Cholesterol is the most abundant sterol in animal tissues, making up as much as 25 percent of cell membranes. Cholesterol may be found free or as part of cholesteryl esters . It is a precursor molecule for many steroid hormones, including glucocorticoids , androgens and estrogen , aldosterone, and mineralocorticoids. A major component of lipoproteins, it is also the precursor for bile salts and bile acids, which are necessary for digestion. Gallstones contain large amounts of cholesterol.
Cholesterol is a seventeen-carbon polycyclic compound made up of three six-membered and one five-membered fused rings. The molecule is relatively nonpolar and hydrophobic , but is slightly polar due to the presence of an alcohol functional group .
Plants contain no cholesterol. In animals cholesterol synthesis occurs in many cells, but most cholesterol synthesis occurs in the liver. Food products from animals contain cholesterol, and the average adult consumes around 450 milligrams (0.016 ounces) per day. Dairy products and egg yolks are particularly rich in cholesterol. Diets low in fat content and high in vegetables, especially those containing polyunsaturated lipids , can help to lower plasma cholesterol levels. Many physicians recommend that their patients try to maintain cholesterol levels below 200 milligrams per deciliter.
Cholesterol in blood plasma is conjugated with other lipid molecules and with carrier proteins. These lipoprotein complexes may form droplets called chylomicrons, but cholesterol is usually transported as part of a number of larger lipoproteins, including low density lipoprotein (LDL), which carries cholesterol from the liver to muscle and other tissues, and high density lipoprotein (HDL), which carries cholesterol to the liver for conversion to bile acids. Physicians are especially concerned when patients have high levels of LDL (the so-called bad cholesterol) in blood; moderate exercise and low-cholesterol diets help to increase HDL (the so-called good cholesterol). Either high fat intake or problems with the transport of cholesterol to and from cells can lead to atherosclerosis (hardening of the arteries), which in turn can contribute to heart attack (myocardial infarction) or stroke.
Humans do not oxidize cholesterol for energy. Instead, cholesterol is converted to bile acids such as cholic acid and deoxycholic acid in liver tissue. Bile acids and salts are secreted into bile, which passes into the intestine and emulsifies fats for digestion. Although some bile acids may be reabsorbed in the intestine along with lipids, much cholesterol leaves the body in feces in the form of metabolites such as bile acids and salts.
Diets rich in oatmeal or other vegetable products are believed to help to lower plasma cholesterol levels. Soluble fibers from the vegetable materials absorb cholesterol and help to prevent absorption in the intestine.