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Intramuscular fat content in meat-producing animals: development, genetic and nutritional control, and identification of putative markers.

Intramuscular fat (IMF) content plays a key role in various quality traits of meat. IMF content varies between species, between breeds and between muscle types in the same breed. Other factors are involved in the variation of IMF content in animals, including gender, age and feeding. Variability in IMF content is mainly linked to the number and size of intramuscular adipocytes. The accretion rate of IMF depends on the muscle growth rate. For instance, animals having a high muscularity with a high glycolytic activity display a reduced development of IMF. This suggests that muscle cells and adipocytes interplay during growth. In addition, early events that influence adipogenesis inside the muscle (i.e proliferation and differentiation of adipose cells, the connective structure embedding adipocytes) might be involved in interindividual differences in IMF content. Increasing muscularity will also dilute the final fat content of muscle. At the metabolic level, IMF content results from the balance between uptake, synthesis and degradation of triacylglycerols, which involve many metabolic pathways in both adipocytes and myofibres. Various experiments revealed an association between IMF level and the muscle content in adipocyte-type fatty acid-binding protein, the activities of oxidative enzymes, or the delta-6-desaturase level; however, other studies failed to confirm such relationships. This might be due to the importance of fatty acid fluxes that is likely to be responsible for variability in IMF content during the postnatal period rather than the control of one single pathway. This is evident in the muscle of most fish species in which triacylglycerol synthesis is almost zero. Genetic approaches for increasing IMF have been focused on live animal ultrasound to derive estimated breeding values. More recently, efforts have concentrated on discovering DNA markers that change the distribution of fat in the body (i.e. towards IMF at the expense of the carcass fatness). Thanks to the exhaustive nature of genomics (transcriptomics and proteomics), our knowledge on fat accumulation in muscles is now being underpinned. Metabolic specificities of intramuscular adipocytes have also been demonstrated, as compared to other depots. Nutritional manipulation of IMF independently from body fat depots has proved to be more difficult to achieve than genetic strategies to have lipid deposition dependent of adipose tissue location. In addition, the biological mechanisms that explain the variability of IMF content differ between genetic and nutritional factors. The nutritional regulation of IMF also differs between ruminants, monogastrics and fish due to their digestive and nutritional particularities.

Energy requirements, utilization and dietary supply to salmonids

Abstract Energy needs of fasting salmonids are about 30–80 kJ/kg body weight/day, depending mainly on water temperature. Maintenance energy requirements are estimated as 75–100 kJ/kg/day, about 10- to 20-fold lower than those of terrestrial vertebrates. Under negative N-balance or maintenance conditions, body protein oxidation accounts for a greater part of the energy supply in fish than in higher animals. Energy requirements for body growth and tissue deposition have been studied to some extent and current estimates vary between 15 and 18 MJ of DE/kg of live weight gain, being affected by water temperature, genotype, body size and growth rate. While the apparent digestibility coefficients of protein and fat from most commonly used feed ingredients are high, those of carbohydrates vary widely. The additive nature of DE values appears to make the use of DE values reliable in feed formulation and feeding and the use of metabolizable energy (ME) values, especially those drawn from poultry nutrition, bears little practical significance. Due to the relatively high contribution of protein and amino acids as preferential energy substrates in fish, much attention is paid to the digestible protein to digestible energy ( DP DE ) ratios, but precise, optimal values are not available for all species. Heat increment of feeding appears to be mainly related to protein intake. Few attempts have been made in the recent past to draw feeding tables based on DE requirements for maintenance and growth, the validity of which needs confirmation under practical farming conditions.

Effect of dietary carbohydrate levels on growth, body composition and glycaemia in rainbow trout, Oncorhynchus mykiss, reared in seawater

