Fatty acids

The fatty acids are simple organic compounds, formed by carbon, hydrogen and oxygen. Every fatty acid molecule has at one extremity (alpha) a carboxyl group (COOH) and at the other (omega) a non-functional methyl group (CH3). The classification of these compounds depends upon the chain length and the number, position and configuration of the double bonds. There are two distinct nomenclatures for naming fatty acids, however the most used one considers the position of the double bond nearest to the terminal methyl group, this is so because it is thought that nutritional characteristics of a fatty acid depend much more on the chemical composition in the vicinity of the methyl group than the composition near the carboxyl extremity (Table I).

Fatty acid chemical formulas are commonly presented in a simplified way, following the formula n1:n2wn3, where n1 indicates the number of carbon atoms, n2 corresponds to the number of double bonds and n3 indicates the number of the carbon atom involved in the double bond nearest to the terminal methyl group. Fatty acids are found mainly in triacylglycerols and phospholipids, but some may well be esterified with cholesterol. Moreover, concerning the double bond configuration, the cis isomers are the most common in nature.

In seafood, the fatty acid profile varies considerably with temperature, salinity and food availability, which in turn are influenced by geographical area and year's season.

 

Saturated fatty acids

Except the carboxyl group, there are no double bonds in the saturated fatty acids thus they are named in accordance with the number of carbon atoms, taking the common -oic suffix of all carboxylic acids. They have the general formula, CH3-(CH2)n-COOH, varying n from 2 to 22. For instance, the fatty acid of sixteen carbons CH3(CH2)14COOH is named hexadecanoic acid as established by IUPAC; however, most frequently is used the trivial name palmitic acid whilst in the simplified nomenclature it is represented as 16:0.

The saturated fatty acids most abundant in seafood lipids are: the myristic acid (C14), 20 to 30 %; the palmitic acid (C16), 60 to 70 % and the stearic acid (C18), 5 to 10 %.

Unsaturated fatty acids

When double bonds are present, fatty acids are said unsaturated. Moreover, they are said monounsaturated if only one double bond is present or polyunsaturated if they have two or more double bonds.

For the monounsaturated fatty acids, the number of carbon atoms varies between ten and thirty. Oleic acid is the most abundant monounsaturated fatty acid, with the structural formula CH3(CH2)7CH=CH(CH2)7COOH, also named 9-octadecenoic and represented in the shorthand nomenclature as 18:1w9 (or 18:1n9).

In seafood lipids, the most abundant monounsaturated fatty acids are typically palmitoleic acid (16:1w7) and oleic acid (18:1w9).

The polyunsaturated fatty acids have typically an even number of carbon atoms and from two to seven cis double bonds, which are the most frequently separated from each other by a single methylene group. As an example, linoleic acid CH 3 (CH2)4(CH=CH-CH2)2(CH2)6COOH, is represented as 18:2w6 (or 18:2n6), where 18 is the number of carbon atoms, 2 indicates the number of double bonds, 6 is the number of the carbon atom of the first double bond and the letter w (or n) indicates that the double bonds are counted from the terminal methyl group.

It is usual to classify unsaturated fatty acids into families, three of which deserve special mention: w3, w6 and w9 (or n3, n6 and n9).

Concerning fish lipids, 20:5w3 and 22:6w3 are the most important polyunsaturated fatty acids, making up about 90 % of all these components.

 

Essential fatty acids

There are two polyunsaturated fatty acids that cannot be synthesized in the human body: linoleic acid (18:2w6) and a-linolenic acid (18:3w3). They must be provided by diet and, therefore, are known as essential fatty acids. Within the body both can be converted to other important polyunsaturated fatty acids such as arachidonic acid (20:4w6), eicosapentaenoic acid or EPA (20:5w3) and docosahexaenoic acid or DHA (22:6w3). In the body polyunsaturated fatty acids are important for maintaining the membranes of all cells and for making prostaglandins which regulate many organic processes such as inflammation and blood clotting.

In Table I is presented the classification of the main fatty acids in seafood, while in Table II is shown their shorthand nomenclature and chemical structure.

Table I. Classification of the main fatty acids in seafood.

(go back)