While there are no absolutes, Sheldon identified three basic body types, ectomorphs, mesomorphs and endomorphs. Ectomorphs are the lean, lanky types. These people can eat like a horse and never gain an ounce. They are usually very thin, with fast metabolisms and extremely low body fat. They are muscular and lean and are natural athletes.
Mesomorphs lose fat and gain muscle with ease.
Endomorphs have slow metabolisms, they are often carbohydrate sensitive, they gain fat quickly if they eat poorly, they gain fat if they don't exercise, and they may stubbornly retain body fat, even on a healthy diet. The tendency of endomorphs to store fat easily can be partly attributed to metabolic problems that are genetically transmitted. For example, endomorphs often metabolize carbohydrates inefficiently. Endomorphs produce too much insulin when they eat carbohydrates and this leads to increased fat storage and difficulty in losing existing fat. Scientists claim that obesity may also be due to chemical imbalances in the brain.