Human beings need energy to survive: to breathe, to move, and to pump blood. Where we get this energy comes from food.
The number of calories contained in a food is a measure of how much potential energy that food possesses. In other words: a gram of
carbohydrates has 4 calories, a gram of protein has 4 calories, and a gram of fat has 9 calories.
Foods are a made up of these three building blocks. So if you know how many carbohydrates, fats and proteins are in any given food, you can figure out how many calories
(energy) that food contains.
Actually, food calories are kilocalories (one thousand
Our bodies "burn" the calories through the metabolic process, by which enzymes break the carbohydrates into glucose and other sugars, the fats into glycerol and fatty acids and the proteins into amino acids.
These molecules are then transported through the bloodstream to the cells, where they are either absorbed for immediate use or sent on to the final stage of metabolism in which they are reacted with oxygen to release their stored energy.