The food we eat has two main purposes:

• to provide energy.
• metabolism i.e. building and enabling functions of the body.

Similar to other fuels, when food is oxidized it produces energy, or heat. This energy is measured in units known as calories. In the field of human nutrition the caloric, or energy, values of food are actually measured in kilocalories (1,000 of these units), but in popular usage the "kilo" has been dropped. The international unit of energy is the joule (1 calorie = 4.184 joules). Different foods supply different amounts of energy, and so have different caloric values. Fat supplies 9 calories per gram, carbohydrates and proteins supply 4 calories per gram, and water and cellulose (fiber) no

calories. That is why foods high in fat are highest in calories, and foods high in cellulose and water (such as fresh vegetables) are lowest. When a diet supplies just the amount of fuel or calories the body needs, all the food energy will be used. But when the diet supplies more calories than are needed, the excess fuel is stored in the body as fat. This is also true when the excess is carbohydrate or protein; the liver converts the excess to fat. When the diet supplies fewer calories than are needed, the body converts its fat stores to energy to make up the difference-and weight is lost. A desirable body weight depends on balancing energy intake-determined by food choices-with energy output-determined by one's basal metabolism (energy needs of the body at rest) plus one's level of physical activity. The number of calories an individual will need every day depends on age, sex, size, and level of physical activity. We should not only stay within optimum calorie levels to maintain ideal body weight, but if we are cutting down on calories, we should also select foods of greater nutrient density (most nutrients per calorie). The way to control body weight is by balancing food intake against physical activity. This is especially important for children, whose weight problems may set a pattern for life" and for the elderly, whose low energy needs may lead to a diet deficient in many of the nutrients needed for good health if nutrient-dense foods are not adequately used.