Fat, or lipids, is a family of chemical compounds stored by plants and animals as a source of energy. Fats are stored in special cells that tend to form pads of tissue under the skin and around certain organs and joints. Research has shown that fat distribution in men and women serves different purposes: abdominal fat, found more often in men, provides fuel for quick energy, while the fat in the thighs and buttocks of women provides energy for pregnancy and nursing. Stored fat, or adipose tissue, serves as a fuel reserve for metabolism. Fat protects the body from shocks and jolts and provides insulation. In plants, fats in the form of oil are found in the stems, seeds, and fruit. All fats are made up of units of glycerol and fatty acids.
The kind of fatty acids eaten can affect a person's health. Saturated fatty acids found in butter, milk, and other animal products can raise the level of cholesterol in the blood, thus leading to arteriosclerosis. Unsaturated fats found in vegetable oils can reduce high levels of blood cholesterol. Fat is a concentrated source of energy. The breakdown of storage fat during metabolism yields fatty acids, the source of metabolic energy for muscle contraction. The body's adipose tissue is in a constant state of buildup and breakdown, thus ensuring a continual supply of fatty acids.
A normal human male has about 15 kg (33 lb) of stored fat, which theoretically can support life for about two months. Obese people may have 100 kg (220 lb) of fat--a year's supply.