The food we eat has two main purposes: to act as fuel and building blocks for the activities and life-sustaining functions of the body. Similar to other fuels, when food is oxidized (in this case, burned) 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 accepted 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 beused. 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.