cholesterol: Definition and Much More from Answers.com

n.

A white crystalline substance, C₂₇H₄₅OH, found in animal tissues and various foods, that is normally synthesized by the liver and is important as a constituent of cell membranes and a precursor to steroid hormones. Its level in the bloodstream can influence the pathogenesis of certain conditions, such as the development of atherosclerotic plaque and coronary artery disease.

[cholester(in), former name for cholesterol (CHOLE– + Greek stereos, solid + –IN) + –OL¹ (so called because it was first found in gallstones).]

A cyclic hydrocarbon alcohol commonly classified as a lipid because it is insoluble in water but soluble in a number of organic solvents. It is the major sterol in all vertebrate cells and the most common sterol of eukaryotes. In vertebrates, the highest concentration of cholesterol is in the myelin sheath that surrounds nerves and in the plasma membrane that surrounds all cells.

Cholesterol can exist either in the free (unesterified) form (see structure below) or in the esterified form, in which a fatty acid is bound to the hydroxyl group of cholesterol by an ester bond. The free form is found in membranes. Cholesteryl esters are normally found in lipid droplets either within the cells of steroidogenic tissues, where it can be converted to free cholesterol and then to steroid hormones, or in the middle of spherical lipid-protein complexes, called lipoproteins, that are found in blood.

Cholesterol, together with phospholipids and proteins, is important in the maintenance of normal cellular membrane fluidity. At physiological temperatures, the cholesterol molecule interacts with the fatty acids of the membrane phospholipids and causes increased packing of the lipid molecules and hence a reduction of membrane fluidity. Thus, all vertebrate cells require cholesterol in their membranes in order for the cell to function normally. Cholesterol is also important as a precursor for a number of other essential compounds, including steroid hormones, bile acids, and vitamin D. See also Steroid.

Cellular cholesterol is obtained both from the diet, following its absorption in the intestine, and from synthesis within all cells of the body. Foods that are particularly high in cholesterol include eggs, red meat, and organs such as liver and brain. About 40–50% of the dietary cholesterol is absorbed from the intestine per day. In contrast, plant sterols are very poorly absorbed. Cholesterol synthesis occurs in all vertebrate cells but is highest in the liver, intestine, and skin, and in the brain at the time of myelination.

Cholesterol and cholesteryl esters are essentially insoluble in water. In order to transport these compounds around the body in the blood, the liver and intestine produce various lipid-protein complexes, called lipoproteins, which serve to solubilize them. Lipoproteins are large, complex mixtures of cholesterol, cholesteryl esters, phospholipids, triglycerides (fats), and various proteins. The major lipoproteins include chylomicrons, very low density lipoprotein (VLDL), low-density lipoprotein, and high-density lipoprotein (HDL).

Total plasma cholesterol levels of less than 200 mg/100 ml are considered desirable. Values of 200–239 or greater than 239 mg per 100 ml are considered, respectively, borderline high or high risk values, indicating the potential for a heart attack. High levels of low-density lipoprotein in the plasma are associated with increased risk of atherosclerosis, (“hardening of the arteries”), which involves deposition of cholesterol and other lipids in the artery wall. Diets low in cholesterol and saturated fats often result in a reduction in total plasma and LDL cholesterol levels. Such changes in blood cholesterol levels are thought to be beneficial and to reduce the incidence of heart attacks.

— Alan W. Cuthbert

See also bile; fats; gall bladder.

The principal sterol in animal tissues, an essential component of cell membranes and the precursor for the formation of the steroid hormones. It is transported in the plasma lipoproteins. Not a dietary essential, since it is synthesized in the body. Eggs contain about 450 mg, milk 14 mg, cheese 70-120 mg, brain 2.2 mg, liver and kidney 300-600 mg, poultry 70-100 mg, and fish 50-60  mg/100 g.

An elevated plasma concentration of cholesterol is a risk factor for atherosclerosis. The synthesis of cholesterol in the body is increased by a high intake of saturated fats, but apart from people with a rare genetic defect in the regulation of cholesterol synthesis, dietary intake of cholesterol does not affect the plasma concentration very much, since there is normally strict control over the rate of synthesis. See also hypercholesterolaemia; hyperlipidaemia; HMG CoA reductase inhibitors;

.

Some people with cholesterol levels just below 200 mg/dl may also be at risk of cardiovascular diseases because of the type of lipoprotein they possess (lipoproteins carry cholesterol and fats in the blood; see lipoprotein).

Several cholesterol test-kits can be bought over the counter. If used properly, these can give quite accurate readings, but interpreting the results is not easy, and the results may make a person unnecessarily anxious. A number of factors can affect blood cholesterol levels temporarily. Exercising or eating before the test can raise levels by more than 15 per cent. It is normal for cholesterol levels to fluctuate, so at least two tests, separated by one or two months, are needed to give a reliable average. People who are worried about their cholesterol level should have it checked properly by a doctor.

Although many people with high cholesterol levels have an increased risk of coronary heart disease, the effects of treatment are not straightforward. Treatment has been linked to increases in deaths from other causes (notably violent deaths). Cholesterol testing may be advisable for those at greatest risk (such as those with a family history of high cholesterol levels or coronary heart disease) and who are prepared to change their lifestyle. But many doctors believe that automatic screening and self-testing may turn healthy people into patients chronically anxious about their cholesterol levels. As one medical textbook states ‘It may be healthier not to know’.

(kəles′tərôl)
n

A lipid common to all animal, but not plant, cells. As a sterol, it contains the cyclopentanophenanthrene nucleus. High levels are found in nerve tissue, atheromas, gallstones, and cysts. Normally 140 to 220 mg are present in 100 ml of blood.

