Influence of dietary conjugated linoleic acid (CLA) on lipid and fatty acid composition in liver and flesh of Atlantic salmon (Salmo salar)

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Abstract

The aim of the present study was to determine the effects of conjugated linoleic acid (CLA) on lipid and fatty acid metabolism in Atlantic salmon. The overall objective being to test the hypotheses that CLA has beneficial effects in salmon including growth enhancement, improved flesh quality through decreased adiposity and lipid deposition thereby minimising detrimental effects of feeding high fat diets, and increased nutritional quality through increased levels of beneficial fatty acids including n  3 highly unsaturated fatty acids (HUFA) and CLA itself. Salmon smolts were fed diets containing two levels of fish oil (low, ∼18% and high, ∼34%) containing three levels of CLA (a 1 : 1 mixture of 9-cis,trans-11 and trans-10,cis-12. at 0, 1 and 2% of diet) for 3 months and the effects on growth performance, liver and muscle (flesh) lipid contents and class compositions, and fatty acid compositions determined. The diets were also specifically formulated to investigate whether the effects of CLA, if any, were more dependent upon absolute content of CLA in the diet (as percentage of total diet) or the relative level of CLA to other fatty acids. Dietary CLA in salmon smolts had no effect on growth parameters or biometric parameters. However, there was a clear trend of increased total lipid and triacylglycerol contents in both liver and flesh in fish fed CLA, particularly in fish fed the high oil diets. Finally, CLA was incorporated into tissue lipids, with levels in flesh being 2-fold higher than in liver, but importantly, incorporation in liver was at the expense of saturated and monounsaturated fatty acids whereas in flesh it was at the expense of n  3HUFA.

Introduction

Lipids and their constituent fatty acids are, along with proteins, the major organic constituents of fish, with carbohydrates being quantitatively much less prominent in most fish than in mammals (Dabrowski and Guderley, 2002). Indeed the lipid (oil) content of fish can markedly exceed the protein content, reflecting the major role lipids and specifically their constituent fatty acids play as sources of metabolic energy in fish, for growth including reproduction and movement including migration (Tocher, 2003). Dietary lipid is also required to provide the necessary essential fatty acids (EFA) for formation of new cell membranes (Sargent et al., 1989). Thus lipid and fatty acids can either be incorporated into cell membranes and thus the flesh of the fish, or they can be oxidised to provide energy, or lipid can be deposited in adipose tissue as an energy store (Tocher, 2003). The more energy supplied by dietary lipid, the less dietary protein will be used for energy, and so more protein can be “spared” for synthesis of new tissue/flesh (Wilson, 1989, Bell, 1998). This phenomenon has been exploited in aquaculture and, in recent years, technical advances in extruded feed production have enabled the lipid or fat content of pelleted diets to increase greatly but, although protein sparing by dietary lipid is well documented, the limits to its effectiveness have not been accurately defined for any fish species (Company et al., 1999). Despite this, recent dietary formulations have tended to continue the upward trend in dietary lipid, particularly in the case of Atlantic salmon (Salmo salar) (Tocher, 2003). In many cases this has successfully increased weight gains, but several studies have shown that a potential and, perhaps, detrimental effect of high fat diets is the deposition of excess lipid in tissues, specifically flesh in the case of salmon (Sargent et al., 2002, Tocher, 2003). As well as dietary lipid levels, lipid deposition can be related to a number of factors including size, season and disease (Shearer, 1994, Sargent et al., 2002). Consequently, it is important to gain a clearer understanding of the physiological mechanisms that control energy metabolism, and that determine lipid and fatty acid homeostasis in fish.

Conjugated linoleic acid (CLA) is a term used to describe positional and geometric isomers of linoleic acid (18 : 2 n  6; LA), the two main naturally occurring isomers being cis-9,trans-11 and trans-10,cis-12, that are known have physiological effects and health benefits including anticarcinogenic and immune enhancing properties in mammals (Belury, 2002). However, in addition, CLA also has several beneficial effects on lipid metabolism in mammals (Delany and West, 2000), particularly in relation to body composition (Wang and Jones, 2004). Specifically, CLA decreased body fat and increased lean body mass in mice (Ohnuki et al., 2001, Terpstra et al., 2002), rats (Yamasaki et al., 2003) and pigs (Thiel-Cooper et al., 2001, Tischendorf et al., 2002). Decreased body fat has also been observed in human studies (Smedman and Vessby, 2001, Thom et al., 2001, Riserus et al., 2001) although the effect was much less than that observed with mice (Terpstra, 2004). CLA is also known to decrease the activity and gene expression of mammalian stearoyl CoA Δ9 desaturase (SCD) (Choi et al., 2001, Choi et al., 2002) and may also suppress Δ6 and Δ5 desaturase (Chuang et al., 2001a, Eder et al., 2002) and elongase (Chuang et al., 2001b). Further studies have suggested that CLA may enhance growth and feed efficiency in young rodents (Pariza et al., 2001) although this has not been consistently observed.

The aims of the present study were to determine the effects of CLA on lipid and fatty acid composition in Atlantic salmon. The overall objective being to test the hypotheses that CLA has beneficial effects in Atlantic salmon including growth enhancement, improved flesh quality through decreased adiposity and lipid deposition thereby minimising detrimental effects of feeding high fat diets, and increased nutritional quality through increased levels of beneficial fatty acids including n-3HUFA and CLA itself. In the trial described, salmon smolts were fed diets containing two levels of fish oil containing three levels of CLA for 3 months and the effects on growth performance, liver and flesh fat contents and fatty acid compositions determined.

Section snippets

Diets and animals

Photoperiod Atlantic salmon (S. salar L.) smolts (S1/2) were obtained from a commercial salmonid farm (Howietoun Fish Farm, Sauchieburn, Scotland) in late October and transported to the Stirling University, Institute of Aquaculture, Marine Environmental Research Laboratory, Machrihanish, Scotland. The fish were maintained in stock tanks for 3 weeks at ambient water temperature of around 10–11 °C to acclimatize during which time the fish were fed standard salmon diet, before being randomly

Diet composition

Inclusion of CLA in the low oil diets resulted in levels of total CLA of 5.9% and 9.5% of total fatty acids at the 1% and 2% inclusion levels, respectively (Table 2). Obviously, in the high oil diets, inclusion of CLA at 1% and 2% resulted in lower levels of total CLA, at 3.3% and 5.8% of total fatty acids, respectively. Note that the levels of CLA in relative terms were identical in the L1 and H2 diets with an overall rank order for CLA content of L2 > L1 = H2 > H1 > L0 / H0.

Growth and biometry

There were no effects of

Discussion

The primary aim of the present trial was to determine if dietary CLA had any important effects on lipid metabolism and growth parameters in Atlantic salmon. In a recent study, no effect of dietary CLA on growth rate or proximate composition was observed in Atlantic salmon fry fed diets containing up to 2% CLA (Berge et al., 2004). Similarly, no effects on weight gain or feed efficiency were noted in juvenile yellow perch (Perca flavescens) or catfish (Ictalurus punctatus) fed diets containing

Acknowledgements

This work and SRK was supported by a Biotechnology and Biological Science Research Council (BBSRC) CASE studentship award (BioMar Ltd., Grangemouth, Scotland).

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