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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 perform all of their functions, including building proteins and other substances needed by the body. The energy can be used immediately or stored to be used later.

When the supply of energy—that is, 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 ½ pound, mostly as fat.

When the intake of energy is insufficient 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. Converting fat to energy is a slower process, so 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. For normally nourished people who experience total starvation (who do not consume any food to supply energy), 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, and physical activity. Typically, to maintain body weight, sedentary women, young children, and older adults need about 1,600 calories a day; older children, active adult women, and sedentary men need about 2,000 calories; and active adolescent boys and young men need about 2,400 calories. However, division of caloric intake by a 24-hour period (daily intake) is arbitrary. Furthermore, the needs of the body vary depending on its activity at any particular time. Vigorous activity, especially aerobic exercise, increases needs substantially, and a lack of activity decreases needs.

How Are Calories in Foods Measured? Food labels always contain the number of calories per serving. But how is this number determined? The answer is surprisingly simple: The food is burned. A sample of the food is placed in an insulated, oxygen-filled chamber that is surrounded by water; this chamber is called a bomb calorimeter. The sample is burned completely. The heat from the burning increases the temperature of the water, which is measured and which indicates the number of calories in the food. For example, if water temperature increases by 20 degrees, the food contains 20 calories. This method of measuring calories is called direct calorimetry.

Last full review/revision February 2003