protein will be converted to a fuel source instead of being used for maintaining muscle tissue.

For those beginning a strength-training program in which the goal is to put on large amount of muscle tissue, experts may recommend up to 1.7–2 grams of protein per kilogram of body weight. This is up to two and a half times the recommended amount. Keep in mind that the body can only process a certain amount of protein for muscular growth. Any proteins above the amount the body can use to increase muscle protein synthesis will be used for energy needs.

An athlete can eat up to 30 percent of his or her daily intake from fats and oils. These fats should be rich in unsaturated fats and limited in saturated and trans fats.

The vitamin and mineral needs of the athlete are slightly higher than that of a sedentary person. Because athletes eat so much food, they tend to get plenty of vitamins and minerals in their diets. (Unless an athlete is trying to lose weight and is on a restricted calorie diet.) Some experts recommend taking a multivitamin with antioxidants due to the fact that muscle damage is occurring during exercise and many vitamins and minerals can be lost through exertion and heavy sweating.