A calorie is a measure of energy. Foods have calories. That is, foods supply the body with energy, which is released when foods are broken down during digestion. Energy enables cells to do all of their functions, including building proteins and other substances needed by the body. The energy can be used immediately or stored for use later.
Foods may not be completely absorbed. In such cases, the body may not be able to use all of the calories in foods as energy.
When the supply of energy—the number of calories consumed in foods—exceeds the body’s immediate needs, the body stores the excess energy. Most excess energy is stored as fat. Some is stored as carbohydrates, usually in the liver and muscles. As a result, weight is gained. An excess of only 200 calories per day for 10 days is likely to result in a weight gain of nearly 1/2 pound, mostly as fat. However, the gain may be slightly more or less.
When too few calories are consumed for the body’s needs, the body begins to use carbohydrates stored in the liver and muscle. Because the body mobilizes stored carbohydrates quickly and because water is usually excreted as carbohydrates are mobilized, weight loss tends to be fast initially. However, the small amount of stored carbohydrates provides energy for only a short time. Next, the body uses stored fat. Because fat contains more energy per pound, weight loss is slower as the body uses fat for energy. However, the amount of fat stored is much larger and can, in most people, provide energy for a long time.
Only during prolonged, severe shortages of energy, does the body break down protein. If normally nourished people experience total starvation (when no food is consumed), death occurs in 8 to 12 weeks.
Energy requirements vary markedly from about 1,000 to more than 4,000 calories a day depending on age, sex, weight, physical activity, disorders present, and the rate at which people burn calories (metabolic rate). However, generally, the number of calories needed per day to maintain body weight is about
The number of calories needed increases as activity level increases, and generally, boys and men need more calories than girls and women.
Estimated Number of Calories Needed Based on Age*, Sex, and Activity Level†
These calorie counts are only general guidelines, partly because the needs of the body vary depending on its activity at any particular time. Also, the division of caloric intake by a 24-hour period (daily intake) is arbitrary. Because fewer than 10% of Americans get as much physical activity as recommended, they tend to need fewer calories than those listed above for active people. Vigorous activity, especially aerobic exercise, increases needs substantially, and a lack of activity decreases needs.