Triglycerides (TG), or triacylglycerols, make up by far the largest proportion of dietary lipids consumed by humans. A TG is composed of three FAs esterified to a glycerol molecule in one of three stereochemically distinct bonding positions: sn-1, sn-2, and sn-3. Variation in the type of FA and their bonding pattern to glycerol further increases the heterogeneity of TG composition. For most dietary oils, approximately 90% of the TG mass consists of FA. These FA are generally unbranched hydrocarbon chains with an even number of carbons, ranging from 4 to 26 carbon atoms. Smaller quantities of longer-chain FA have also been identified in mammalian tissues and thus may exist in human diets (4). Very long-chain fatty acids (VLCFA) predominate in the brain and specialized tissues, such as retina and spermatozoa (5). Adipose tissue contains FA of varying lengths.
In addition to differences in chain length, FA vary in the number and arrangement of double bonds along the hydrocarbon chain. Major FA are given in Hgure.,4.,..l. Systems for identifying the position of double bonds along the hydrocarbon chain entail carbon-counting from either end of the molecule. The less common "delta" system of identification of double bonds counts from the carboxyl end of the fatty acyl chain. More commonly used is identification of the position of the first carbon of a double bond relative to the methyl terminus of the FA. Double bonds identified relative to the methyl end use the terms "n" or "w" to indicate distance of the first bond along the carbon chain. For example, any FA described as n-6, or w-6, has the initial double bond situated between the 6th and 7th carbon atoms from the methyl end. To contain a single double bond, a FA must be at least 12 carbon atoms in length. These monounsaturated fatty acids (MUFA) typically possess a double bond at the n-9 or n-7 position. Addition of further double bonds produces a PUFA. Each subsequent double bond invariably occurs three carbon atoms further along the carbon chain from the bond preceding it. Therefore, the number of double bonds within a FA is restricted by its chain length. FA with 18 carbon atoms or more that possess more than a single double bond will contain the first bond of their series only at the n-9, n-6, or n-3 position. For a 16-carbon atom FA, the first double bond may be located at the n-7 position. A maximum of 6 double bonds occurs in dietary Fa.
Figure 4.1. Names, codes, and formulas of fatty acids mentioned in this chapter.
The essentiality of a FA depends on the distance of the first double bond from the methyl terminus. During de novo FA formation, human biosynthetic enzymes can insert double bonds at the n-9 position or higher; however, these enzymes cannot insert double bonds at any position closer to the methyl end. For this reason, FA with double bonds at the n-6 and n-3 positions are, as individual classes, considered essential. These EFA must therefore be obtained from plants or other organisms that possess the enzymatic pathways for their construction. Mammalian tissues contain four families of PUFA (n-3, n-6, n-7, n-9) designated according to the number of carbon atoms from the terminal methyl group to the first carbon of the first double bond. Among all FA, only those of n-6 and n-3 classes are essential to the diet. All other FA can be synthesized by humans from an excess of dietary energy.
Double bonds in foods we consume most commonly occur in the cis configuration. Trans bonds, also present, are a result of hydrogenation, the process used to increase the viscosity of oils, and the microbial metabolism in ruminants. Trans bonds reduce internal rotational mobility of the fatty acyl chain and are less reactive to electrophilic additions such as halogenation, hydration, and hydrogenation ( 6, 7). Most dietary trans FA are monoenes, 18 carbons in length. The major trans FA, elaidic acid (C18:1n-9 trans) has a melting point of 44°C, versus 13°C for oleic acid (C18:1n-9).
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