audio|Triglyceride.ogg|Triglyceride (more properly known as audio|Triacylglycerol.ogg|triacylglycerol, TAG or triacylglyceride) is
glyceridein which the glycerolis esterified with three fatty acids. [cite web | title=Nomenclature of Lipids |publisher=IUPAC-IUB Commission on Biochemical Nomenclature (CBN) | url=http://www.chem.qmul.ac.uk/iupac/lipid/ |accessdate=2007-03-08] It is the main constituent of vegetable oiland animal fats.
Triglycerides are formed from a single molecule of glycerol, combined with three fatty acids on each of the OH groups, and make up most of fats digested by humans. Ester bonds form between each fatty acid and the glycerol molecule. This is where the enzyme pancreatic lipase acts, hydrolysing the bond and ‘releasing’ the fatty acid. In triglyceride form, lipids cannot be absorbed by the duodenum. Fatty acids, monoglycerides (one glycerol, one fatty acid) and some diglycerides are absorbed by the duodenum, once the triglycerides have been broken down.
The chemical formula is RCOO-CH2CH(-OOCR')CH2-OOCR", where R, R', and R" are longer
alkylchains. The three fatty acids RCOOH, R'COOH and R"COOH can be all different, all the same, or only two the same.
Chain lengths of the fatty acids in naturally occurring triglycerides can be of varying lengths but 16, 18 and 20
carbonsare the most common. Natural fatty acids found in plants and animals are typically composed only of even numbers of carbon atoms due to the way they are bio-synthesised from acetyl CoA. Bacteria, however, possess the ability to synthesise odd- and branched-chain fatty acids. Consequently, ruminantanimal fat contains odd numbered fatty acids, such as 15, due to the action of bacteriain the rumen.
Most natural fats contain a complex mixture of individual triglycerides; because of this, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, one of which contains palmitic, oleic and stearic acids in that order. This gives rise to a fairly sharp melting point, causing
chocolateto melt in the mouth without feeling greasy.
fatty acid metabolism"Triglycerides, as major components of very low density lipoprotein(VLDL) and chylomicrons, play an important role in metabolismas energy sources and transporters of dietary fat. They contain more than twice as much energy (9 kcal/g) as carbohydrates and proteins. In the intestine, triglycerides are split into glycerol and fatty acids (this process is called lipolysis) (with the help of lipases and bilesecretions), which are then moved into the cells lining the intestines (absorptive enterocytes). The triglycerides are rebuilt in the enterocytes from their fragments and packaged together with cholesteroland proteins to form chylomicrons. These are excreted from the cells and collected by the lymph system and transported to the large vessels near the heart before being mixed into the blood. Various tissues can capture the chylomicrons, releasing the triglycerides to be used as a source of energy. Fat and liver cells can synthesize and store triglycerides. When the body requires fatty acids as an energy source, the hormone glucagonsignals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. As the braincannot utilize fatty acids as an energy source (unless converted to a ketone), the glycerolcomponent of triglycerides can be converted into glucose, via gluconeogenesis, for brain fuel when it is broken down. Fat cells may also be broken down for that reason, if the brain's needs ever outweigh the body's.
Triglycerides cannot pass through cell membranes freely. Special enzymes on the walls of blood vessels called lipoprotein lipases must break down triglycerides into fatty acids and glycerol. Fatty acids can then be taken up by cells via the fatty acid transporter (FAT).
Role in disease
:"See also the main article
hypertriglyceridemia"In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, and, by extension, the risk of heart disease and stroke. However, the relative negative impact of raised levels of triglycerides compared to that of LDL:HDL ratios is as yet unknown. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level.
Another disease caused by high triglycerides is
American Heart Associationhas set guidelines for triglyceride levels: [ [http://www.americanheart.org/presenter.jhtml?identifier=183 What Your Cholesterol Levels Mean ] ] Please note that this information is relevant to triglyceride levels as tested after fasting 8 to 12 hours. Triglyceride levels remain temporarily higher for a period of time after eating.
When some fatty acids are converted to
ketonebodies, overproduction can result in ketoacidosisin diabetics.Fact|date=September 2008
Reducing triglyceride levels
Moderating the consumption of fats, alcohol and carbohydrates and partaking of aerobic exercise are considered essential to reducing triglyceride levels.Fact|date=June 2008
Omega-3 fatty acidsfrom fish, flax seed oil or other sources, (up to 3g per day in US, but up to 2g in Europe where it should be associated with Omega-6 with a ideal ω_6/ω_3 ratio, unless under physician care) [http://www.americanheart.org/presenter.jhtml?identifier=4632 Fish and Omega-3 Fatty Acids] . American Heart Association.] , Omega-6 fatty acids, one or more grams of niacin(mega-dose vitamin B-3) per day and some statins reduce triglyceride levels.
Unlike Japan, it is generally admitted that most populations in western countries are lacking omega-3 nutritional sources. As a result ingesting of excessively high levels of saturated or monounsaturated fatty acids in order to assimilate enough omega-6 fatty acids is common. The ideal ratio ω_6/ω_3 is almost never met and is most often too high (about 12 in France, up to 80 in the caucasian population of the US and Canada), and unused high levels of saturated or monounsaturated fatty acids accumulate in the body in the form of triglycerides that do not participate in the needed syntheses in the body.Fact|date=September 2008
In some cases
Fibrateshave been used as they can bring down TGs substantially. However they are not used as a first line measure as they can have unpleasant or dangerous side effects. In one case due to an increase in mortality, clofibratewas withdrawn from the North American market.Fact|date=March 2008
Alcohol abuse can cause elevated levels of triglycerides.Fact|date=September 2008
Triglycerides are also split into their components via
transesterificationduring the manufacture of biodiesel. The fatty acid mono alkyl estercan be used as fuel in diesel engines. The glycerin has many uses, such as in the manufacture of food and in the production of pharmaceuticals.Other examples are the Triglyceride process in the decaffeination of coffee beans.
Staining for fatty acids, triglycerides, lipoproteins, and other lipids is done through the use of
lysochromes (fat-soluble dyes). These dyes can allow the qualification of a certain fat of interest by staining the material a specific color. Some examples: Sudan IV, Oil Red O, and Sudan Black B.
Diglyceride acyltransferase- enzyme responsible for triglyceride biosynthesis
Medium chain triglycerides
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