It is a component of membranes in body cells and is required for normal development of the brain and nervous tissue. Cholesterol frequently exists in foods and body tissues esterified to one fatty acid per molecule. Phospholipids are important structural components of brain and nervous tissue, of membranes throughout body tissues, and of lipoproteins-the carriers of cholesterol and fats in the blood.Ĭholesterol and plant sterols, such as sitosterol, are high-molecular-weight alcohols with a characteristic cyclic nucleus and are unrelated to the structure of fats or phospholipids.
Lecithin, for example, is made up of glycerol, two fatty acids (one saturated, usually), phosphate, and choline. Phospholipids contain glycerol, fatty acids, phosphate, and, with such exceptions as phosphatidylglycerol and phosphatidylinositol, a nitrogenous component. This latter process occurs naturally in the rumen of ruminants. Naturally occurring geometric isomers in food are mainly cis isomers, but hydrogenation of oils in the manufacture of margarine and shortening results in formation of some trans isomers. Unsaturated fatty acids form geometric isomers, i.e., the carbon chains are on the same side of the double bond in a cis isomer and on opposite sides of the bond in a trans isomer. These fatty acids appear to play distinctive roles in the structure and function of biologic membranes in the retina and central nervous system (Neuringer and Connor, 1986). Chief among these ω-3 fatty acids are eicosapentaenoic acid (EPA), which has 20 carbons and 5 double bonds (C20:5, ω-3), and docosahexaenoic acid (DHA), which has 22 carbons and 6 double bonds (C22:6, ω-3).Ī growing body of evidence from studies in animals, including nonhuman primates, indicates that α-linolenic acid, or its longer-chain derivates EPA and DHA, are essential in the diet. Under this classification system, oleic acid (C18:1, ω-9) belongs to the ω-9 group, and the PUFAs in fish oils currently receiving much attention belong to the ω-3 group. Since linoleic acid has 18 carbon atoms and 2 double bonds, it is usually represented in shorthand as C18:2, ω-6. Linoleic acid (18 carbons with 2 double bonds) and arachidonic acid (20 carbons with 4 double bonds) belong to the omega(ω)-6 group of fatty acids, since the first double bond, counting from the methyl end of the molecule, occurs at carbon number 6. Arachidonic acid is also required by the body but can be synthesized from linoleic acid, which is abundant in oils from corn, soybeans, and safflower seeds. Linoleic acid is classified as an essential nutrient, since the body requires it but cannot synthesize it. Monounsaturated fatty acids (MUFAs), such as oleic acid, contain one double bond per molecule, whereas polyunsaturated fatty acids (PUFAs), such as linoleic acid, contain more than one. Other animal and vegetable fats contain predominantly longer-chain SFAs (more than 14 carbons in length) and are found chiefly in meats, butterfat, and some vegetable oils.
Short- and medium-chain saturated fatty acids (SFAs) (4 to 12 carbons in length) are found in milk fat, palm oil, and coconut oil. The number of carbons in the chain varies, and the compound may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds). They also contribute satiety, flavor, and palatability to the diet.įatty acids generally consist of a straight alkyl chain, terminating with a carboxyl group. They are important in the diet as energy sources and as sources of essential fatty acids and fat-soluble vitamins, which tend to associate with fats. Lipids that are important to our discussion include fats and oils (triglycerides or triacyglycerols), fatty acids, phospholipids, and cholesterol.įats and oils are esters of glycerol and three fatty acids. Lipids are compounds that are insoluble in water but are soluble in organic solvents such as ether and chloroform.
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