Triglycerides are lipids that include fats and oils. You may have heard about triglycerides in relation to medicine, as high levels of triglycerides are a common sign of various health issues. However, there is another side to triglycerides: triglycerides as energy powerhouses! Both their structure and function make them such useful energy storage molecules.

Triglycerides are often referred to simply as fats and are the most common lipids found in living organisms. A lot of them come from foods we eat often, like butter and vegetable oils.

The structure of triglycerides

The building blocks of triglycerides are fatty acids and glycerol. The term triglyceride comes from the fact that they have three (tri-) fatty acids attached to glycerol (glyceride).

Depending on the bonds between carbon atoms in the chain, fatty acids can be saturated and unsaturated: mono-unsaturated and poly-unsaturated. Saturated fatty acids have only single bonds. Unsaturated fatty acids have one or more double bonds between carbon atoms: mono-unsaturated have one double bond, while poly-unsaturated have two or more. That is why you will hear fats referred to as saturated and unsaturated fats.

Fig. 1 - Simplified structure of a triglyceride with one saturated (palmitic acid), one mono-unsaturated (oleic acid), and one poly-unsaturated fatty acid (alpha-linolenic acid) attached to a glycerol backbone

How are triglycerides formed?

Triglycerides are formed during the condensation reaction of fatty acids and glycerol.

Glycerol has three –OH groups to which three fatty acids attach during condensation. A covalent bond called the ester bond forms between glycerol and fatty acids.

The formation of triglycerides is a condensation reaction. The carboxyl group of each fatty acid loses one hydrogen atom, and glycerol loses three –OH groups. This results in the release of not one but three water molecules since three fatty acids attach to glycerol, and therefore three ester bonds form.

Like all biological macromolecules, triglycerides go through hydrolysis when they need to be broken down into their building blocks of fatty acids and glycerol. For instance, the breakdown of fats stored in fat cells during hunger. During hydrolysis, ester bonds between fatty acids and glycerol break using three water molecules. This results in the breaking down of triglycerides and the release of energy.

Fig. 2 - Hydrolysis of triglycerides (left) results in one molecule of glycerol (blue) and three fatty acids (right). The red bonds are three hydrolysed ester bonds

The function of triglycerides

The primary function of triglycerides is energy storage and providing energy to the body. They are obtained via the food we eat or are released from the liver. They are then transported via blood plasma, providing nutrients to different body parts.

• Triglycerides are excellent energy storage molecules because they are composed of long hydrocarbon chains (chains in fatty acids) with many bonds between carbon and hydrogen atoms. These bonds hold a large amount of energy. This energy is released when fatty acids are broken down (a process called fatty acid oxidation).

• Triglycerides have a low mass to energy ratio, which means a significant amount of energy can be stored in a small volume. Triglycerides are energy powerhouses - they hold more energy per gram than carbohydrates and proteins!

• Triglycerides are large and insoluble in water (hydrophobic). This means that triglycerides can be stored in cells without affecting their osmosis. This, too, makes them excellent energy storage molecules.

• Triglycerides are stored as oils in plants, specifically in seeds and fruits. In animals, triglycerides are stored as fats in the liver and adipose tissue (the connective tissue that serves as the primary lipid storage in mammals).

Other functions of triglycerides include:

• Insulation - Triglycerides stored beneath the body surface insulate mammals from the environment, keeping their bodies warm. In aquatic animals, a thick layer of fat underneath their skin keeps them warm and dry.

• Protection - Triglycerides are stored in adipose tissue, which serves as a protective shield around vital organs.

• Providing buoyancy - Aquatic mammals (e.g., seals) have a thick layer of fat underneath their skin to prevent them from sinking whenever they are underwater.

Triglycerides can prove to have a negative effect on our health. If you remember, plants store excess glucose in the form of starch, and animals store it as glycogen. The same thing happens with triglycerides. We don’t need triglycerides short-term, so we store them as body fat. However, human bodies often store an excessive amount of triglycerides, mainly around the organs.

Therefore, hypertriglyceridemia (high triglyceride levels) may occur. It is a serious indication that our bodies are not functioning well and may lead to conditions such as heart disease and stroke. It can be an indication of diabetes as well. Read more on this disease in the article Diabetes.

A piece of general advice is to limit the intake of so-called “bad fats”, i.e. the food high in saturated fats, such as starchy food, baked goods, fast food and other high-calorie food, and even alcohol. This advice extends to include the intake of healthier fats, including fish, white chicken meat, whole grains, low-fat dairy, and vegetable oils like olive and rapeseed oil.