Cholesterol is a lipid found in the cell membranes of all animal tissues, and it is transported in the blood plasma of all animals.

The name originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -ol for an alcohol, as researchers first identified cholesterol in solid form in gallstones by François Poulletier de la Salle in 1769. However, it is only in 1815 that chemist Eugène Chevreul named the compound "cholesterine".

Most of the cholesterol in the body is synthesized by the body and some has dietary origin. Cholesterol plays a central role in many biochemical processes, such as the composition of cell membranes and the synthesis of steroid hormones.

Cholesterol is more abundant in tissues which either synthesize more or have more abundant densely-packed membranes, for example, the liver, spinal cord and brain.

Since cholesterol is insoluble in blood, it is transported in the circulatory system within lipoproteins, complex spherical particles which have an exterior composed mainly of water-soluble proteins; fats and cholesterol are carried internally. There is a large range of lipoproteins within blood, generally called, from larger to smaller size: chylomicrons, very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), low density lipoprotein (LDL) and high density lipoprotein (HDL). The cholesterol within all the various lipoproteins is identical.

Primary Functions

Cholesterol is required to build and maintain cell membranes; it regulates membrane fluidity over a wide range of temperatures.  Some research indicates that cholesterol may act as an antioxidant.] Cholesterol also aids in the manufacture of bile (which is stored in the gallbladder and helps digest fats), and is also important for the metabolism of fat soluble vitamins, including vitamins A, D, E and K. Cholesterol is the major precursor for the synthesis of vitamin D and of the various steroid hormones (which include cortisol and aldosterone in the adrenal glands, and the sex hormones progesterone, the various estrogens, testosterone, and derivatives).

Cholesterol and Heart Disease

According to the lipid hypothesis, abnormally high cholesterol levels (hypercholesterolemia), or, more correctly, higher concentrations of LDL and lower concentrations functional HDL are strongly associated with heart disease because these promote atheroma development in arteries (atherosclerosis). In contrast, HDL particles (especially large HDL) have been identified as a mechanism by which cholesterol and inflammatory mediators can be removed from atheroma.

Increased concentrations of HDL correlate with lower rates of atheroma progressions and even regression. The relation of cholesterol to CHD, termed the "lipid hypothesis," is still hotly debated.

Atherosclerosis leads to myocardial infarction (heart attacks), stroke and peripheral vascular disease. Since higher blood concentrations of LDL, especially the smaller and denser LDL particles, contribute to this process, they are often termed "bad cholesterol" because they have been linked to atheroma formation, while high concentrations of functional HDL, which can remove cholesterol from cells and atheroma, offer protection. These balances are mostly genetically determined but can be changed by body build, medications, food choices and other factors.

Cholesterol Testing

Most testing methods for LDL do not actually measure LDL in their blood, much less particle size. For cost reasons, LDL values have long been estimated using the Friedewald formula: [total cholesterol] − [total HDL] − 20% of the triglyceride value = estimated LDL. The basis of this is that Total cholesterol is defined as the sum of HDL, LDL, and VLDL. Ordinarily just the total, HDL, and triglycerides are actually measured. The VLDL is estimated as one-fifth of the triglycerides. It is important to fast for at least 8-12 hours before the blood test because the triglyceride level varies significantly with food intake.

A blood sample taken after fasting is taken by a doctor or a home cholesterol monitoring device to determine a lipoprotein profile. This measures total cholesterol, LDL (bad) cholesterol, HDL (good) cholesterol and triglycerides. It is recommended to have cholesterol tested more frequently than 5 years if a person: has total cholesterol of 200 mg/dL or more, is a man over age 45 or a woman over age 50, has HDL (good) cholesterol less than 40 mg/dL, or other risk factors for heart disease and stroke.

It is recommended by the American Heart Association to test cholesterol every 5 years for people aged 20 years or older

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