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. In addition, cholesterol is an important component for the manufacture of bile acids, steroid hormones, 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 eukaryotes such as plants and fungi. It is almost completely absent among prokaryotes, i.e. bacteria.
Function
Cholesterol is required to build and maintain membranes; it modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group on cholesterol interacts with the polar head groups of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty acid chain of the other lipids. Through the interaction with the phospholipid fatty acid chains, cholesterol increases membrane packing, which reduces membrane fluidity. In this structural role, cholesterol reduces the permeability of the plasma membrane to neutral solutes, protons, (positive hydrogen ions) and sodium ions.
Within the cell membrane, cholesterol also functions in intracellular transport, cell signaling and nerve conduction.
Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including caveola-dependent and clathrin-dependent endocytosis. The role of cholesterol in such endocytosis can be investigated by using methyl beta cyclodextrin (MβCD) to remove cholesterol from the plasma membrane. Recently, cholesterol has also been implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane. Lipid raft formation brings receptor proteins in close proximity with high concentrations of second messenger molecules. In many neurons, a myelin sheath, rich in cholesterol, since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses.
Within cells, cholesterol is the precursor molecule in several biochemical pathways. In the liver, cholesterol is converted to bile, which is then stored in the gallbladder. Bile contains bile salts, which solubilize fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat-soluble vitamins, A, D, E, and K. Cholesterol is an important precursor molecule for the synthesis of vitamin D and the steroid hormones, including the adrenal gland hormones cortisol and aldosterone, as well as the sex hormones progesterone, estrogens, and testosterone, and their derivatives.
Some research indicates cholesterol may act as an antioxidant.
Dietary sources of
CholesterolAnimal fats are circuitous mixtures of triglycerides, with bottom amounts of phospholipids and cholesterol. As a consequence, all foods absolute beastly fat accommodate cholesterol to capricious extents. Major comestible sources of cholesterol cover cheese, egg yolks, beef, pork, poultry, fish, and shrimp. Human breast milk aswell contains cogent quantities of cholesterol.
From a comestible perspective, cholesterol is not begin in cogent amounts in bulb sources.In addition, bulb articles such as beat seeds and atom accommodate cholesterol-like compounds alleged phytosterols, which are believed to attempt with cholesterol for assimilation in the intestines. Phytosterols can be supplemented through the use of phytosterol-containing anatomic foods or nutraceuticals that are broadly accustomed as accepting a accurate LDL cholesterol-lowering efficacy. Current added guidelines acclaim doses of phytosterols in the 1.6-3.0 grams per day ambit (Health Canada, EFSA, ATP III,FDA) with a contempo meta-analysis demonstrating an 8.8% abridgement in LDL-cholesterol at a beggarly dosage of 2.15 gram per day. However, the allowances of a diet supplemented with phytosterol has been questioned.
Total fat assimilation aswell plays a role in claret cholesterol levels. This aftereffect is anticipation to appear about by changes in the abundance of cholesterol and lipoproteins that are actinic by the body. In particular, saturated, monounsaturated and polyunsaturated fats accept been apparent to access HDL-based cholesterol levels, with saturated fats aswell accretion LDL-based cholesterol levels. Trans fats accept been apparent to abate levels of HDL whilst accretion levels of LDL. Based on such affirmation and affirmation implicating low HDL and top LDL levels in cardiovascular ache (see Hypercholesterolemia), abounding bloom authorities apostle abbreviation LDL cholesterol through changes in diet in accession to added affairs modifications. The USDA for archetype recommends that those adulatory to abate their cholesterol through a change in diet should aim to absorb beneath than 7% of their circadian activity needs from saturated fat and beneath than 200 mg of cholesterol per day. An another appearance is that any abridgement to comestible cholesterol assimilation could be counteracted by the organs compensating to try to accumulate claret cholesterol levels constant.
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