Cholesterol (ko-LES-ter-all) is a biological substance made by the liver. It's used throughout the body to insulate from cold, protect from injury, and build important structures like cell membranes, nerve cell coverings, hormones, and Vitamin D. Naturally a waxy-like material, cholesterol doesn't mix well with blood so the body coats cholesterol molecules with protein to make them travel easily through the bloodstream. This encapsulated cholesterol then becomes either a high-density lipoprotein (LIP-o-pro-teen) or HDL (H-D-L), or a low-density lipoprotein, LDL (L-D-L). HDL is known as 'good' cholesterol, because it has the role of removing extra LDL or 'bad' cholesterol from the bloodstream when a person eats high cholesterol foods. HDL's job is important because excess circulating LDL is generally deposited along arterial (art- EAR- ee-al) walls, which often leads to many forms of heart disease. A person's blood level of HDL can be measured best after a 12 hour fast. A physician can then take a small blood sample and measure the milligrams of each type of lipoprotein per deciliter of blood. In a healthy individual, the HDL level should make up at least 25 percent of the total cholesterol. This would generally correspond to about 40 milligrams of HDL per deciliter. HDL levels can be raised if necessary through exercise, which both reduces LDL and increases HDL simultaneously. If you're concerned about your cholesterol level, or have questions about good cholesterol, contact a health care provider.