Cholesterol, is a fatty substance found in animal tissues. The human body produces cholesterol, but this substance also enters the body in food. Meats, egg yolks, and milk products, such as butter and cheese, contain cholesterol. Such organs as the brain and liver contain much cholesterol.

Cholesterol is a type of lipid, one of the classes of chemical compounds essential to human health. Cholesterol makes up an important part of the membranes of each cell in the body. In addition, the liver uses cholesterol to make bile acids, which aid digestion. The body also uses cholesterol to produce vitamin D and certain hormones, including sex hormones.

Cholesterol and triglycerides, another lipid, are two of the major fatty substances in the blood. Triglycerides may be used by cells for energy, or they may be stored for later use. Doctors often measure the amount of cholesterol and triglycerides in blood to help determine a patient's overall health. High levels of cholesterol, particularly if accompanied by high levels of triglycerides, increase the risk of heart disease.

Both cholesterol and triglycerides are carried through the bloodstream in large molecules called lipoproteins. There are two chief types of cholesterol-carrying lipoproteins, low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Cholesterol in blood can thus be identified as either LDL-cholesterol or HDL-cholesterol, depending on which lipoprotein carries it. High levels of LDL-cholesterol in blood are a primary cause of heart attacks. LDL can be found in the wall of heart arteries. Low levels of HDL-cholesterol also increase the risk of heart attack. Scientists believe that HDL's help remove cholesterol from tissues.