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Cholesterol is a steroid lipid, found in the cell membranes of all body tissues, and transported in the blood plasma, of all animals. Most cholesterol is produced internally, not dietary in origin. It is present in higher concentrations in tissues which either produce more or have more densely packed membranes; for example the liver, spinal cord, brain and atheroma. Cholesterol plays a central role in many biochemical processes, but is best known for the association of cardiovascular disease with various lipoprotein cholesterol transport patterns in the blood.
The name originates from the Greek chole- (bile) and stereos (solid), as researchers first identified cholesterol in solid form in gallstones.
Cholesterol is primarily synthesized from acetyl CoA through the HMG-CoA reductase pathway in many cells/tissues. About 20-25% of total daily production (~1 g/day) occurs in the liver, other sites of higher synthesis rates include the intestines, adrenal glands and reproductive organs.
Cholesterol is minimally soluble in water; it cannot dissolve and travel in the water-based blood stream. Instead, it is transported in the blood stream by lipoproteins; protein 'molecular-suitcases' which are water soluble and carry cholesterol and fats internally. The proteins forming the surface of the given lipoprotein particle determine from what cells cholesterol will be removed and to where it will be supplied.
The largest lipoproteins, which primarily transport fats from the intestinal mucosa to the liver are called chylomicrons. They carry mostly triglyceride fats and cholesterol (both from food and especially internal cholesterol secreted by the liver into the bile). In the liver, chylomicron particles give up triglycerides and some cholesterol and are converted into low-density lipoprotein (LDL) particles which carry triglycerides and cholesterol on to other body cells. In healthy individuals the low-density lipoprotein (LDL) particles are large and relatively few in number. Conversely, large numbers of small low-density lipoprotein (LDL) particles are strongly associated with promoting atheromatous disease within the arteries. (Lack of information on low-density lipoprotein (LDL) particle number and size is one of the major problems of conventional lipid tests.)
High-density lipoprotein (HDL) particles transport cholesterol back to the liver for excretion, but vary considerably in their effectiveness for doing this. Having large numbers of large HDL particles correlates with better health outcomes. Conversely, having small amounts of large HDL particles is strongly associated with atheromatous disease progression within the arteries. (Note that the concentration of total HDL does not indicate the actual number of functional large HDL particles, one of the major problems of conventional lipid tests.)
The cholesterol in LDL cholesterol and the cholesterol in HDL cholesterol are identical. The only difference between the two is the carrier protein molecules (i.e. the lipoprotein).
The production is directly regulated by the cholesterol levels present, though the homeostatic mechanisms involved are only partly understood. A higher intake in food leads to a net decrease in endogenous production and visa-versa. The main regulatory mechanism is the sensing of intracellular cholesterol in the endoplasmic reticulum by the protein SREBP (Sterol Regulatory Element Binding Protein 1 and 2). In the presence of cholesterol, SREBP is bound to two other proteins: SCAP (SREBP-cleavage activating protein) and Insig-1. When cholesterol levels fall, Insig-1 dissociates from the SREBP-SCAP complex, allowing the complex to migrate to the Golgi apparatus, where SREBP is cleaved by S1P and S2P (site 1/2 protease), two enzymes that are activated by SCAP when cholesterol levels are low. The cleaved SREBP then migrates to the nucleus and acts as a transcription factor to bind to the "Sterol Regulatory Element" of a number of genes to stimulate their transcription. Amongst the genes transcribed are the LDL receptor and HMG-CoA reductase. The former scavenges circulating LDL from the bloodstream, while HMG-CoA reductase leads to an increase of endogenous production of cholesterol.
A large part of this mechanism was clarified by Dr Michael S. Brown and Dr Joseph L. Goldstein in the 1970s. They received the Nobel Prize in Physiology or Medicine for their work in 1985.
The average amount of blood cholesterol varies with age, typically rising gradually until one is about 60 years old. A study by Ockrene et al showed that there are seasonal variations in cholesterol levels in humans, more on average in winter.
Cholesterol is an important component of the membranes of cells, providing stability. It is the major precursor for the synthesis of vitamin D, of the various steroid hormones, including cortisol, cortisone, and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Further recent research shows that cholesterol has an important role for the brain synapses as well as in the immune system, including protecting against cancer.
Cholesterol is excreted from the liver in bile and reabsorbed from the intestines. Under certain circumstances, when more concentrated, as in the the gallbladder, it crystallises and is the major constituent of most gallstones, although lecitin and bilirubin gallstones also occur less frequently.
See also the main article hypercholesterolemia
In conditions featuring elevated LDL, cholesterol allegedly promotes atheroma plaque deposits in the walls of arteries, a condition known as atherosclerosis, which is a major contributor to coronary heart disease and other forms of cardiovascular disease. (Conversely, HDL particles have been the only identified mechanism by which cholesterol can be removed from atheroma.)
The provides a set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease:
| Level mg/dL | Level mmol/L | Interpretation |
| <200 | <5.2 | Desirable level corresponding to lower risk for heart disease |
| 200-239 | 5.2-6.2 | Borderline high risk |
| >240 | >6.2 | 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 (1.9-3.3 mmol/L), and a ratio of total cholesterol to HDL—arguably the most useful measure—of less than 5:1 is thought to be healthy.
Increasing clinical evidence has strongly supported the greater predictive value of still more sophistical testing which directly measures both LDL and HDL particle concentrations and size as opposed to the more usual estimates/measures of the total cholesterol carried within LDL particles or the total HDL concentration. There are three commercial labs in the United States which offer more sophisticated analysis, different methodologies. The real key is cholesterol transport which is determined by both the proteins which form the lipoprotein particles and the proteins on cell curfaces with which they interact.
Some cholesterol derivatives, (among others simple cholesteric lipids) are known to generate liquid crystalline phase called "cholesteric". The cholesteric phase is in fact a chiral nematic phase and changes colour when its temperature changes. Therefore cholesterol derivatives are commonly used as temperature sensitive dyes, in liquid crystal thermometers, and in temperature sensitive paints.
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