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Lipids are generally known as fats and oils in food and nutrition. They are unique in nutrition, as they are in all of biology, in that they are not soluble in water. Early work on the chemistry of living organisms led to the discovery that fatty substances were soluble in organic solvents, such as chloroform, ethyl ether, alcohols, and hydrocarbons. Those solubility characteristics are dependent on the neutral or polar attributes of particular lipids and define the structural and functional aspects of lipids in living systems. This article presents the classification of lipids in their chemical groupings, their characteristic chemical and physical properties, and their nomenclature. Major groups of lipids include fatty acids, acylglycerols, phospholipids and sphingolipids, and sterols. Some lipid compounds, such as fat-soluble vitamins and waxes, are not included.

the lipid, extraction and identification methods, and attempts at classification. Table 1 lists fatty acids important in food and nutrition. The accepted shorthand description shows the number of carbons: number of double bonds, location of double bonds from the carbon at the methyl (n or omega) position, and cis or trans configuration (Figure 2). Saturated fatty acids (SFAs) have hydrogen atoms at every possible carbon site, and unsaturated fatty acids have double bonds. Fatty acids with double bonds may occur in isomeric forms. Geometric isomers are referred to as cis and trans rather than the convention Z and E preferred by chemists. For example, linoleic acid is 18:2 n-6, having 18 carbons, two double bonds located at the n (or omega) minus 6 and n minus 9 positions on the chain. Conventional carbon numbering is from the carboxyl end; therefore, linoleic acid can be written as cis 18:2 A9,12. Delta indicates numbering from the carboxyl carbon and the atom number from the carboxyl is sometimes used (C-9). Desaturase enzymes are named according to the delta number (i.e., A-9-desaturase). Commonly, the cis configuration is not noted because almost all natural fatty acids are in the cis configuration. Also, unless otherwise specified, the

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