Technically, one calorie is the amount of energy required to raise the temperature of 1 gram of water 1 degree Centigrade (from 14.5 to 15.5).   The "calorie" measure used commonly to discuss the energy content of food is actually a kilocalorie or 1000 real calories; this is the amount of energy required to raise one kilogram of water (about 2.2 pounds) one degree Centigrade.  Different foods can be used by the body to produce different amounts of energy -- which is why a small piece of chocolate can have hundreds more calories than a similarly sized piece of lettuce.

However, since calories are a measure of energy, there cannot be, as some diet books claim, different "types" of calories.  A fat calorie has the same amount of energy as a protein calorie by definition.

Functions

The amount of calories in a diet refers to how much energy the diet can provide for the body. A well-balanced diet is one that delivers an adequate amount of calories while providing the maximum amount of nutrients.

The body breaks down food molecules to release the energy stored within them. This energy is needed for vital functions like movement, thought, growth -- anything that you do requires the use of fuel. The body stores energy it does not need in the form of fat cells for future use.

The process down food for use as energy is called metabolism. Increased activity results in increased metabolism as the body needs more fuel. The opposite is also true. With decreased activity the body continues to store energy in fat cells and does not use it  up. Therefore, weight gain is the result of increased intake of food, decreased activity or both.

The nutrition labels on food packages indicate the amount of calories contained in the food.

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