Stable weight depends on an even balance between energy intake from food and energy expenditure. Energy expenditure occurs during the day in three ways:

* As energy expended during rest
* As energy used to metabolize food
* As energy expended during physical activity

Energy used during rest accounts for about two-thirds of expended energy, which is generally used to maintain body temperature and muscle contractions in the heart and intestine.

When a person consumes more calories than energy that is used, the body stores the extra calories in fat cells. Fat cells function as energy reservoirs. They enlarge or contract depending on how people use energy. If people do not balance energy input and output by eating right and exercising, fat can builds up. This can lead to weight gain.