Fatty acids are hydrocarbon chains with a carboxylic acid at one end and a methyl group at the other. The dietary fatty acids are a diverse group of lipids and as a result they can be classified in a number of different ways. The simplest way to classify dietary fatty acids is via their chain length (including the methyl and carboxylic acid groups), with the major groupings being short, medium and long chain fatty acids. Short chain fatty acids have chain lengths of less than 6 carbon atoms (e.g. butyric acid, C:4), medium chain fatty acids have chain lengths of between 6 and 12 carbons (e.g. capric acid, C:10) , whereas long chain fatty acids have chain length over 14 carbons (e.g. palmitic acid, C:16). Sometime fatty acids with chains over 22 carbons (e.g. behenic acid, C:22) are designated very long chain fatty acids. All fatty acids have even numbers of carbon atoms. Triglycerides composed of medium chain fatty acids may be readily transported to the liver via the portal vein where they undergo β-oxidation, and this may explain their substantial thermogenic properties.
Dietary fatty acids can further be classified based on their saturation. Saturated fatty acids possess no double bonds in their carbon chains, and as a result are ‘saturated’ with hydrogen. In contrast, unsaturated fatty acids possess double bonds which necessitates the removal of hydrogen atoms from the hydrocarbon chain. Monounsaturated fatty acids have a single double bond, whereas polyunsaturated fatty acids have multiple double bonds. The presence of double bonds causes structural changes to the shape of the molecule because of the removal of positive charges in the form of hydrogens. As a result unsaturated fatty acids tend to have a kinked shape, in contrast to the straight shape of the saturated fatty acids. If the double bonds in unsaturated fatty acids have hydrogens on the same side of the molecule they are said to be in a cis configuration, whereas if the hydrogens are on opposite sides of the molecule they are said to be in the trans configuration. Trans fatty acids retain the straight shape of the saturated fatty acids due to charge repulsion.