Definition

Cholesterol is a fatty substance found in animal tissue and is an important component to the human body. It is manufactured in the liver and carried throughout the body in the bloodstream. Problems can occur when too much cholesterol forms an accumulation of plaque on blood vessel walls, which impedes blood flow to the heart and other organs. The highest cholesterol content is found in meat, poultry, shellfish, and dairy products.

Description

Cholesterol is the Dr. Jekyll and Mr. Hyde of medicine, since it has both a good side and bad side. It is necessary to digest fats from food, make hormones, build cell walls, and participate in other processes for maintaining a healthy body. When people talk about cholesterol as a medical problem, they are usually referring to high cholesterol. This can be somewhat misleading, since there are four components to cholesterol. These are:

• LDL, the so-called bad cholesterol
• HDL, the so-called good cholesterol
• triglycerides, a blood fat lipid that increases the risk for heart disease
• total cholesterol

The U.S. Food and Drug Administration (FDA) estimates that 90 million American adults, roughly one-half of the adult population, have elevated cholesterol levels. High LDL (low-density lipoprotein) is a major contributing factor of heart disease. The cholesterol forms plaque in the heart's blood vessels, which restricts or blocks the supply of blood to the heart, and causes a condition called atherosclerosis. This can lead to a heart attack, resulting in damage to the heart and possibly death.

In 2001, chemical researchers found a link between cholesterol and Alzheimer's disease. Reducing the amount of cholesterol in the cells appears to block attachment of senile plaques to the brain's neurons. (The plaques begin the process that eventually kills brain neurons.) More study remains to test the effects of cholesterol on Alzheimer's.

The population as a whole is at some risk of developing high LDL cholesterol in their lifetimes. Specific risk factors include a family history of high cholesterol, obesity, heart attack or stroke, alcoholism, and lack of regular exercise. The chances of developing high cholesterol increase after the age of 45. One of the primary causes of high LDL cholesterol is too much fat or sugar in the diet, a problem especially true in the United States. Cholesterol also is produced naturally in the liver and overproduction may occur even in people who limit their intake of high cholesterol food. Low HDL and high triglyceride levels also are risk factors for atherosclerosis.

Causes & Symptoms

There are no readily apparent symptoms that indicate high LDL or triglycerides, or low HDL. The only way to diagnose a problem is through a simple blood test. However, one general indication of high cholesterol is obesity. Another is a high-fat diet. In 2001, new research involving twins demonstrated that both genetic factors and diet contribute to cholesterol levels.

Diagnosis

High cholesterol often is diagnosed and treated by general practitioners or family practice physicians. In some cases, the condition is treated by an endocrinologist or cardiologist. Total cholesterol, LDL, HDL, and triglyceride levels as well as the cholesterol to HDL ratio are measured by a blood test called a lipid panel. The cost of a lipid panel is generally $40-100 and is covered by most health insurance and HMO plans, including Medicare, providing there is an appropriate reason for the test. Home cholesterol testing kits are available over the counter but test only for total cholesterol. The results should only be used as a guide and if the total cholesterol level is high or low, a lipid panel should be performed by a physician. In most adults the recommended levels, measured by milligrams per deciliter (mg/dL) of blood, are: total cholesterol, less than 200; LDL, less than 130; HDL, more than 35; triglycerides, 30-200; and cholesterol to HDL ratio, four to one. However, the recommended cholesterol levels may vary, depending on other risk factors such as hypertension, a family history of heart disease, diabetes, age, alcoholism, and smoking.

Doctors have always been puzzled by why some people develop heart disease while others with identical HDL and LDL levels do not. New studies indicate it may be due to the size of the cholesterol particles in the

bloodstream. A test called a nuclear magnetic resonance (NMR) LipoProfile exposes a blood sample to a magnetic field to determine the size of the cholesterol particles. Particle size also can be determined by a centrifugation test, where blood samples are spun very quickly to allow particles to separate and move at different distances. The smaller the particles, the greater the chance of developing heart disease. It allows physicians to treat patients who have normal or close to normal results from a lipid panel but abnormal particle size.

Treatment

The primary goal of cholesterol treatment is to lower LDL to under 160 mg/dL in people without heart disease and who are at lower risk of developing it. The goal in people with higher risk factors for heart disease is less than 130 mg/dL. In patients who already have heart disease, the goal is under 100 mg/dL, according to FDA guidelines. Also, since low HDL levels increase the risks of heart disease, the goal of all patients is more than 35 mg/dL.

In both alternative and conventional treatment of high cholesterol, the first-line treatment options are exercise, diet, weight loss, and stopping smoking. Other alternative treatments include high doses of niacin, soy protein, garlic, algae, and the Chinese medicine supplement Cholestin (a red yeast fermented with rice).

Diet and Exercise

Since a large number of people with high cholesterol are overweight, a healthy diet and regular exercise are probably the most beneficial natural ways to control cholesterol levels. In general, the goal is to substantially reduce or eliminate foods high in animal fat. These include meat, shellfish, eggs, and dairy products. Several specific diet options are beneficial. One is the vegetarian diet. Vegetarians typically get up to 100% more fiber and up to 50% less cholesterol from food than non-vegetarians. The vegetarian low-cholesterol diet consists of at least six servings of whole grain foods, three or more servings of green leafy vegetables, two to four servings of fruit, two to four servings of legumes, and one or two servings of non-fat dairy products daily.

A second diet is the Asian diet, with brown rice being the staple. Other allowable foods include fish, vegetables such as bok choy, bean sprouts, and black beans. It allows for one weekly serving of meat and very few dairy products. The food is flavored with traditional Asian spices and condiments, such as ginger, chilies, turmeric, and soy sauce.

Another regimen is the low glycemic or diabetic diet, which can raise the HDL (good cholesterol) level by as much as 20% in three weeks. Low glycemic foods promote a slow but steady rise in blood sugar levels following a meal, which increases the level of HDL. They also lower total cholesterol and triglycerides. Low glycemic foods include certain fruits, vegetables, beans, and whole grains. Processed and refined foods and sugars should be avoided.

