All extra energy contained in the body (other than a small amount of glucose stored as glycogen) becomes fat. Some of this fat is found in blood plasma and other cells throughout the body, but the greatest amount is in the body’s adipose – fat storage – cells. This body fat is the storage form for all the extra energy (calories) taken in, regardless of the source: fat, carbohydrate, protein, or alcohol.
Adipose tissue is active – continually changing in response to our energy needs. Cells throughout the body, other than erythrocytes (red blood corpuscles) and the cells in the central nervous system, are able to use fatty acids directly for energy.
In addition to storing energy, adipose tissue insulates the body. About one half of body fat is the layer just below the skin that protects us from changes in external temperature and helps to maintain a fairly constant internal temperature. Other adipose deposits surround and cushion such vital organs as the kidneys and those of the reproductive system. There are also fat pads in the cheeks, palms of the hands, and balls of the feet.