The amount of fat in the human body is regulated by a number of mechanisms. The most important of these is the balance between energy intake and energy expenditure. Energy intake is controlled by the hypothalamus, which regulates hunger and satiety signals in the brain. Energy expenditure is determined by the amount of physical activity and the efficiency with which energy is used by the body.
The amount of fat stored in the body also depends on hormonal factors. Insulin promotes fat storage, while glucagon and adrenalin promote lipolysis, or break down of stored fats. Other hormones that affect fat storage include cortisol, thyroid hormone, and sex hormones such as testosterone and estrogen.
In addition to these regulatory mechanisms, there are also a number of genetic factors that influence an individual’s propensity to store fat. Some people are simply more efficient at storing energy as fat than others. This difference is due in part to different levels of enzymes involved in lipid metabolism, as well as different numbers of adipocytes (fat cells).