Exercise is an extremely important part of lowering bad cholesterol and raising good cholesterol. It should consist of 20-30 minutes of vigorous aerobic exercise at least three times a week. Exercises that cause the heart to beat faster include fast walking, bicycling, jogging, roller skating, swimming, and walking up stairs. There also are a wide selection of aerobic programs available at gyms or on videocassette.

Garlic

A number of clinical studies have indicated that garlic can offer modest reductions in cholesterol. A 1997 study by nutrition researchers at Pennsylvania State University found men who took garlic capsules for five months reduced their total cholesterol by 7% and LDL by 12%. Another study showed that seven cloves of fresh garlic a day significantly reduced LDL, as did a daily dose of four garlic extract pills. Other studies in 1997 and 1998 back up these results. However, two more recent studies have questioned the effectiveness of garlic in lowering "bad cholesterol."

Cholestin

Cholestin hit the over-the-counter market in 1997 as a cholesterol-lowering dietary supplement. It is a processed form of red yeast fermented with rice, a traditional herbal remedy used for centuries by the Chinese. Two studies released in 1998 showed Cholestin lowered LDL cholesterol by 20-30%%. It also appeared to raise HDL and lower triglyceride levels. Although the supplement contains hundreds of compounds, the major active LDL-lowering ingredient is lovastatin, a chemical also found in the prescription drug Mevacor. The FDA banned Cholestin in early 1998 but a federal district court judge lifted the ban a year later, ruling the product was a dietary supplement, not a drug. It is not fully understood how the substance works and patients may want to consult with their physician before taking Cholestin. No serious side effects have been reported, but minor side effects, including bloating and heartburn, have been reported.

Other Treatments

A study released in 1999 indicated that blue-green algae contains polyunsaturated fatty acids that lower cholesterol. The algae, known as alga Aphanizomenon flosaquae (AFA) is available as an over-the-counter dietary supplement. Niacin, also known as nicotinic acid or vitamin B₃, has been shown to reduce LDL levels by 10-20%, and raise HDL levels by 15-35%. It also can reduce triglycerides. But because an extremely high dose of niacin (2-3 grams) is needed to treat cholesterol problems, it should only be taken under a doctor's supervision to monitor possible toxic side effects. Niacin also can cause flushing when taken in high doses. Soy protein with high levels of isoflavones also have been shown to reduce bad cholesterol by up to 10%. A daily diet that contains 62 mg of isoflavones in soy protein is recommended, and can be incorporated into other diet regimens, including vegetarian, Asian, and low glycemic. In 2003, research revealed that policosanol, a substance made from sugar cane wax or beeswax, lowered LDL cholesterol nearly 27% in study subjects in a Cuban study.

Allopathic Treatment

A wide variety of prescription medicines are available to treat cholesterol problems. These include statins such as Mevacor (lovastatin), Lescol (fluvastatin), Pravachol (pravastatin), Zocor (simvastatin), Baycol (cervastatin), and Lipitor (atorvastatin) to lower LDL. A group of drugs called fibric acid derivatives are used to lower triglycerides and raise HDL. These include Lopid (gemfibrozil), Atromid-S (clofibrate), and Tricor (fenofibrate).

A new class of drugs was identified late in 2001 that work differently from the statin drugs. These drugs rely on compounds that bind to a sterol that regulates protein (called SCAP) and speds up removal of cholesterol from the plasma (the fluid part of the blood.) Doctors decide which drug to use based on the severity of the cholesterol problem, side effects, and cost.

Expected Results

High cholesterol is one of the key risk factors for heart disease. Left untreated, too much bad cholesterol can clog the blood vessels, leading to chest pain (angina), blood clots, and heart attacks. Heart disease is the number one killer of men and women in the United States. By reducing LDL, people with heart disease may prevent further heart attacks and strokes, prolong and improve the quality of their lives, and slow or reverse cholesterol buildup in the arteries. In people without heart disease, lowering LDL can decrease the risk of a first heart attack or stroke.

Prevention

The best way to prevent cholesterol problems is through a combination of healthy lifestyle activities, a primarily low-fat and high-fiber diet, regular aerobic exercise, not smoking, and maintaining an optimal weight. In a small 2003 Canadian study, people who ate a low-fat vegetarian diet consisting of foods that are found to help lower cholesterol dropped their levels of LDL cholesterol as much as results from some statin drugs. But for people with high risk factors for heart disease, such as a family history of heart disease, diabetes, and being over the age of 45, these measures may not be enough to prevent the onset of high cholesterol. There are studies being done on the effectiveness of some existing anti-cholesterol drugs for controlling cholesterol levels in patients who do not meet the criteria for high cholesterol but no definitive results are available.

Resources

Books

Bratman, Steven and David Kroll. Natural Pharmacist: Natural Treatments for High Cholesterol. Roseville, CA: Prima Publishing, 2000.

Ingels, Darin. The Natural Pharmacist: Your Complete Guide to Garlic and Cholesterol. Roseville, CA: Prima Publishing, 1999.

Murray, Michael T. Natural Alternatives to Over-the-Counter and Prescription Drugs New York: William Morrow & Co., 1999.

Trubo, Richard. Cholesterol Cures: From Almonds and Antioxidants to Garlic, Golf, Wine and Yogurt. Emmaus, PA: Rodale Press, 1996.

Periodicals

"Both Genetics and Diet Influence Cholesterol Levels." Heart Disease Weekly (October 14, 2001).

Carter, Ann. "Cholesterol in Your Diet." Clinical Reference Systems (July 1, 1999): 282.

"Chemical Engineers Suggest Alzheimer Onset Tied to Cholesterol." Pain and Central Nervous System Week (December 24, 2001):3.

