Lipids are unique organic compounds, each serving key roles and performing specific functions within the body. As we discuss the various types of lipids (triglycerides, phospholipids, and sterols) in further detail, we will compare their structures and functions and examine their impact on human health.

As previously highlighted, lipids are a family of organic compounds that are mostly insoluble in water, which includes fats and oils. The three main types of lipids are triglycerides (approx. 95% of the fat we obtain in the diet), phospholipids, and sterols.

Triglycerides Structure and Functions

Triglycerides are the main form of lipids found in the body and the diet. Fatty acids and glycerol are the building blocks of triglycerides. Glycerol is a thick, smooth, and syrupy compound often used in the food industry. To form a triglyceride, a glycerol molecule is joined by three fatty acid chains via condensation reactions. Triglycerides contain varying mixtures of fatty acids.

Fatty Acids

Fatty acids consist of a carboxylic acid (−COOH) group on one end of a carbon chain and a methyl group (−CH3) on the other end. They can be classified by their chain length and saturation (which influences their shape). Saturation and shape play a role in determining if the compound is solid or liquid at room temperature.

It’s All in the Chain

Fatty acids have different chain lengths and compositions. Foods have fatty acids with chain lengths between four and twenty-four carbons, with most containing an even number of carbon atoms. When the carbon chain length is shorter, the melting point of the fatty acid becomes lower—and the fatty acid becomes more liquid. Short-chain fatty acids are those with less than 6 carbons, medium-chain fatty acids are those with 6-12 carbons, and long-chain fatty acids are those with more than 12 carbons.

Degrees of Saturation and Shape

Fatty acid chains are held together by carbon atoms that attach to each other and hydrogen atoms. The term saturation refers to whether or not a fatty acid chain is filled (or “saturated”) to capacity with hydrogen atoms. If each available carbon bond holds a hydrogen atom, we call this a saturated fatty acid chain. All carbon atoms in such a fatty acid chain are bonded with single bonds. Sometimes, the chain has a place where hydrogen atoms are missing. This is referred to as the point of unsaturation.

When there are one or more double bonds (C=C) between carbon atoms, that fatty acid is called an unsaturated fatty acid, as it has one or more points of unsaturation. Any fatty acid that has only one double bond is a monounsaturated fatty acid. Monounsaturated fats help regulate blood cholesterol levels, thereby reducing the risk of heart disease and stroke. A polyunsaturated fatty acid is a fatty acid with two or more double bonds or two or more points of unsaturation. Olive oil contains a high amount of monounsaturated fatty acids, and walnuts contain high amounts of polyunsaturated fatty acids. Both monounsaturated fats and polyunsaturated fats provide nutrition that is essential for normal cell development and healthy skin.

Carbon bonding affects the shape of fatty acids. Double carbon bonds, such as those in unsaturated fatty acids, cause kinks in the fatty acids, which makes it harder for them to pack together. On the other hand, saturated fatty acids tend to be straighter, which makes it easier for them to pack together.  Therefore, foods that have a high percentage of saturated fatty acids tend to be solid at room temperature. Examples of these are fats found in chocolate (e.g., stearic acid which is an eighteen-carbon saturated fatty acid) and meat. Foods rich in unsaturated fatty acids, such as olive oil (e.g., oleic acid which is an eighteen-carbon unsaturated fatty acid) tend to be liquid at room temperature. Flaxseed oil is rich in alpha-linolenic acid, which is an unsaturated fatty acid that becomes a thin liquid at room temperature.

Knowing the connection between chain length, degree of saturation, and the state of the fatty acid (solid or liquid) is important for making food choices. If you decide to limit or redirect your intake of fat products, choosing unsaturated fats is more beneficial than choosing saturated fats. This choice is easy enough to make because unsaturated fats tend to be liquid at room temperature (ie. olive oil), whereas saturated fats tend to be solid at room temperature (ie. butter). While butter is a well-known solid saturated fat, it’s important to note that various cultures have their own traditional solid fats. For instance, ghee, commonly used in South Asian cuisine, is also a solid fat with similar properties. Despite being more solid, avocados are actually rich in unsaturated fats. Most vegetable and fish oils contain high quantities of polyunsaturated fats. Olive oil and canola oil are also rich in monounsaturated fats. Conversely, tropical oils are an exception to this rule as they are liquid at room temperature yet high in saturated fats. Palm oil (often used in food processing) is highly saturated and has been proven to raise blood cholesterol. Shortening, margarine, and commercially prepared products (in general) report using only vegetable-derived fats in their processing. But even so, much of the fat they use may be in the saturated and trans fat categories.

Cis or Trans Fatty Acids?

The introduction of a carbon double bond in a carbon chain, as in an unsaturated fatty acid, can result in different structures for the same fatty acid composition. When the hydrogen atoms are bonded to the same side of the carbon chain, it is called a cis fatty acid. Because the hydrogen atoms are on the same side, the carbon chain has a bent structure. Naturally occurring fatty acids usually have a cis configuration.

In a trans fatty acid, the hydrogen atoms are attached on opposite sides of the carbon chain. Trans fatty acids can be found naturally in foods such as dairy products but most trans fatty acids are industrially produced through a process called hydrogenation. Hydrogenation is the process of adding hydrogen to the carbon double bonds, thus making the fatty acid saturated (or less unsaturated, in the case of partial hydrogenation). This is how vegetable oils are converted into semisolid fats for manufacturing uses.