Body composition is the ratio of fat to muscle tissue in the body, and it is often used to determine a person's level of health. Three main eneralized body types exist, according to the Center for Wellness Without Borders at the University of Houston. All three are determined by the person's body composition, which in turn partially is determined by the person's DNA.
Endomorphs lack both muscle and fat, comparatively, so they're very thin in appearance with oversized joints and prominent ribs. Ectomorphs are predisposed to having both fat and muscle tissue, but don't look particularly muscular, having rounded, softer features instead. Mesomorphs fall between the two, but are commonly thought of as more muscular. The classic 'hero's build' is a mesomorph body type. Body composition can be further divided into the ratios of different kinds of fat, such as subcutaneous fat, which is fat below the skin's surface, and visceral fat, which is fat in the abdominal cavity.
Several methods exist to test body fat percentage, such as a multi-location skinfold caliper test, a water displacement test or a full-body X-ray. It's useful to know how much body fat you have at a given time, as too much can cause health problems. At the same time, too little can be dangerous, causing organ failure or hypothermia. After all, the primary purpose of body fat is to provide insulation and a source of energy during periods of starvation.