Cholesterol is a soft, waxy substance found in the bloodstream and in all body's cells. It is used to form cell membranes, some hormones and is needed for other functions. But a high level of cholesterol in the blood, which is called hypercholesterolemia, is a major risk factor for coronary heart disease, which can lead to heart attack. Cholesterol and other fats are transported to and from the cells by special carriers called lipoproteins.

There are several kinds, but the most common ones to be studied are low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is known as “bad” cholesterol, because if too much LDL cholesterol circulates in the blood, it can slowly build up in the walls of the arteries feeding the heart and brain. Together with other substances it can form plaque, a thick, hard deposit that can clog those arteries. This condition is known as atherosclerosis. HDL cholesterol is known as "good" cholesterol because it carries cholesterol away from the arteries and back to the liver, where it is passed from the body. A high HDL level appears to protect against heart attack. The opposite is also true: a low HDL level (less than 40 mg/dL in men; less than 50 mg/dL in women) indicates a greater risk.