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Body composition typically describes the molecular components that form the human body. Experts divide body composition into two separate components: the fat component and the fat-free component.

The fat component is made up of two types of fat:

• Essential fat. Essential fat is necessary for normal functioning in the body, including providing important metabolic fuel for energy production. Essential fat includes fat in the heart, intestines, kidneys, liver, lungs, marrow of the bones, muscles, spleen and lipid-rich tissues found throughout the central nervous system (brain and spinal cord).
  
  
• Storage fat. Includes the fat that accumulates in adipose tissue to both protect the body and to help conserve body heat. It is located around internal organs (internal storage fat) and just beneath the skin (subcutaneous storage fat).

The fat-free component includes everything in the lean body mass, which is the total weight of water, muscles, bones, ligaments, tendons and internal organs (and excluding the essential fat found in the marrow of bones and internal organs).

Research indicates that fat found in certain areas of the body presents greater risks than fat found in other parts of the body. For example, fat that accumulates around the waist creating an “apple-shaped” body (android obesity) tends to present greater health risks than fat that accumulates around the thighs and buttocks creating a “pear-shaped” body (gynoid obesity).

Being too thin (underweight) also poses potential health risks. People who do not receive enough calories may not have the energy they need to maintain good health and may be at risk for conditions such as infertility, increased vulnerability to infections and osteoporosis.