"Eating a Vegetarian Diet that Includes Cholesterol-lowering Foods may Lower Lipid Levels as Much as Some Medications." Environmental Nutrition (March 2003):8.

"Researchers Identify New Class of Cholesterol-Lowering Drugs." Heart Disease Weekly (December 23, 2001):14.

Sage, Katie. "Cut Cholesterol with Policosanol: This Supplement Worked Better than a Low-fat Diet in One Study." Natural Health (March 2003):32.

Marandino, Cristin. "The Case for Cholesterol." Vegetarian Times (August 1999): 10.

Schmitt, B.D. "Treating High Cholesterol Levels." Clinical Reference Systems (July 1, 1999): 1551.

VanTyne, Julia, and Davis, Lori. "Drop Your Cholesterol 25 to 100 Points." Prevention (November 1999): 110.

Organizations

National Cholesterol Education Program. NHLBI Information Center, P.O. Box 30105, Bethesda, MD 20824-0105. http://www.nhlbi.nih.gov.

[Article by: Ken R. Wells; Teresa G. Odle]

Waxy organic compound found in blood and all animal tissues. It is a steroid, with molecular formula C₂₇H₄₆O, containing four rings in its structure. Cholesterol is essential to life; it is a primary component of cell membranes and a starting or intermediate material from which the body makes bile acids, other steroid hormones, and vitamin D. It is made in the liver and some other organs, in greater or lesser amounts depending on the amount recently consumed in the diet. It circulates in the blood in compounds called lipoproteins, since it is not water-soluble alone. Excess cholesterol in the blood forms deposits in arteries (see arteriosclerosis), which can lead to coronary heart disease. Michael Brown (born 1941) and Joseph Goldstein (born 1940) won a Nobel Prize in 1985 for their work in discovering this process. Since the body makes cholesterol from fats, blood cholesterol cannot be reduced by limiting only the amount of cholesterol in the diet; the amount of fat, especially saturated fat (see saturation, fatty acid) must also be reduced. See also triglyceride.

For more information on cholesterol, visit Britannica.com.

A lipid-related compound found in tissues and manufactured in the liver. Cholesterol has a number of essential functions: it is important for body tissue repair; it is a constituent of cell membranes which it helps strengthen; it forms the starting point of steroid manufacture and is involved in the formation of several hormones; it forms bile salts; and it is the raw material for vitamin D. Dietary sources of cholesterol include animal products such as eggs, meat, and cheese. A high-fat diet can increase blood cholesterol levels. Nicotine (from tobacco smoking) increases deposition of cholesterol in arterial walls, but hereditary factors are also important determinants of blood cholesterol levels. High blood cholesterol levels are associated with cardiovascular diseases. Blood cholesterol levels can be decreased by taking regular aerobic exercise and eating a low-fat diet. A deficiency of cholesterol is rare. See also high-density lipoproteins, low-density lipoproteins.

(kəlĕs'tərōl') , fatty lipid found in the body tissues and blood plasma of vertebrates; it is only sparingly soluble in water, but much more soluble in some organic solvents. A steroid, cholesterol can be found in large concentrations in the brain, spinal cord, and liver. The liver is the most important site of cholesterol biosynthesis, although other sites include the adrenal glands and reproductive organs. By means of several enzymatic reactions, cholesterol is synthesized from acetic acid; it then serves as 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. The insolubility of cholesterol in water is also a factor in the development of atherosclerosis (see arteriosclerosis), the pathological deposition of plaques of cholesterol and other lipids on the insides of major blood vessels, a condition associated with coronary artery disease. This buildup of cholesterol in the blood vessels may constrict the passages considerably and inhibit the flow of blood to and from the heart. Recent research has shown that the relative abundance of certain protein complexes, called lipoproteins, to which cholesterol becomes attached may be the real cause of cholesterol buildup in the blood vessels. High-density lipoprotein (HDL) carries cholesterol out of the bloodstream for excretion, while low-density lipoprotein (LDL) carries it back into the system for use by various body cells. Researchers believe that HDL and LDL levels in the bloodstream may be at least as important as cholesterol levels, and now measure both to determine risk for heart disease. Reducing consumption of foods containing cholesterol and saturated fat has been found to lower blood cholesterol levels. Cholesterol levels can also be reduced with drugs, most especially with HMG-CoA reductase inhibitors (commonly called “statins”), such as lovastatin (Mevacor) and atorvastatin (Lipitor), and by regular exercise.

Cholesterol is one of the most widely disseminated organic compounds in the animal kingdom. Almost three hundred years ago, Antonio Vallisnieri observed that gallstones were soluble in turpentine or alcohol. Poulletier de la Salle, some thirty years later, demonstrated that the main constituent of gallstones could be crystallized from alcohol. This substance was thought to be a wax until 1815, when Michel Eugène Chevreul showed that it was not saponifiable and gave it the name "cholesterine" derived from the Greek chole, bile, and steros, solid. Soon thereafter, it was isolated from blood, brain, tumors, and egg yolk. The isolated compounds were shown to be identical. In 1843 Vogel found it in atherosclerotic arteries.

The chemical structure of cholesterol was elucidated over the years beginning in 1859. The compound was shown to contain a secondary hydroxyl group and a double bond. The exact empirical formula (C₂₇H₄₆O) was established in 1888 by Friedrich Reinitzer. Proof of structure was obtained chiefly through the brilliant work of Adolf Windaus and Heinrich Wieland. The structure of cholesterol suggested by Windaus and Wieland in the 1920s was incorrect, but that does not detract in any way from their contribution. The true structure was established in the 1930s based on X-ray diffraction data.

