Your body needs energy in order to function correctly. It gets this energy from food. If the amount of energy taken in is equal to the amount used up, your weight stays the same. If you take in more than you use up, energy is left over: This is known as a positive energy balance.

Your body stores this excess energy in the form of fatty tissue, as a reserve for times in the future when you might not have enough food. Everyone needs a different amount of food: some people burn up the energy that they take in more quickly than others do.