Triglyceride

Triglycerides (also known as triacylglycerols or triacylglycerides) are glycerides in which the glycerol is esterified with three fatty acids. They are the main constituent of vegetable oil and animal fats.

Chemical structure

CH2COOR-CHCOOR'-CH2-COOR"
where R, R', and R" are long alkyl chains; 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 from 3 to 22 carbon atoms, but 16 and 18 are most common. Shorter chain lengths may be found in some substances (butyric acid in butter). Typically, plants and animals have natural fatty acids that comprise only of even numbers of carbon atoms due to the way they are bio-synthesised from acetyl CoA, however bacteria possess the ability to synthesise odd- and branched-chain fatty acids. Consequently, ruminant animal fat contains significant proportions of branched-chain fatty acids, due to the action of bacteria in 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 comprises only of 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 chocolate to melt in the mouth without feeling greasy.

Metabolism

Triglycerides play an important role in metabolism as energy sources. 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 bile secretions), which can then move into blood vessels. The triglycerides are rebuilt in the blood from their fragments and become constituents of lipoproteins, which deliver the fatty acids to and from fat cells among other functions. Various tissues can release the free fatty acids and take them up as a source of energy. Fat cells can synthesize and store triglycerides. When the body requires fatty acids as an energy source, the hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. As the brain can not utilize fatty acids as an energy source, the glycerol component of triglycerides can be converted into glycogen 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.

Role in disease

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 negative impact of raised levels of triglycerides is lower than that of LDL:HDL ratios. The risk can be partly accounted for a strong inverse relationship between triglyceride level and HDL-cholesterol level.

Other diseases caused by high triglycerides include pancreatitis.

Guidelines

The American Heart Association has set guidelines for triglyceride levels:

Levelmg/dL <150 Levelmmol/L (<1.69mmol/L) Interpretation Normal range, lowest risk

Levelmg/dL 150-199 Levelmmol/L (1.70-2.25mmol/L) Interpretation Borderline high

Levelmg/dL 200-498 Levelmmol/L (2.25-5.63mmol/L) Interpretation High

Levelmg/dL >500 Levelmmol/L (>5.65 mmol/L) Interpretation Very high, increased risk

Please note that this information is relevant to triglyceride levels as tested after fasting. Triglyceride levels remain temporarily higher for a period of time after eating.

Reducing triglyceride levels

Cardiovascular exercise and low-moderate carbohydrate diets containing essential fatty acid are recommended for reducing triglyceride levels. When these fail, fibrate drugs, niacin, and some statins are registered for reducing triglyceride levels. Prior alcohol intake can cause elevated levels of triglycerides.

Industrial uses

Triglycerides are also split into their components via transesterification during the manufacture of biodiesel. The fatty acid monoalkyl ester can be used as fuel in diesel engines. The glycerin can be used for food and in pharmaceutical production, among others.

Staining

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.

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