Abstract Three experimental diets were fed to near-satiation twice a day for 9 weeks to triplicate groups of 40 rainbow trout (mean initial weight 230 g) reared in seawater (36.5 ppt) at 9°C. The diets were formulated to have the same levels of nitrogen (crude protein 43% DM) and energy (digestible energy: around 15 kJ/g DM) but they contained different amounts of digestible carbohydrate (8–24.4%) and fat (5.7–11.1%). Digestibility was measured using chromic oxide as a marker. Plasma glucose concentrations were measured in 48-h fasted trout and at selected intervals after a single meal. Digestibility of starch and energy was lowest in trout fed the diet containing the highest level of crude starch. Apparent digestibility values of protein (86.0–87.4%) and fat (76.8–81.5%) were similar for the three diets. Fish growth and body composition were not affected significantly ( P > 0.05) by the non-protein energy sources of the diets. However, the amounts of digestible protein and digestible energy required per kilogram production were higher for fish fed the high-carbohydrate diet (475 ± 7 g DP and 18.8 ± 0.3 MJ DE) than for fish fed the high-fat diet (419 ± 21 g DP and 16.0 ± 0.8 MJ DE). Liver size and liver lipid content increased in direct relationship to the dietary level of digestible carbohydrate. The highest percentage of muscle lipid was found in fish fed the diet containing 25% carbohydrate and 10% lipid as non-protein energy sources. Glycaemia was increased by elevated digestible carbohy-drate intake. In conclusion, 230 g rainbow trout were able to utilize carbohydrate efficiently for growth in seawater (up to 25% of digestible carbohydrate in diet) under the conditions of this study. However, dietary lipid had a slightly better protein-sparing effect than digestible carbohydrate at a similar level of digestible energy intake. High dietary levels of digestible carbohydrate induced lipogenesis and enhanced lipid deposition in the liver.

Effect of digestible carbohydrates on protein/energy utilization and on glucose metabolism in rainbow trout (Salmo gairdneri R.)

Abstract Diplicate groups of rainbow trout were fed low-protein (38%) diets containing high levels (38%) of five different carbohydrate sources (raw starch, extruded corn, extruded wheat, extruded corn starch or extruded wheat starch). A modified pair-feeding method was used to allow equal intakes of protein and of digestible energy by each group of fish. Growth parameters were followed over a period of 18 weeks; the digestibility of the diets, nitrogen and energy balances, and the respiratory and ammonia quotients of fish fed the different diets were determined. To study the long-term effects of dietary carbohydrates, plasma glucose profile and its control by bovine insulin was followed in fish fed over 30 weeks with selected test diets. The results show that inclusion of extruded cereals or extruded starch improves availability of dietary energy. High levels of carbohydrates to not adversely affect overall growth or nutrient retention efficiencies. Long-term feeding with carbohydrate-rich d diets does not confer on trout any adaptive capacity to regulate postprandial glycemia levels. Respirometric measurements appear to provide interesting data on the metabolic utilization of body substrates.

The effects of ovine growth hormone on protein turnover in rainbow trout

Ovine growth hormone (oGH) was administered to rainbow trout via an intraperitoneal cholesterol implant. After 21 days, plasma oGH levels were recorded as control group, less than 2 ng ml-1, i.e., not detectable, and oGH group, 19.2 +/- 2.8 ng ml-1. oGH-treated fish exhibited significantly increased whole-body growth rates, whole-body protein accretion rates, stimulated tissue protein synthesis, and tissue protein accretion rates. A dramatic decrease in white muscle protein concentration was also observed after oGH treatment. In some tissues (liver and stomach), elevated protein synthesis rates were the result of higher RNA/protein ratios. However, in other tissues (gill and ventricle), increased RNA activity accounted for the differences in rates of protein synthesis. The growth promoting effects of oGH on both whole-body and tissue protein turnover were generally accompanied with no change in the efficiency of deposition of newly synthesized protein. For the same ration size, the oGH group showed higher retentions of ingested nitrogen. It is concluded that oGH significantly enhances whole-body growth rates as a result of the stimulatory effect on protein synthesis rates with little effect on protein degradation.

Hepatic protein kinase B (Akt)-target of rapamycin (TOR)-signalling pathways and intermediary metabolism in rainbow trout (Oncorhynchus mykiss) are not significantly affected by feeding plant-based diets.

The aim of the present study was to analyse the effects of partial or total replacement of fish meal (FM) and fish oil (FO) by a mixture of plant protein (PP) and a mixture of vegetable oils (VO) on the hepatic insulin-nutrient-signalling pathway and intermediary metabolism-related gene expression in rainbow trout (Oncorhynchus mykiss). Triplicate groups of fish were fed four practical diets containing graded levels of replacement of FM and FO by PP and VO for 12 weeks: diet 0/0 (100 % FM, 100 % FO); diet 50/50 (50 % FM and 50 % PP, 50 % FO and 50 % VO); diet 50/100 (50 % FM and 50 % PP, 100 % VO); diet 100/100 (100 % PP, 100 % VO). Samplings were performed on trout starved for 5 d then refed with their allocated diet. In contrast to partial substitution (diet 50/50), total substitution of FM and FO (diet 100/100) led to significantly lower growth compared with diet 0/0. The insulin-nutrient-signalling pathway (protein kinase B (Akt), target of rapamycin (TOR), S6 protein kinase 1 (S6K1) and S6) was characterised in trout liver and found to be activated by refeeding. However, changes in diet compositions did not differentially affect the Akt-TOR-signalling pathway. Moreover, expression of genes encoding fructose-1,6-biphosphatase, mitochondrial phosphoenolpyruvate carboxykinase, glucokinase, pyruvate kinase and carnitine palmitoyl transferase 1 were not affected by refeeding or by dietary changes. Refeeding down- and up-regulated the expression of gluconeogenic glucose-6-phosphatase isoform 1 and lipogenic fatty acid synthase genes, respectively. Expression of both genes was also increased with partial replacement of FM and total replacement of FO (diet 50/100). These findings indicate that plant-based diets barely affect glucose and lipid metabolism in trout.

