Effect of dietary fatty acids on incorporation of long chain polyunsaturated fatty acids and conjugated linoleic acid in lamb, beef and pork meat: a review

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Abstract

Inspired by the health conscious consumer, several studies in meat producing domesticated animals have been completed that have aimed at increasing the polyunsaturated fatty acid content, and in particular the n-3 or ω-3 long chain fatty acids, as well as the conjugated linoleic acid (CLA) content in intramuscular fat of beef, lamb and pork meat. Increasing the n-3 content in animal meats can be achieved by including fish oil/fish meal in the diet (i.e. rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)), linseed (oil) and/or forages (i.e. rich in linolenic acid (LNA)). Diets rich in LNA result in an increased level of LNA, EPA and docosapentaenoic acid (DPA) in the meat, while in most cases no effect on intramuscular DHA level was observed. Increasing DHA contents in meat was mainly achieved when fish oil/fish meal was included in the animals diet. In most studies, an increased n-3 content in the intramuscular fat was accompanied with a decreased n-6 deposition, mainly due to a lower n-6 dietary supply between the treatments. This resulted in a more favourable n-6/n-3 ratio in the meat while the polyunsaturated fatty acid/saturated fatty acid (P/S) ratio was less affected.

Attempts to increase the intramuscular c9t11CLA content was accomplished by feeding ruminants n-3 rich diets (linseed either or not in combination with grass (silage)), fish oil or high concentrate diets rich in linoleic acid (LA). The c9t11CLA content in beef and lamb meat varied between 0.2 and 1.0 g/100 g of total fatty acids and was not increased to a high extent by nutritional factors. In contrast, including CLA-oils consisting of several CLA isomers in the diet of monogastrics markedly increased the intramuscular CLA content.

Introduction

The fatty acid composition of meat has long been studied but still receives a lot of attention in research because of its implications for human health. Besides a lower total fat intake, human nutritionists are recommending a higher intake of polyunsaturated fatty acids (PUFA), and especially of n-3 or ω-3 fatty acids at the expense of n-6 or ω-6 fatty acids (e.g. Department of Health, 1994, Voedingsaanbevelingen voor Belgie, 2000). Numerous animal feeding trials have been carried out using different species and breeds aiming at bringing the polyunsaturated fatty acid/saturated fatty acid (P/S) ratio of meat closer to the recommended value (>0.7), as well as for the n-6/n-3 ratio (<5). Besides the beneficial effects of PUFA for human health (see recent reviews of Connor, 2000, Williams, 2000), the conjugated linoleic acid (CLA) isomers, in particular c9t11CLA and t10c12CLA, have received much attention for their health promoting effects (see recent reviews of Jahreis et al., 2000, Pariza et al., 2000, Pariza et al., 2001, Roche et al., 2001).

In the following text, emphasis is laid on the effects of nutrition on the intramuscular fatty acid composition of lamb, beef and pork meat only. This restriction in the data has been chosen since intramuscular fat is irreversibly connected with meat and it cannot be removed before human consumption, as is the case for visible fat, such as subcutaneous fat and backfat. In addition, given the more polyunsaturated fatty acid composition of intramuscular fat compared to removable depot fats, the relevance of intramuscular fat for the intake of long chain PUFA may be larger than expected at first sight. In this respect intramuscular fat may influence human health. On the other hand, the major fat intake from meat products by humans in western societies probably originates from backfat of pigs which is present in many processed meat products. This is in contrast with subcutaneous fat from ruminants, which is generally not or much less consumed by humans. No data are available to our knowledge on the contribution of intramuscular and removable animal fats separately to total fatty acid intake.

Several reviews have been published covering studies describing manipulation of the fatty acid composition of animal meat (Nurnberg et al., 1998, Demeyer and Doreau, 1999, Jakobsen, 1999, Wood et al., 1999), but paying less attention to long chain PUFA. Therefore, it was chosen to incorporate only these, mainly refereed, studies describing the intramuscular fat composition in particular the long chain n-3 or ω-3 (linolenic acid (LNA) C18:3n-3; eicosapentaenoic acid (EPA) C20:5n-3; docosapentaenoic acid (DPA) C22:5n-3 and docosahexaenoic acid (DHA) C22:6n-3) and the long chain n-6 or ω-6 fatty acids (linoleic acid (LA) C18:2n-6; arachidonic acid (ARA) C20:4n-6 and C22:4n-6) or studies determining CLA isomers (c9t11 or t10c12). Indeed the recent, and rapidly increasing interest, in long chain PUFA and CLA isomers in meat motivated this review.

All data are presented as g/100 g of total fatty acids to obtain a better comparison of results originating from studies with large differences in fat content. Therefore, some data had to be recalculated from the original references. When fatty acid composition was expressed on an absolute basis (e.g. mg/100 g meat), data were converted to g/100 g of total fatty acids taking into account the total fat or fatty acid content. Several references reported fatty acid composition of phospholipid and triacylglycerol fractions separately. Total fatty acid composition was then reconstituted based on the total fatty acid content in these fractions. The P/S ratio was always calculated as (C18:2n-6 + C18:3n-3)/(C14:0 + C16:0 + C18:0), while the n-6/n-3 ratio was calculated as (C18:2n-6 + C20:3n-6 + C20:4n-6 + C22:4n-6)/(C18:3n-3 + C20:5n-3 + C22:5n-3 + C22:6n-3).

Section snippets

Fatty acid composition of intramuscular fat

Intramuscular fat refers to the fatty acids present in the intramuscular adipose tissue and in the muscle fibres. The intramuscular adipose tissue is comprised of fat cells, isolated or in clusters, along the fibres and in the interfascicular area and contains mainly triacylglycerols, while the lipids of the fibres are cytosolic droplets of triacylglycerols, phospholipids and cholesterol. The amount of triacylglycerols in the fibres is only a minor part of the total intramuscular

Dietary supplementation of fish oil or fish meal

Long chain n-3 PUFA, EPA and DHA have a wide range of biological effects, which are believed to be beneficial for human health (Kromhout, 1989, Barlow et al., 1990). These fatty acids are only found in significant amounts in fish oil, fish meal and some algae products (Nettleton, 1991, Givens et al., 2000). Increasing the human intake of n-3 fatty acids is mainly achieved by supplementing the diet with encapsulated fish oil, by increasing the consumption of fish rich in n-3 fatty acids or by

Increasing the conjugated linoleic acid content in meat

A number of studies have recently been conducted with special emphasis on the amount of CLA in animal products and its effects on human health. The term CLA refers to a group of positional and geometric isomers of octadecaenoic acid with a conjugated double bond system, of which the c9t11 and t10c12 isomers are the most abundant. Both isomers are naturally found in ruminant derived food products (Chin et al., 1992), as ruminants metabolise PUFA in the rumen, i.e. lipolysis and biohydrogenation,

Conclusion

For both ruminants and monogastrics, fish oil or fish meal seem to be the only effective way to increase the deposition of DHA. While linseed or linseed oil inclusion in the animals diet supplies LNA, the conversion of LNA to its longer chain metabolites EPA and DPA seems to be limited, resulting in only a small increase in the deposition of EPA and DPA in intramuscular fat. Feeding linseed or linseed oil to ruminants and monogastrics seems to have no effect on the intramuscular DHA

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