The foods we eat contain nutrients that provide energy and other substances the body needs. Most of the nutrients in food fall into three major groups: proteins, fats, and carbohydrates. The body uses fat as a fuel source, and fat is the major storage form of energy in the body. Fat also has many other important functions in the body, and a moderate amount is needed in the diet for good health. Fats in food come in several forms, including saturated, monounsaturated, and polyunsaturated. Too much fat or too much of the wrong type of fat can be unhealthy.