There were many suggestions regarding the biological synthesis of cholesterol. The biosynthetic pathway became accessible with the introduction of radioactive carbon in the 1940s. The biosynthetic scheme was generally elucidated by the work of Konrad Bloch, George Popjak, and John Cornforth. It was first shown that cholesterol could be synthesized in mammals and ergosterol in yeast from small organic molecules. Eventually it was shown that all twenty-seven carbon atoms of cholesterol were derived from the two carbon atoms of acetate. The methyl group of acetate contributed fifteen of the twenty-seven carbons of cholesterol and the carboxyl group contributed twelve. The pathway began with the condensation of two acetate residues to give acetoacetate and addition of one more two-carbon moiety to yield hydroxymethylglutaric acid (HMG). HMG lost a carbon atom and the resulting compound rearranged to provide an isoprene unit. Two five-carbon units combined to give a geranyl derivative that added another isoprene to give a farnesyl unit. Two farnesyl units united to provide squalene (C₃₀H₅₀), a hydrocarbon found in the livers of some species of shark that cyclyzed to yield lanosterol, a thirty-carbon atom sterol also found in sheep wool. In a series of rearrangements and demethylations, lanosterol yielded cholesterol. The key step in this complex synthetic pathway involves the reduction of HMG-CoA. Inhibition of HMG-CoA reductase is the basis of a number of potent new serum cholesterol-lowering drugs.

Cholesterol represents about 0.2 percent of the weight of the human body. As Table 1 shows, the bulk of the body's cholesterol is present in two tissues; one is the brain and nerve tissue, the other is muscle. In the brain, cholesterol is thought to act as an insulator, but there have been relatively few studies of the metabolism of brain cholesterol. The next large reservoir of cholesterol is muscle. Between them, nervous tissue and muscle carry 44 percent of the body's cholesterol. The cholesterol in these reservoirs turns over slowly.

Cholesterol is ubiquitous in the human body, where it plays structural and metabolic roles. Together with phospholipid, cholesterol is present in every cell membrane. In the adrenals, cholesterol is converted to adrenocortical hormones such as cortisone. In the gonads, cholesterol is converted to the appropriate sex hormone—estradiol in women, testosterone in men. The cholesterol in skin is the precursor of 7-dehydrocholesterol, which is ultimately converted to vitamin D. The major catabolic products of cholesterol are the bile acids—cholic and chenodeoxycholic. These are designated as the primary bile acids; they are metabolized in the liver to deoxycholic and lithocholic acids. It has been estimated that over 90 percent of biologically synthesized cholesterol is metabolized to bile acids. In general, the body synthesizes more cholesterol than it ingests.

Table 1

In 1912 Nicolai Anitschkow showed that cholesterolfed rabbits developed aortic deposits similar to early human atherosclerosis. His experiments presented a possible explanation of human atherosclerosis and that particular debate has not yet abated. Simultaneously with Anitschkow's studies, A. I. Ignatowski demonstrated the atherogenic potential of animal protein, but compared to work on cholesterol and fat there has only been a desultory interest in protein effects.

Since Anitschkow's results were obtained by dietary manipulation, the view that dietary cholesterol was implicated in atherogenesis was accepted generally. With development of simple, rapid methods of cholesterol analysis, it became possible to screen populations for blood cholesterol content. Large epidemiological studies were launched and their results helped to develop the concept of risk factors for heart disease. Currently, the major risk factors are hypercholesterolemia, hypertension, smoking, obesity, and maleness. However, emerging data suggest that homocysteinemia and inflammation (due to infection with cytomegalovirus or chlamydia pneumoniae) are also important factors.

When cholesterol is ingested, it is emulsified with phospholipid and absorbed. The absorbed lipid circulates in the blood as a water soluble lipid-protein complex called lipoprotein. Initially, absorbed cholesterol is part of a large, triglyceride-rich particle called the chylomicron. In the course of circulation, the triglyceride is removed by activity of cellular lipases and the particles become smaller and their cholesterol content increases. The cholesterol-containing, lipid-protein complex consists of several fractions that are separable by virtue of their hydrated densities. In general terms, the four major fractions are the triglyceride-rich chylomicrons and very low density (VLDL), the cholesterol-rich low density (LDL), and the protein-rich high density (HDL).

Due to development by John Gofman of methods for ultracentrifugal separation of lipoproteins, researchers have been able to isolate and study lipoproteins. The cholesterol-rich low density lipoproteins (LDL) are thought to be major risk factors for coronary disease. It was demonstrated that oxidized LDL is the real villain in coronary disease. It also was shown that LDL can be subfractionated into small, dense and large "fluffy" particles. The small particles appear to infiltrate the artery preferentially. Researchers also know that the process of atherogenesis is not simple and is mediated by an array of small proteins. The high-density lipoproteins are about 50 percent protein. In the simplest terms, LDL facilitates entry of cholesterol into cells and HDL facilitates its removal. LDL receptors on the cell surface facilitate LDL uptake. The proteins of lipoproteins are very important because they provide recognition by cells, and it is now becoming evident that genetic differences in apolipoproteins may dictate susceptibility to disease as well as chances for the efficacy of medication.

The effects of dietary cholesterol became a concern shortly after Anitschkow's observation and warnings regarding excess levels of cholesterol intake, which constitute one of the foundations of dietary therapy. Since cholesterol occurs only in food of animal origin, it was a simple extension to seek an explanation of the role of cholesterol by examining the lipids of food from animal sources. Although no dietary fat is totally saturated or unsaturated, attention also turned to effects of fat saturation.

