We evaluated the effects of dietary fat type on fat metabolism and deposition in broiler chickens. Birds were fed diets containing either 8 g dietary saturated (beef tallow) or polyunsaturated fat (sunflower oil)/100 g for 32 d. The abdominal fat deposition of chickens fed the sunflower oil-enriched diet was significantly lower than that of chickens fed the tallow-enriched diet (2.63 +/- 0.47 versus 3.03 +/- 0.44 g/100 g live wt.; P = 0.033). The specific activities of heart carnitine palmitoyltransferase I and L-3-hydroxyacyl-CoA dehydrogenase were higher (P < or = 0.03) in chickens fed the sunflower oil-enriched diets, indicating a greater rate of beta-oxidation. Liver fatty acid synthetase activity was lower (P = 0.01) in chickens fed the sunflower oil-enriched diet, suggesting reduced hepatic lipogenesis in this group. Postprandial plasma triglyceride levels were significantly lower (P < 0.05) in birds fed the sunflower oil-enriched diet, indicating a higher rate of dietary lipid clearance from the bloodstream to tissues. In conclusion, the lower fat deposition observed in broilers fed sunflower oil-enriched diets appears to be the net result of an increased rate of lipid catabolism and lower rate of fatty acid synthesis despite higher dietary fat absorption.

Rainbow trout (Oncorhynchus mykiss) hepatocytes were cultured under simulated conditions of varying nutritional status to explore the short-term modulation by dietary substrates of the main lipogenic enzymes: glucose-6-phosphate dehydrogenase (G6PD), malic enzyme (ME), ATP-citrate lyase (ACL), acetyl-CoA carboxylase (ACoAC) and fatty acid synthetase (FAS). Primary cultures were individually exposed to varying amounts of glucose, hydrolysed casein and long-chain polyunsaturated fatty acids (PUFA) for 12 h. A second set of experiments was designed to evaluate the effects of mixing different relative amounts of these macronutrients in the culture medium. Glucose concentrations of up to 20–25 mM SHOWED A STIMULATORY EFFECT ON G6PD, ME, ACL AND ACOAC ACTIVITY (P<0·05) WHILE AN EARLIER INHIBITORY EFFECT ON FAS WAS OBSERVED AT 10–20 Mm glucose (P<0·05). The use of hydrolysed casein as a nutritional source of amino acids inhibited the activity of FAS and ME (P<0·05), and stimulated G6PD, ACoAC and ACL activity (P<0·05). Low levels of linolenic acid exerted a stimulatory effect on all the lipogenic enzymes assayed (P<0·05) with the exception of FAS, and increased amounts showed some inhibition of lipogenic activities (P<0·05). Eicosapentaenoic acid and docosahexaenoic acid showed a similar effect, although the former strongly inhibited FAS activity while the latter showed greater potential to inhibit ACoAC and G6PD. A complete change in the relative levels of glucose, hydrolysed casein and PUFA in turn led to changes in the enzyme activity patterns observed. The present study shows the feasibility of exploring the direct regulation of lipogenesis in isolated fish cells by varying the relative amounts of main macronutrients, mimickingin vivodietary conditions. It is felt that such an approach may serve to investigate the macronutrient regulation of other metabolic pathways.

The objective of this experiment was to test the effect of total or partial substitution of dietary fish oil (FO) by linseed oil (LO) in Atlantic salmon feeding on performance, liver and muscle fatty acid composition, selected lipogenic and lipolytic enzyme activities, and flesh oxidative stability. For 12 wk, fish (220 +/- 12 g of initial BW) were fed five experimental diets in which the FO was serially replaced by 25, 50, 75, and 100% LO. Total FO replacement by LO did not (P = 0.20) affect fish final weight, biometric indices, or i.m. fat contents. Liver and muscle neutral lipid (NL) composition responded to dietary treatments in different ways. Whereas the sum of n-3 PUFA in muscle followed a linear and quadratic pattern with increasing levels of LO, a linear (P = 0.005) effect was observed in the liver NL fraction. Total n-3 and n-6 PUFA contents in the polar lipid fraction (PL) were unaffected (P = 0.356) by dietary input of LO in muscle. Activity of liver glucose-6-P-dehydrogenase (G6PD) was greater with increasing levels of LO (P = 0.004). A time effect (P < 0.001) was observed in the concentration of lipid peroxidation products, expressed as thiobarbituric acid reactive substances, in fish flesh stored under refrigeration for 9 d; however, the progressive inclusion of LO in the feed did not affect (P = 0.125) flesh oxidation stability. In summary, LO can totally replace FO in Atlantic salmon feed without affecting growth performance and muscle susceptibility to lipid oxidation. Fatty acid metabolism in the liver was affected by LO, promoting G6PD activity and eicosatetraenoic acid accumulation; however, a 100% LO replacement decreased (P < 0.001) concentrations of eicosapentaenoic and docosahexaenoic acids in salmon muscle.

