Monounsaturated fatty acids most frequently occur in higher concentrations in plant foods such as olive oil, most nuts, and avocados. Although marrow from animal bones is infrequently consumed in the U.S., it is also a good source of monounsaturated fats. When contrasted to saturated fatty acids, dietary monounsaturated fatty acids lower blood cholesterol concentrations. Below is a list of the common and numeric names for monounsaturated fatty acids that may occur in foods. The most common monounsaturated fatty acid in both plant and animal foods is oleic acid (18:1). As was the case with saturated fatty acids, most monounsaturated fatty acids are even numbered. Odd numbered monounsaturated fatty acids are infrequently present in foods.
Monounsaturated fatty acids contain a single double (=) bond between two carbon atoms. The naming scheme for monounsaturated fatty acids follows that for saturated fatty acids. Oleic acid (18:1) means that the fatty acid is 18 carbon atoms in length and there is a single (:1) double bond. When a single double bond occurs in a fatty acid, it is useful to know the position of the double bond. Hence oleic acid can be further labeled 18:1n-9, meaning that there is a single double bond, 9 carbon atoms from the omega (n) end of the molecule. Note that the numbering scheme for double bonds starts at the omega or methyl end of the fatty acid whereas the numbering of carbon atoms starts from the carboxyl end of the carbon backbone. Below is a schematic diagram of oleic acid or 18:1n-9.
As was the case with saturated fatty acids, the schematic diagram above is not precisely correct because it doesn’t show the correct angle of the carbon to carbon bonds, which really is 109 degrees rather than 180 degrees. In the diagram below, you see a geometrically correct diagram of oleic acid.
Notice in both diagrams of oleic acid that the hydrogen atoms occur on the same side of the double bond, rather than on opposite sides. When this configuration occurs, we must add another dimension to the naming scheme. Oleic acid now becomes 18:1n-9 cis, meaning that the two hydrogen atoms fall on the same side of the carbon to carbon double bond. Notice that the “cis” double bond causes a bend or “kink” in the carbon atom backbone. These kinks determine the shape of the fatty acid and become more and more important to the molecule’s function in our bodies as we consider polyunsaturated fatty acids. Before we get to polyunsaturated fatty acids, an additional factor in naming fatty acids must be considered.
Oleic acid is the “normal” form of this monounsaturated fatty acid because it occurs most frequently and in the highest concentration in foods. However there are other variants of 18:1 called isomers. Isomers are 2 molecules (fatty acids) with the same molecular weight but with differing structure. Fatty acids can have geometric isomers or positional isomers or both. Let’s first consider geometric isomers – a variant in which the hydrogen atoms occur on opposite sides of the carbon-to-carbon double bond. Below is a diagram of 18:1n-9, trans. Notice that compared to the normal, oleic acid, the hydrogen atoms are on the opposite (trans) side of the double bond. The common name for 18:1n-9,trans is trans elaidic acid. When people talk about “trans fatty acids” in margarine, shortening and processed foods, this is the specific fatty acid identified. Below is a schematic diagram of both oleic acid (18:1n-9, cis) and trans elaidic acid (18:1n-9, trans).
Notice that the incorporation of the “trans” double bond in 18:1 causes the normally “kinked” fatty acid to become straightened out similar to saturated fatty acids. This change in shape is the primary reason why a good fatty acid (oleic) becomes a bad fatty acid (trans elaidic) in its effect upon blood cholesterol levels and cellular metabolism.
The other kind of isomer that fatty acids can form is called a positional isomer, in which the “cis” double bond presents itself in a position other than the normal position along the carbon backbone. An example of this would be 18:1n-12, cis, in which the double bond appears at the 12th carbon atom down from the omega (n) end of the molecule instead of the n-9 position that normally occurs in oleic acid. Finally, isomers of fatty acids can simultaneously contain both positional and geometric isomers such as 18:1n-11, trans, which is called trans vaccenic acid. This is one of the few naturally occurring trans fatty acids and is present in the fats derived from ruminant mammals. Butter, cheese, milk and beef fat contain between 3-5 % trans vaccenic acid. However, unlike 18:1n-9 trans (trans elaidic acid), 18:1n-11, trans appears to be beneficial because in the body it is converted to another fatty acid called conjugated linoleic acid (CLA), which has anti-cancer and cardiovascular protective effects.