Merck Manual

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Adrienne Youdim

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Aug 2019| Content last modified Aug 2019
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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 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.

Did You Know...

  • After the first few pounds are lost, weight loss slows down when the body has burned all its stored carbohydrates and starts burning stored fat.

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.

How Are Calories in Foods Measured?

Food labels 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.

Recommended Number of Calories

Energy requirements vary markedly from about 1,000 to more than 3,200 calories a day depending on age, sex, weight, physical activity, disorders present, and the rate at which people burn calories (metabolic rate). Caloric intake in the higher range is required by those who participate in activities outside the normal activities of daily living that require extra energy. However, generally, the number of calories needed per day to maintain body weight is about

  • For young children: 1,000 to 1,800

  • For older children and adolescents: 1,200 to 3,200

  • For adults: 1,600 to 3,000

The number of calories needed increases as activity level increases, and generally, boys and men need more calories than girls and women.

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.


Estimated Number of Calories Needed Based on Age*, Sex, and Activity Level†




Young children (age 2–6 years)







Older children and adolescents (age 7–18 years)







Adults (age 19–60 years)







Adults (age 61 years and over)







* The number of calories needed increases as people age up until age 20 years. After age 20, the number of calories needed starts to decrease.

† The more active a person is, the higher the number of calories needed.

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