Fats are made up of the same elements as carbohydrates - carbon, hydrogen and oxygen - but the way the atoms are linked together is different. Fats can be found in both plants and animal's and are insoluble in water. They are grouped in three categories:

Simple fats (triglycerides)
Compound fats (phospholipids, lipoproteins)
Derived fats (cholesterol)

Fats in the body serve three basic functions:
1. They provide the major source organs.
2. They serve to cushion and protect the major organs.
3. They act as an insulator, preserving body heat and protecting against excessive cold.

Fat is the most calories dense of any nutrient. A pound of fat contains about 3,500 calories, as opposed to 1,600 calories stored in a pound of protein or carbohydrate.

When you exercise, assuming you stay within your aerobic capacity (don't get out of breath), the body uses fats and carbohydrates for energy on about a 50-50 basis. But the longer you continue to exercise, the higher the percentage of fat used. After three hours or so, the body may derive as much as 80 percent of its energy from fat.

Fat molecules differ biochemical in their composition, being either saturated, unsaturated or polyunsaturated. In addition to other factors, diets high in saturated fat tend to raise the cholesterol level of the blood. Therefore, health experts recommend that something like two-thirds of your fat intake be polyunsaturated fats.