The amount of cholesterol in the average American diet is in the range of 300–350 mg/day. It used to be much higher. The levels of cholesterol in a number of common animal foods are given in Table 2. It is evident that most muscle contains about the same amount of cholesterol, 81 ± 7 mg/100g. Cholesterol content of butter (per 100 g) is high, but we rarely eat more than 5–10 g of butter per meal. Shrimp is high in cholesterol but very low in fat. Eggs are also high in cholesterol. Continuing research nevertheless indicates that the cholesterol level of a food per se has little effect on serum cholesterol levels. The cholesterolemic effect is a function of dietary fat saturation. It has been shown that the absorption of cholesterol is more a function of the accompanying dietary fat than of cholesterol itself. Saturated dietary fat leads to higher cholesterol levels than does unsaturated fat. This observation is true for most people who are called "non-responders" (to dietary cholesterol). A small number of people are "responders," meaning they absorb more cholesterol, regardless of accompanying fat. In the late 1960s, Keys and Hegsted developed formulas for estimating changes in serum cholesterol based upon changes in dietary fat. There have been a number of more complex formulas developed, but the originals are referred to most often today. Essentially, they found saturated fatty acids to be hypercholesterolemic and unsaturated fatty acids to lower cholesterol. Stearic acid was considered neutral. The polyunsaturated fats lower cholesterol across the board so that HDL cholesterol (the "good" cholesterol) falls as does LDL cholesterol. Oleic acid seems to affect only LDL cholesterol. The reduction in total cholesterol may not be as profound, but the LDL/HDL cholesterol ratio is improved. Recent findings show that the structure of individual triglycerides may also influence their atherogenicity.

In summary, cholesterol is a substance that appears in all cells and also has a number of metabolic functions.

Table 2

It is synthesized in the body and is part of every cell membrane. Cholesterol is metabolized to adrenocortical or sex hormones, bile acids, and vitamin D. Levels of serum cholesterol are related to risk of coronary disease, but it should be borne in mind that cardiovascular disease is a metabolic disease, not one of cholesterol deposition. Dietary cholesterol is absorbed, but its effects on serum cholesterol are slight. Generally, there is an increase of about 2 mg of serum cholesterol for every 100 mg ingested. Cholesterol should be viewed as a chemical necessary for life and not as a toxic substance. As with so many other aspects of life, moderation is the key.

Bibliography

Gibbons, G. F., K. A. Mitropoulos, and Nick B. Myant. Biochemistry of Cholesterol. Amsterdam: Elsevier Biomedical Press, 1982.

Howell, Wanda H., et al. "Plasma Lipid and Lipoprotein Responses to Dietary Fat and Cholesterol: A Meta Analysis." American Journal of Clinical Nutrition 65 (1997): 1747–1764.

Keys, Ancel, Joseph T. Anderson, and Francisco Grande. "Serum Cholesterol Response to Changes in Diet, IV: Particular Fatty Acids in the Diet." Metabolism 14 (1965): 776–787.

Kritchevsky, David. Cholesterol. New York: Wiley, 1958.

Kritchevsky, David. "Food Lipids and Atherosclerosis." In Food Lipids and Health, edited by Richard E. McDonald and David B. Min. New York: M. Dekker, 1996.

Leinoneu, M. "Chlamydia pneumoniae and Other Risk Factors for Atherosclerosis." Journal of Infectious Diseases 181, Suppl. 3 (2000): S414–S416.

Myant, Nick B. The Biology of Cholesterol and Related Steroids. London: Heinemann Medical Books, 1981.

Myant, Nick B. Cholesterol Metabolism, LDL, and the LDL Receptor. San Diego, Calif.: Academic Press, Inc., 1990.

—David Kritchevsky

(kuh-les-tuh-rawl, kuh-les-tuh-rohl)

A white soapy substance found in the tissues of the body and in certain foods, such as animal fats, oils, and egg yolks. Cholesterol has been linked to heart disease and atherosclerosis. (It collects on the walls of arteries and interferes with the flow of blood.) High levels of cholesterol in the blood are considered to be unhealthy. (See saturated fats, HDL, and LDL.)

A steroid alcohol found in animal fats and oils, bile, blood, brain tissue, milk, egg yolk, myelin sheaths of nerve fibers, liver, kidneys and adrenal glands. It is a necessary component of all cell surface and intracellular membranes and a constituent of myelin in nervous tissue; it is a precursor of bile acids and steroid hormones, and it occurs in the most common type of gallstone, in atheroma of the arteries, in various cysts, and in carcinomatous tissue. Most of the body's cholesterol is synthesized, but some is obtained in the diet.
The preoccupation in human medicine with the relationship between cholesterol and the development of atheromatous plaques in the coronary arteries is not reflected in veterinary medicine. The importance of cholesterol to the veterinarian is limited to the measurement of blood cholesterol levels as an indicator of liver disease or thyroid activity.

• c. pneumonia — see endogenous-lipid pneumonia.

IN BRIEF: A waxy substance found in the body and in certain foods.

Having high cholesterol is unhealthy for many people.

Cholesterol
IUPAC name	(10R,13R)-10,13-dimethyl-17-(6-methylheptan-2-yl)- 2,3,4,7,8,9,11,12,14,15,16,17-dodecahydro-1H- cyclopenta[a]phenanthren-3-ol
Identifiers
CAS number	57-88-5
PubChem	5997
SMILES	CC(C)CCCC(C)C1CCC2C1 (CCC3C2CC=C4C3(CCC(C4)O)C)C
Properties
Molecular formula	C27H46O
Molar mass	386.654
Appearance	white crystalline powder [1]
Melting point	148-150 °C [1]
Boiling point	360 °C (decomposes)
Solubility in water	0.095 mg/l (30 °C)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Cholesterol is a sterol (a combination steroid and alcohol), a lipid found in the cell membranes of all tissues, and is transported in the blood plasma of all animals. Because cholesterol is synthesized by all eukaryotes, trace amounts of cholesterol are also found in membranes of plants and fungi.

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".^[2]

Most of the cholesterol is synthesized by the body and some has dietary origin. 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. It plays a central role in many biochemical processes, such as the composition of cell membranes and the synthesis of steroid hormones. Cholesterol is insoluble in blood, but is transported in the circulatory system bound to one of the varieties of lipoprotein, spherical particles which have an exterior composed mainly of water-soluble proteins. The main types, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) carry cholesterol from and to the liver.

