Food scientists measure food energy in calories. A calorie is the amount of energy it takes to raise the temperature of one gram of water 1 degree Celsius.
Calories measure the amount of energy a food or drink can provide to the body. Different foods have different amounts of calories and different nutrients. The body uses calories for all its processes. Digestion changes foods and drinks to substances that are usable by the body.
If all the calories consumed are not used up immediately, the body stores this energy as fat to use later. When the body has a surplus of calories, the person gains weight. If insufficient calories are consumed, the body burns fat and the person loses weight.