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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.
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