According to the lipid hypothesis, abnormally high cholesterol levels (hypercholesterolemia) and abnormal proportions of LDL and HDL are associated with cardiovascular disease by promoting atheroma development in arteries (atheroscleroris). This disease process leads to myocardial infarction (heart attack), stroke and peripheral vascular disease. As high LDL contributes to this process, it is termed "bad cholesterol", while high levels of HDL ("good cholesterol") offer a degree of protection. The balance can be redressed with exercise, a healthy diet, and sometimes medication.

Physiology

Function

Cholesterol is required to build and maintain cell membranes; it regulates membrane fluidity over a wide range of temperatures. The hydroxyl group on cholesterol interacts with the phosphate head of the membrane, while the bulky steroid and the hydrocarbon chain is embedded in the membrane. Some research indicates that cholesterol may act as an antioxidant.^[3] 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. It 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).

Recently, cholesterol has also been implicated in cell signalling processes, where it has been suggested that it forms lipid rafts in the plasma membrane. It also reduces the permeability of the plasma membrane to hydrogen ions (protons) and sodium ions.^[4]

Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including the caveolae-dependent endocytosis and clathrin-dependent endocytosis. The role of cholesterol in caveolae-dependent and clathrin-dependent endocytosis can be investigated by using methyl beta cyclodextrin (MβCD) to remove cholesterol from the plasma membrane.

Synthesis and intake

The HMG-CoA reductase pathway

Cholesterol is required in the membrane of mammalian cells for normal cellular function, and is either synthesized in the endoplasmic reticulum, or derived from the diet, in which case it is delivered by the bloodstream in low-density lipoproteins. These are taken into the cell by LDL receptor-mediated endocytosis in clathrin-coated pits, and then hydrolysed in lysosomes.

Cholesterol is primarily synthesized from acetyl CoA through the HMG-CoA reductase pathway in many cells and 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. For a person of about 150 pounds (68 kg), typical total body content is about 35 g, typical daily internal production is about 1 g and typical daily dietary intake is 200 to 300 mg in the United States and societies adopting its dietary patterns. Of the cholesterol input to the intestines via bile production, 92-97% is reabsorbed in the intestines and recycled via enterohepatic circulation.

Konrad Bloch and Feodor Lynen shared the Nobel Prize in Physiology or Medicine in 1964 for their discoveries concerning the mechanism and regulation of the cholesterol and fatty acid metabolism.

Regulation

Biosynthesis of cholesterol is directly regulated by the cholesterol levels present, though the homeostatic mechanisms involved are only partly understood. A higher intake from food leads to a net decrease in endogenous production, while lower intake from food has the opposite effect. 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 Insig1. 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 SRE (sterol regulatory element) of a number of genes to stimulate their transcription. Among the genes transcribed are the LDL receptor and HMG-CoA reductase. The former scavenges circulating LDL from the bloodstream, whereas HMG-CoA reductase leads to an increase of endogenous production of cholesterol.^[5]

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.^[5]

The average amount of blood cholesterol varies with age, typically rising gradually until one is about 60 years old. There appear to be seasonal variations in cholesterol levels in humans, more, on average, in winter.^[6]

Excretion

Cholesterol is excreted from the liver in bile and reabsorbed from the intestines. Under certain circumstances, when more concentrated, as in the gallbladder, it crystallises and is the major constituent of most gallstones, although lecithin and bilirubin gallstones also occur less frequently.

Body fluids

Cholesterol is minimally soluble in water; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream by lipoproteins - protein "molecular-suitcases" that are water-soluble and carry cholesterol and triglycerides internally. The apolipoproteins 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 fats in the form of triglycerides and cholesterol. In the liver, chylomicron particles release triglycerides and some cholesterol. The liver converts unburned food metabolites into very low density lipoproteins (VLDL) and secretes them into plasma where they are converted to low-density lipoprotein (LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, the relatively few LDL particles are large. In contrast, large numbers of small dense LDL (sdLDL) particles are strongly associated with the presence of atheromatous disease within the arteries. For this reason, LDL is referred to as "bad cholesterol".

The 1987 report of National Cholesterol Education Program, Adult Treatment Panels suggest the total blood cholesterol level should be: <200 mg/dl normal blood cholesterol, 200-239 mg/dl borderline-high, >240 mg/dl high cholesterol.^[7]

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, and hence it is commonly called "good cholesterol". In contrast, having small amounts of large HDL particles is independently associated with atheromatous disease progression within the arteries.

Clinical significance

Hypercholesterolemia

Conditions with elevated concentrations of oxidized LDL particles, especially "small dense LDL" (sdLDL) particles, are associated with atheroma formation in the walls of arteries, a condition known as atherosclerosis, which is the principal cause of coronary heart disease and other forms of cardiovascular disease. 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.

Elevated levels of the lipoprotein fractions, LDL, IDL and VLDL are regarded as atherogenic (prone to cause atherosclerosis).^{[citation needed]} Levels of these fractions, rather than the total cholesterol level, correlate with the extent and progress of atherosclerosis. Conversely, the total cholesterol can be within normal limits, yet be made up primarily of small LDL and small HDL particles, under which conditions atheroma growth rates would still be high. In contrast, however, if LDL particle number is low (mostly large particles) and a large percentage of the HDL particles are large, then atheroma growth rates are usually low, even negative, for any given total cholesterol concentration.^{[citation needed]}

Multiple human trials utilizing HMG-CoA reductase inhibitors, known as statins, have repeatedly confirmed that changing lipoprotein transport patterns from unhealthy to healthier patterns significantly lowers cardiovascular disease event rates, even for people with cholesterol values currently considered low for adults. As a result, people with a history of cardiovascular disease may derive benefit from statins irrespective of their cholesterol levels,^[8] and in men without cardiovascular disease there is benefit from lowering abnormally high cholesterol levels ("primary prevention").^[9] Primary prevention in women is practiced only by extension of the findings in studies on men,^[10] since in women, none of the large statin trials has shown a reduction in overall mortality or in cardiovascular end points.^[11]

The American Heart Association provides a set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease:^[12]

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 separately, this simplistic view has become somewhat outdated. The desirable LDL level is considered to be less than 100 mg/dL (2.6 mmol/L)^{[citation needed]} , although a newer target of <70 mg/dL can be considered in higher risk individuals based on some of the above-mentioned trials. A ratio of total cholesterol to HDL — another useful measure — of far less than 5:1 is thought to be healthier. Of note, typical LDL values for children before fatty streaks begin to develop is 35 mg/dL.

