Fatty Acids

Fatty acids are hydrocarbon chains with a methyl and carboxyl end. The majority of dietary fatty acids have an even number of carbons. The range in chain length of common dietary fatty acids is broad. Fatty acids with 16 and 18 carbons make up the majority of fatty acids present in plants and animals. However, they are by no means the most metabolically active. Long-chain unsaturated fatty acids, such as arachidonic acid (C20:4), are common precursors of regulatory compounds.

Essential nutrients are those that the body cannot synthesize or cannot synthesize in amounts adequate to meet needs. Linoleic acid (18:2) and/or fatty acids that can be derived from linoleic acid are essential fatty acids. These specific fatty acids are essential because humans cannot introduce a double bond above the ninth carbon from the carboxyl end of the acyl chain. To maintain optimal health, they must be supplied by the diet of humans. The metabolism of linoleic acid is represented in Figure 1.

A wide range of fatty acids occur in nature. There are a number of features of fatty acids that distinguish one from another. In addition to chain length, they also vary with regard to degree of saturation and location of the double bond(s). Fatty acids with a single double bond are referred to as monounsa-turated fatty acids, and those with two or more double bonds are referred to as polyunsaturated

Linoleic Acid 18:2n-6

desaturase ▼

alpha-Linolenic Acid 18:3n-6

desaturase ▼

Dihomo- gamma-linolenic Acid 20:3n-6

Arachidonic Acid 20-4n-6

desaturase ▼

Docosatetraenoic Acid 22:4n-6

22:5n-6

Figure 1 Metabolism of linoleic acid.

fatty acids (Figure 2). The double bonds within unsaturated fatty acids can either be in the cis (hydrogen atoms on the same side of the acyl chain) or trans (hydrogen atoms on opposite sides of the acyl chain) conformation (Figure 3). The cis conformation is most commonly found in nature. Double bonds can also vary with regard to location within the acyl chain. The presence of double bonds, per se, and their number, position, and conformation, dictates the physical properties of the fatty acids.

Unsaturated fatty acids of the same length with an identical number of double bonds can occur in multiple forms due to variation in the conformation of one or more of the double bonds (cis versus trans). They are referred to as geometric isomers (Figure 3). A common example is oleic acid (18:1c-9) and elaidic acid (18:1t-9). The presence of a cis relative to a trans double bond results in a greater bend or kink in the hydrocarbon chain. This kink impedes the fatty acids from aligning or packing together, thereby lowering the melting point of the fat. In a cell membrane this will be reflected in increased fluidity. In food this will be reflected in an oil that is liquid or fat that is soft at room temperature.

Unsaturated fatty acids of the same length with an identical number of double bonds and conformation can also occur in multiple forms due to the location of the double bonds within the acyl chain. They are referred to as positional isomers. A common example is alpha-linolenic acid (18:3n-3) and gamma-linolenic acid (18:3n-6). The difference in location of double bonds results in small alterations to the melting point yet large differences in the metabolic properties of the fatty acids. The most common distinction made among positional isomers of fatty acids is the location of the first double bond from the methyl end of the acyl chain. A fatty acid in which the first double bond occurs three carbons from the methyl end is termed an omega-3 fatty acid, frequently denoted n-3 fatty acid. This class of fatty acids is distinguished from the major class of fatty acids in which the first double bond occurs six carbons from the methyl end, termed an omega-6 or n-6 fatty acid. Enzymes that metabolize fatty acids distinguish among both positional and geometric isomers. The metabolic products of the different positional isomers of fatty acids have different and, occasionally, opposite physiological effects.

Most double bonds within fatty acids occur in a nonconjugated sequence, both in the human body and in food. That is, a carbon atom with single carbon-carbon bonds separates the carbons making up the double bonds. Some double bonds occur in

Figure 2 Saturated, monounsaturated, and polyunsaturated (n-3 and n-6) acids.

Cis form

Trans form /

Elaidic acid

Figure 3 Cis and trans double-bond-containing fatty acids. (Copyright © The McGraw-Hill Companies, Inc.)

Oleic acid

Elaidic acid

Figure 3 Cis and trans double-bond-containing fatty acids. (Copyright © The McGraw-Hill Companies, Inc.)

the conjugated form, without an intervening carbon atom separating the double bonds. Conjugated double bonds tend to be more reactive chemically (i.e., more likely to become oxidized). Although there is considerable speculation about the role of conjugated double bond-containing fatty acids and human health, the current state of knowledge is insufficient to draw any firm conclusions.

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