cholesterol

Last reviewed 08/2022

Cholesterol is a sterol, an alcohol derivative of a steroid, which is present in the blood and most cell membranes. It is a type of lipid.

Cholesterol is important as a:

• molecule which increases the fluidity of cell membranes
• precursor of steroid hormones
• precursor of bile salts

Cholesterol is found in animal products, particularly eggs, and is also synthesised de novo in the liver. It is virtually absent from plants.

The average intake of cholesterol is 300-500 mg/day (total dietary fat intake in countries such as the UK and USA is probably 80-100g per day). The average loss is 1000 mg via dead skin, gut epithelial cells and bile salts.

Cholesterol absorption for the gut is incomplete - generally 30-60% of the quantity ingested actually entering the body.

• although cholesterol is absorbed from the gut, it is also secreted back into it as one of the components of bile
• the body synthesizes at least as much as obtained via dietary intake

Cholesterol synthesis:

• almost all tissues can synthesize cholesterol - however much of its synthesis in the adult occurs in the liver, gut and central nervous system
• an important physiological regularotory step occurs when 3-hydroxy-3-methylglutaryl CoA is converted to mevalonic acid. The enzyme responsible, HMG-CoA reductase, can be inhibited by a variety of physiological factors - probably the most important of which is intracellular level of cholesterol - this is the reason that if tissues are supplied an abundant supply of cholesterol from the gut and liver they in turn down-regulate their own synthesis of cholesterol

Cholesterol structure:

• can be free (with no fatty acid group attached to its OH-group), or can have a fatty acyl group bound by an ester link
• esterified cholesterol can hardly interact with water whereas the OH group of free cholesterol can to some extent
• for everyday clinical purposes the distinction between the two forms of cholesterol is not made

Cholesterol is important pathologically in its role in cardiovascular disease and cholesterol gallstones.