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.

Given the well-recognized role of cholesterol in cardiovascular disease, it is surprising that some studies have shown an inverse correlation between cholesterol levels and mortality in subjects over 50 years of age — an 11% increase overall and 14% increase in CVD mortality per 1 mg/dL per year drop in cholesterol levels. In the Framingham Heart Study, the researchers attributed this phenomenon to the fact that people with severe chronic diseases or cancer tend to have below-normal cholesterol levels.^[13] This explanation is not supported by the Vorarlberg Health Monitoring and Promotion Programme, in which men of all ages and women over 50 lower cholesterol levels with very low cholesterol were increasingly likely to die of cancer, liver diseases, and mental diseases. This result indicates that the low cholesterol effect occurs even among younger respondents, contradicting the previous assessment among cohorts of older people that this is a proxy or marker for frailty occurring with age.^[14]

A small group of scientists, united in The International Network of Cholesterol Skeptics, continues to question the link between cholesterol and atherosclerosis.^[15] However, the vast majority of doctors and medical scientists accepts the link as fact.^[16]

Hypocholesterolemia

Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, and while some studies suggest a link with depression, cancer and cerebral hemorrhage it is unclear whether the low cholesterol levels are a cause for these conditions or an epiphenomenon[1].

Food sources

Cholesterol is found in animal fats: all food containing animal fats contains cholesterol; food not containing animal fats either contains no cholesterol or negligible amounts. Major dietary sources of cholesterol include eggs, beef and poultry.^[17]

Plants have trace amounts of cholesterol, so even a vegan diet, which includes no animal foods, has traces of cholesterol. However, the amounts are very small. For example, to ingest the amount of cholesterol in one egg, one would need to drink about 9.6 litres (19.57 pounds) of pure peanut oil.^{[12] [18]}

Plant products (e.g. flax seed, peanut), also contain cholesterol-like compounds, phytosterols, which are suggested to help lower serum cholesterol.^[19]

Cholesteric liquid crystals

Some cholesterol derivatives, (among other simple cholesteric lipids) are known to generate the liquid crystalline cholesteric phase. 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 temperature-sensitive paints.

See also

• Triglycerides
• Diet and heart disease

Additional images

References

External links

• Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults US National Institutes of Health Adult Treatment Panel III
• Aspects of fat digestion and metabolism - UN/WHO Report 1994
• American Heart Association - "About Cholesterol"
• Heart and Stroke Foundation of Canada - "What is blood cholesterol?"

Cholestanes, membrane lipids: sterols
Adosterol - Cholecalciferol/Ergocalciferols - Cholesterol - Dihydrotachysterol - Fusidic acid - Lanosterol - Phytosterols - Solanine

zh-yue:膽固醇

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - kolesterol

Nederlands (Dutch)
cholesterol

Français (French)
n. - cholestérol

Deutsch (German)
n. - Cholesterin, Gallenfett

Ελληνική (Greek)
n. - (βιολ.) χοληστερόλη

Italiano (Italian)
colesterolo

Português (Portuguese)
n. - colesterol (m) (Quím.)

Русский (Russian)
холестерин

Español (Spanish)
n. - colesterol

Svenska (Swedish)
n. - kolesterol

中文（简体） (Chinese (Simplified))
胆固醇

中文（繁體） (Chinese (Traditional))
n. - 膽固醇

한국어 (Korean)
n. - 콜레스테롤

日本語 (Japanese)
n. - コレステロール

العربيه (Arabic)
‏(الاسم) كولسترول, دهن الصفراء, شحم المرارة‏

עברית (Hebrew)
n. - ‮כוהל מוצק שריכוז גבוה שלו בדם עלול לגרום הסתיידות עורקים, כולסטרול‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.

Best of the Web: cholesterol

Some good "cholesterol" pages on the web:

How? home.howstuffworks.com

Copyrights:

		Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
		Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
		World of the Body. The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved.  Read more
		Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved.  Read more
		Food and Fitness. Food and Fitness: A Dictionary of Diet and Exercise. Copyright © 1997, 2003 by Oxford University Press. All rights reserved.  Read more
		Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.  Read more
		Alternative Medicine Encyclopedia. Encyclopedia of Alternative Medicine. Copyright © 2005 by The Gale Group, Inc. All rights reserved.  Read more
		Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
		Sports Science and Medicine. The Oxford Dictionary of Sports Science & Medicine. Copyright © Michael Kent 1998, 2006, 2007. All rights reserved.  Read more
		Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
		Food & Culture Encyclopedia. Encyclopedia of Food and Culture. Copyright © 2003 by The Gale Group, Inc. All rights reserved.  Read more
		Health Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
		Veterinary Dictionary. The Veterinary Dictionary. Copyright © 2007 by Elsevier. All rights reserved.  Read more
		Word Tutor. Copyright © 2004-present by eSpindle Learning, a 501(c) nonprofit organization. All rights reserved. eSpindle provides personalized spelling and vocabulary tutoring online; free trial.  Read more
		Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cholesterol".  Read more
		Translations. Copyright © 2007, WizCom Technologies Ltd. All rights reserved.  Read more