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Food Lipids: Chemistry, Nutrition, and Biotechnology

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3. Polyunsaturated Fatty Acids<br />

Polyunsaturated fatty acids in the diet belong to either the n-6 or the n-3 family.<br />

About 90% of all polyunsaturated fatty acid in the diet is linoleic acid, which is<br />

found in vegetable oils like sunflower oil, corn oil, <strong>and</strong> soybean oil. The mean daily<br />

intake of fatty acids from the n-3 family is only 1–3 g. These polyunsaturated fatty<br />

acids are either from vegetable or animal origin. �-Linolenic acid is found in rapeseed<br />

<strong>and</strong> soybean oils, while the very long chain fatty acids timnodonic or eicosapentaenoic<br />

acid (EPA) <strong>and</strong> cervonic or docosahexaenoic acid (DHA) are only present<br />

in fish oils.<br />

a. n-6 Polyunsaturated Fatty Acids. Earlier studies found that linoleic acid was<br />

hypocholesterolemic as compared with carbohydrates <strong>and</strong> monounsaturated fatty<br />

acids [5,6]. The study of Mattson <strong>and</strong> Grundy [20], however, suggested that, as<br />

compared with monounsaturates, part of the cholesterol-lowering effect of linoleic<br />

acid was due to a decrease in HDL cholesterol (Fig. 7). However, linoleic acid intake<br />

in that study was unrealistically high (28% of energy intake), which may have influenced<br />

the results. Studies at lower intakes found similar effects of linoleic <strong>and</strong><br />

oleic acids on HDL cholesterol, but also on LDL cholesterol [21–23]. Thus, these<br />

more recent studies suggested that replacement of saturated fatty acids in the diet by<br />

monounsaturated fatty acids causes the same favorable change in plasma lipoprotein<br />

cholesterol levels as replacement by polyunsaturated fatty acids. However, it should<br />

be noted that other recent studies found a small favorable effect linoleic acid on<br />

plasma LDL cholesterol as compared with oleic acid [24].<br />

b. n-3 Polyunsaturated Fatty Acids. The effects of �-linoleic acid on the plasma<br />

lipoprotein profile are similar to those of linoleic acid [25]. The highly unsaturated<br />

Figure 7 Effects of palmitic acid <strong>and</strong> linoleic acids on plasma total, LDL cholesterol, HDL<br />

cholesterol, <strong>and</strong> triacylglycerol concentrations relative to those of cis-monounsaturated fatty<br />

acids (oleic acid). Twelve men received three liquid formula diets, each for 4 weeks, in r<strong>and</strong>om<br />

order. The composition of the diets was identical, except for 14% of daily energy intake,<br />

which was provided as palmitic acid (C16:0), linoleic acid (C18:2), or oleic acid (cis-C18:1)<br />

[20].<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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