This tables displays the location and approximate amounts of energy stored as ATP-CP, carbohydrate, fat, and protein in an average young adult male. The total amount of energy stored in the body varies greatly among individuals and is dependent on body size, body composition, physical fitness level, and diet. In the table each energy source is documented as to its gram weight and energy (calorie) value. Carbohydrate, protein, and fat yield 4, 4, and 9 calories per gram, respectively.
Carbohydrate is available as glucose in the blood, glycogen within the liver, and glycogen within muscle fibers. Note that total carbohydrate stores comprise about 2,000 calories. While a similar amount of calories (2,500) are stored in muscle as intramuscular triglycerides, the total caloric value of fat stores in this person is greater than 80,000 calories. Energy stored in adipose tissue is the most variable component of total body energy stores and ranges from approximately 50,000-100,000 calories in men and women with body compositions of 10-30% body fat. Fatty acids and triglycerides circulating in the blood represent the smallest component of fat stores.
While protein stores in muscle account for 30,000 calories, protein is infrequently used as an energy source. Although protein may supply up to 5-10% of the energy needs of an athlete engaged in endurance exercise, all body protein is functional. Thus, protein is not specifically stored for the purpose of generating energy.