Triglyceride

Example of an unsaturated fat triglyceride. Left part: glycerol, right part from top to bottom: palmitic acid, oleic acid, alpha-linolenic acid, chemical formula: C₅₅H₉₈O₆

(more properly known as , TAG or triacylglyceride) is a glyceride in which the glycerol is esterified with three fatty acids. It is the main constituent of vegetable oil and animal fats.

Chemical structure

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.

General structure of a triglyceride

The chemical formula is RCOO-CH₂CH(-OOCR')CH₂-OOCR", where R, R', and R" are longer 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 of varying lengths, but 16, 18 and 20 carbons are 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, ruminant animal fat contains odd numbered fatty acids, such as 15, 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 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 chocolate to melt in the mouth without feeling greasy .

Metabolism

See also fatty acid metabolism

Triglycerides, as major components of very low density lipoprotein (VLDL) and chylomicrons, play an important role in metabolism as 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 monoacylglycerol and free fatty acids in a process called lipolysis, with the secretion of lipases and bile, which are subsequently moved to absorptive enterocytes, cells lining the intestines. The triglycerides are rebuilt in the enterocytes from their fragments and packaged together with cholesterol and 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 glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. As the brain cannot utilize fatty acids as an energy source (unless converted to a ketone), the glycerol component 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 free fatty acids and glycerol. Fatty acids can then be taken up by cells via the fatty acid transporter (FAT).

Role in disease

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 pancreatitis.

Guidelines

The American Heart Association has set guidelines for triglyceride levels:

Level mg/dL	Level mmol/L	Interpretation
<150	<1.69	Normal range, low risk
150-199	1.70-2.25	Borderline high
200-499	2.26-5.65	High
>500	>5.65	Very high: high risk

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 ketone bodies, overproduction can result in ketoacidosis in diabetics.

Reducing triglyceride levels

To lower triglyceride levels, one may reduce consumption of fats, alcohol and carbohydrates, particularly in rice, and engage in aerobic exercise. The American Heart Association notes that diets high in carbohydrates, with carbohydrates accounting for more than 60% of the total caloric intake, can increase triglyceride levels. Carbohydrate consumption increases insulin production, which is associated with increased triglyceride production. Increased exercise and reduced carbohydrate consumption ameliorate one potential cause of insulin overproduction to help maintain sensible triglyceride levels. Triglyceride levels are also reduced by omega-3 fatty acids from fish, flax seed oil and other sources. Recommendation in the U.S. is that one ingest up to 3 grams a day of such oils. In Europe the recommendation is for up to 2 grams. However, omega-3 consumption should be balanced with omega-6 fatty acids, ideally in a ω-6/ω-3 ratio between 1:1 and 4:1 (i.e., no more than four grams omega-6 for every one of omega-3). Ingestion of omega-6 fatty acids, more than 1 gram of niacin, mega-dose vitamin B-3, a day, and statins is also recommended.

It has been found that residents in Western countries do not ingest sufficient quantity of food with omega-3. The ideal ratio ω−6/ω−3 is almost never met, and the ratio is often too high, about 12 in France, up to 80 among whites in the U.S. and Canada. Unused saturated or monounsaturated fatty acids accumulate in the body in the form of triglycerides that do not participate in the metabolism of the body.

In some cases, fibrates have been used to bring down triglycerides substantially.

Alcohol abuse can elevate triglycerides levels.

Carnitine has the ability to lower blood 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 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. Triglycerides are also a major feedstock source for biodiesel.

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.

See also

• Diglyceride acyltransferase - enzyme responsible for triglyceride biosynthesis
• Medium chain triglycerides
• Lipids
• Vertical Auto Profile