Triglyceride (blue: fatty acid; red: glycerol backbone)

Triglycerides 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 fatty acids; the three fatty acids can be all different, all the same, or only two the same.

Chain lengths of the fatty acids in triglycerides can be from 4 to 22 C atoms, but 16 and 18 are most common. Shorter chain lengths are found in butter for instance. Almost without exeption, only even numbers of carbon atoms are found in natural fatty acids - due to the way they are bio-synthesised from acetic acid.

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 mostly a single triglyceride (composed of Palmitic, Oleic and Stearic acids in that order) and has a fairly sharp melting point (so chocolate melts in the mouth without feeling greasy).

Metabolism

(see Fatty acid metabolism)
Triglycerides play an important role in metabolism as energy sources. They contain twice as much energy (8000 kcal/kg) as carbohydrates. In the intestine, triglycerides are split into glycerol and fatty acids (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. 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.

Role in disease

See also the main article hypertriglyceridaemia

In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, and, by extension, to the risk of heart disease and stroke. However, the negative impact of raised levels of triglycerides is lower than that of LDL-cholesterol. 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:

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 (and some statins) are registered for reducing triglyceride levels.

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.