What is Cholesterol?
Cholesterol, from the Greek chole- (bile) and stereos (solid) followed by the chemical suffix -ol for an alcohol, is an organic chemical substance classified as a waxy steroid of fat. It is an essential structural component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity and is thus manufactured by every cell.
In addition to its importance within cells, cholesterol also serves as a precursor for the biosynthesis of steroid hormones, bile acids, and vitamin D. Cholesterol is the principal sterol synthesized by animals; in vertebrates it is formed predominantly in the liver. Small quantities are synthesized in other cellular organisms (eukaryotes) such as plants and fungi. It is almost completely absent among prokaryotes (i.e., bacteria).
François Poulletier de la Salle first identified cholesterol in solid form in gallstones in 1769. However, it was only in 1815 that chemist Eugčne Chevreul named the compound "cholesterine".
Animal fats are complex mixtures of triglycerides, with lesser amounts of phospholipids and cholesterol. As a consequence, all foods containing animal fat contain cholesterol to varying extents. Major dietary sources of cholesterol include cheese, egg yolks, beef, pork, poultry, fish, and shrimp. Human breast milk also contains significant quantities of cholesterol.
From a dietary perspective, cholesterol is not found in significant amounts in plant sources. In addition, plant products such as flax seeds and peanuts contain cholesterol-like compounds called phytosterols, which are believed to compete with cholesterol for absorption in the intestines. Phytosterols can be supplemented through the use of phytosterol-containing functional foods or nutraceuticals that are widely recognized as having a proven LDL cholesterol-lowering efficacy. Current supplemental guidelines recommend doses of phytosterols in the 1.6-3.0 grams per day range (Health Canada, EFSA, ATP III,FDA) with a recent meta-analysis demonstrating an 8.8% reduction in LDL-cholesterol at a mean dose of 2.15 gram per day. However, the benefits of a diet supplemented with phytosterol has been questioned.
Fat intake also plays a role in blood-cholesterol levels. This effect is thought to come about by changes in the quantity of cholesterol and lipoproteins that are synthesized by the body. Isocalorically replacing dietary carbohydrates with monounsaturated and polyunsaturated fats has been shown to lower serum LDL and total cholesterol levels and increase serum HDL levels, while replacing carbohydrates with saturated fat was shown to increase HDL, LDL, and total cholesterol levels. Trans fats have been shown to reduce levels of HDL while increasing levels of LDL. Based on such evidence and evidence implicating low HDL and high LDL levels in cardiovascular disease (see Hypercholesterolemia), many health authorities advocate reducing LDL cholesterol through changes in diet in addition to other lifestyle modifications. The USDA, for example, recommends that those wishing to reduce their cholesterol through a change in diet should aim to consume less than 7% of their daily energy needs from saturated fat and fewer than 200 mg of cholesterol per day. An alternative view is that any reduction to dietary cholesterol intake could be counteracted by the organs compensating to try to keep blood cholesterol levels constant.
However, the The China Study uses epidemiological evidence to claim that casein raises blood cholesterol even more than the ingested saturated fat or cholesterol.
This definition may contain information from Wikipedia.