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Cholesterol is a steroid lipid, found in the body tissues (and blood plasma) of vertebrates. It can be found in large concentrations within the liver, spinal cord, and brain.

The name originates from the Greek chole- (bile) and stereos (solid) - researchers first identified cholesterol in solid form in gallstones.

Cholesterol is primarily synthesized from acetyl CoA in the liver, although other sites include the intestines, adrenal glands and reproductive organs.

Cholesterol is an important component of the membranes of cells, providing stability. It is the major precursor for the synthesis of vitamin D3, of the various steroid hormones, including cortisol, cortisone, and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Cholesterol is excreted from the liver in the form of a secretion known as bile; it sometimes crystallizes in the gall bladder to form gallstones.

Mostly insoluble in water, it travels in the blood stream in the form of lipoproteins. Low-density lipoprotein (LDL) carries cholesterol from the liver and intestines to the body cells, while high-density lipoprotein (HDL) carries it back for excretion.

It is interesting to note that the cholesterol in LDL cholesterol and the cholesterol in HDL cholesterol are identical. The only difference between the two is the carrier molecule (i.e. the lipoprotein).

Cholesterol often forms plaque deposits in the walls of arteries, a condition known as atherosclerosis, which is a major contributor to coronary heart disease when the buildup is such that it inhibits blood flow to the heart. The ratio of HDL to LDL is a commonly performed test, and the higher the ratio of these two, the lower the risk of heart disease.

The American Heart Association provides a set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease: ; Less than 200 mg/dl : Desirable level corresponding to lower risk for heart disease ; Between 200 and 239 mg/dl : Borderline high risk ; 240 mg/dl or greater : High risk

However, as today's testing methods determine LDL ("bad") and HDL ("good") cholesterol separetely, this simplistic view has become somewhat outdated. The desirable LDL level is considered to be 75-130 mg/dl, and a ratio of total cholesterol to HDL—arguably the most useful measure—of less than 5 mg/dl is thought to be healthy.

See also: 7-dehydrocholesterol, triglycerides

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