ABSTRACT
Fatty acids are ubiquitous molecules in biological systems. They occur as components of lipids, notably phospholipids and glycolipids in membranes, and triacylglycerols in seed oils of plants, oily fish, and adipose tissue (fat) in animals. They are present in appreciable quantities in many foodstuffs and although there has been a great deal of concern over the consumption of too much fat, it has to be remembered that fatty acids are essential to our well being. There is a range of different types of fatty acids, varying in chain length and number of double bonds (degree of unsaturation), and any one source normally contains several different types. Fatty acids with even numbers of carbon atoms predominate and there are short-chain (SCFA, C4-C10), medium-chain (MCFA, C12, C14), long-chain (LCFA, C16-C22), and very long-chain (LCFA>C22); sometimes the C20-C24 fatty acids are referred to as LCFA. Common saturated fatty acids (SFA) are palmitic (16:0) and stearic (18:0) acids. Unsaturated fatty acids (UFA) are divided into monounsaturated fatty acids (MUFA) with one double bond and polyunsaturated fatty acids (PUFA) with two or more double bonds; in this review we will define LC-PUFA as containing 20 and 22 carbons. Important UFA in nutrition include the MUFA oleic acid (OA, 18:1v9; the double bond is nine carbons from the terminal methyl end of the molecule), diunsaturated linoleic acid (LA, 18:2v6; containing two methylene-interrupted double bonds, the one nearest the terminal methyl is six carbons down from it), triunsaturated a-linolenic acid (ALA, 18:3v3), tetraunsaturated arachidonic acid (AA, 20:4v6), pentaunsaturated eicosapentaenoic acid (EPA, 20:5v3), and hexaunsaturated docosahexaenoic acid (DHA, 22:6v3). It can be seen from this list that some PUFA are of the v3 (or n-3) type whereas others are v6 (or n-6), the fatty acids in each group being biosynthetically related. LA and ALA are termed essential fatty acids because they cannot be biosynthesized by humans and they must be provided in the diet from vegetable or animal sources. The more unsaturated and longer chain v6 and v3 acids may be biosynthesized from LA and ALA (Figure 2.1), respectively, or they may be obtained from the diet. The double bonds in UFA can exist in two geometrical forms. The cis form overwhelmingly predominates in nature although trans fatty acids occur naturally in milk and ruminant tissues, and are formed in partially hydrogenated vegetable oils (PHVO) used in foods.