To maximize growth, farmed fish are fed high-fat diets, which can lead to high tissue lipid concentrations that have an impact on quality. The intake of conjugated linoleic acid (CLA) reduces body fat in mammals and this study was undertaken to determine the effects of dietary CLA on growth, composition, and postprandial metabolic variables in sea bream. Fish were fed 3 diets containing 48 g/100 g protein and 24 g/100 g fat, including fish oil supplemented with 0 (control), 2, or 4% CLA for 12 wk. Feed intake, specific growth rate, total body fat, and circulating somatolactin concentration were lower in fish fed CLA than in controls. Feed efficiency was greater in fish fed 2% CLA than in controls. Liver triglyceride concentrations were higher in fish fed 4% CLA and muscle triglyceride concentrations were lower in fish fed both CLA diets than in controls. Hepatic fatty acyl desaturase and elongase mRNA levels in fish fed CLA were lower than in controls. Metabolic differences between controls and CLA-fed fish were observed at 6 h but not at 24 h after the last meal, including lower postprandial circulating triglyceride concentrations, higher hepatic acyl-CoA-oxidase, and lower L-3-hydroxyacyl-CoA dehydrogenase activities in CLA-fed fish than in controls. Dietary CLA did not affect enzymes involved in lipogenesis including hepatic fatty acid synthase and malic enzyme, but it decreased glucose 6-phosphate dehydrogenase activity at 24 h, but not at 6 h after feeding. The data suggest that CLA intake in sea bream has little effect on hepatic lipogenesis, channels dietary lipid from adipose tissue to the liver, and switches hepatic mitochondrial to peroxisomal beta-oxidation.

The effects of dietary fish oil and digestible protein (DP) levels on muscle fatty acid composition and susceptibility to lipid peroxidation were studied in two representative fish species for human nutrition, from fresh and seawater, rainbow trout (Oncorhynchus mykiss) and European sea bass (Dicentrarchus labrax). In rainbow trout, higher concentrations of dietary fat and DP led to higher weight gain (g/d) (P = 0.001 and P = 0.043 respectively). Additionally, an interaction effect was observed in this species, since the effect of DP was only evident when the dietary fat concentration was low (P = 0.043). A similar tendency was also observed in European sea bass, although with less marked differences among nutritional treatments. Trout fed on diets with a higher concentration of dietary fat had higher concentrations of intramuscular total and neutral lipids in the dorsal muscle (P = 0.005). Increased levels of dietary DP led to significantly lower concentrations of polar lipids in the dorsal muscle of both rainbow trout (P = 0.005) and European sea bass (P = 0.006). In the neutral fraction of intramuscular lipids of dorsal muscle the concentration of n-3 fatty acids was positively affected by the dietary fat concentration in both rainbow trout (P = 0.04) and sea bass (P = 0.001). Muscle homogenates from trout and sea bass fed on diets rich in fish oil showed a significantly higher susceptibility to oxidation than muscle homogenates from fish fed on low-fat diets (P = 0.001). The higher DP concentration also increased susceptibility to oxidation. Moreover, in rainbow trout an interaction effect was observed where the pro-oxidant effect was of higher magnitude when the dietary concentration of both nutrients, fat and protein, was high (P = 0.004).