Cholesterol is an important body molecule; it is a key component of cell membranes and a precursor to a number of hormones and to vitamin D. Cholesterol is synthesized in the human body by the liver, and may also be consumed as part of the diet; on average, about 75%-85% of one's total body cholesterol is made within the body, and the remainder is obtained from food.
The cholesterol molecule is made up of a series of carbon rings; there are three 6-carbon rings side by side, with a 5-carbon ring on one end. The molecule has the formula C27H46O.
The body transports cholesterol by producing special proteins which attach to the cholesterol molecules; the combination of protein and cholesterol is known as a lipoprotein. HDL, or High Density Lipoproteins, is sometimes called "good" cholesterol, because HDL travels to the liver, where the cholesterol is converted into bile and excreted from the body. This results in lower levels of cholesterol in the bloodstream.
LDL, or Low Density Lipoproteins, work to keep cholesterol circulating in the bloodstream. This can cause problems when the endothelium, or lining of a blood vessel, is damaged. Endothelium damage (which can be caused by a variety of things, including smoking, hypertension, diabetes, or bacterial infection) creates thin spots where LDL can leave the bloodstream and lodge in the wall of the blood vessel. White blood cells may move into the area to try to digest the cholesterol, resulting in even more damage to the endothelium, and over time a plaque can build up. Plaques narrow the arteries and impede blood flow, putting the patient at risk of a heart attack or ischemic stroke.
Cholesterol in the diet comes exclusively from animal sources. Meat, dairy, and egg yolks are the main culprits for dietary cholesterol. People on a low cholesterol diet need to avoid fatty cuts of meat, liver, butter, full-fat milk products, and egg yolks. Lean meats in moderation, skim milk and products made from skim milk (yogurt, etc), and egg whites are acceptable.