Cholesterol, from the Greek chole- (bile) and stereos (solid) followed by the chemical suffix -ol for an alcohol, is an organic chemical substance classified as a waxy steroid of fat. It is an essential structural component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity.
In addition to its importance within cells, cholesterol is an important component in the hormonal systems of the body for the manufacture of bile acids, steroid hormones, and vitamin D. Cholesterol is the principal sterol synthesized by animals; in vertebrates it is formed predominantly in the liver. Small quantities are synthesized in other cellular organisms (eukaryotes) such as plants and fungi. It is almost completely absent among prokaryotes, i.e. bacteria.
Although cholesterol is important and necessary for human health, high levels of cholesterol in the blood have been linked to damage to arteries and cardiovascular disease.
François Poulletier de la Salle first identified cholesterol in solid form in gallstones, in 1769. However, it was only in 1815 that chemist Eugène Chevreul named the compound "cholesterine".
Since cholesterol is essential for all animal life, it is primarily synthesized from simpler substances within the body. However, high levels in blood circulation, depending on how it is transported within lipoproteins, are strongly associated with progression of atherosclerosis. For a person of about 68 kg (150 pounds), typical total body cholesterol synthesis is about 1 g (1,000 mg) per day, and total body content is about 35 g. Typical daily additional dietary intake in the United States is 200–300 mg. The body compensates for cholesterol intake by reducing the amount synthesized.
Cholesterol is recycled. It is excreted by the liver via the bile into the digestive tract. Typically about 50% of the excreted cholesterol is reabsorbed by the small bowel back into the bloodstream. Phytosterols (similar substances produced within plants) can compete with cholesterol reabsorption in the intestinal tract, thus reducing cholesterol reabsorption
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