Hepatic gene expression profiles in juvenile rainbow trout (Oncorhynchus mykiss) fed fishmeal or fish oil-free diets

Reducing the reliance on fishery by-products as amino acid and fatty acid sources in feeds for farmed fish is a major objective today. We evaluated the effect of dietary fish oil or dietary fishmeal replacement by vegetable oils and plant proteins respectively through analysis of hepatic transcriptomes in rainbow trout (Oncorhynchus mykiss). Fish were fed right from first feeding with diets based on plant by-products before being killed. We analysed the hepatic gene profile using trout cDNA microarrays (9K). Our data showed that seventy-one and seventy-five genes were affected after fish oil and fishmeal replacement respectively. The major part of modified gene expression coding for proteins of the metabolic pathways was as follows: (i) a lower level of expression for genes of energy metabolism found in fish after fishmeal and fish oil replacement; (ii) a lower level of gene expression for fatty acid metabolism (biosynthesis) in fish fed with vegetable oils; (iii) a differential expression of actors of detoxification metabolism in trout fed with vegetable oils; (iv) a lower level of expression of genes involved in protein metabolism in fish fed with plant proteins. Overall, our data suggest that dietary fish oil replacement is linked to a decreased capacity of fatty acid biosynthesis (fatty acid synthase) and variation of detoxification metabolism (cytochrome P450s) whereas dietary fishmeal replacement may depress protein metabolism in the liver as reflected by glutamine synthetase.

Insulin Stimulates Lipogenesis and Attenuates Beta-Oxidation in White Adipose Tissue of Fed Rainbow Trout

As lipid deposition tissue in fish, the white adipose tissue (WAT) has important functions related to reproduction and the challenges of long-term fasting. In the study reported here, we infused fish fed a high-carbohydrate diet with two doses of insulin for 5xa0days in order to explore the effects of this hormone on lipogenesis and beta-oxidation-related enzymes. We demonstrated the presence of some of the main lipogenic enzymes at molecular, protein and activity levels (ATP-citrate lyase and fatty acid synthase). However, while ATP-citrate lyase was unexpectedly down-regulated, fatty acid synthase was up-regulated (at protein and activity levels) in an insulin dose-dependent manner. The main enzymes acting as NADPH donors for lipogenesis were also characterized at biochemical and molecular levels, although there was no evidence of their regulation by insulin. On the other hand, lipid oxidation potential was found in this tissue through the measurement of gene expression of enzymes involved in β-oxidation, highlighting two carnitine palmitoyltransferase isoforms, both down-regulated by insulin infusion. We found that insulin acts as an important regulator of trout WAT lipid metabolism, inducing the final stage of lipogenesis at molecular, protein and enzyme activity levels and suppressing β-oxidation at least at a molecular level. These results suggest that WAT in fish may have a role that is important not only as a lipid deposition tissue but also as a lipogenic organ (with possible involvement in glucose homeostasis) that could also be able to utilize the lipids stored as a local energy source.

Studies on the nutrition of Siberian sturgeon, Acipenser baeri. II. Utilization of dietary non-protein energy by sturgeon

Abstract Duplicate groups of juvenile Siberian sturgeons (initial body weight 49 g) were fed two experimental diets (crude protein 51% DM; gross energy 22 kJ/g DM) containing either 21.8% lipid and 9.9% crude starch (diet F) or 12.5% lipid and 20% gelatinized starch (diet M) as non-protein energy sources. Growth parameters were followed over a period of 8 weeks; the digestibility of the diets, nitrogen and energy balances and body composition of sturgeons fed the two diets were determined. The digestibility of lipid decreased when the diet contained a high level of fat, so that the available digestible energy (DE) was higher in the diet containing the lower fat level with digestible carbohydrates. This increase in DE led to an increase in fat deposition, even in the muscle. In spite of the difference in DE, metabolic nitrogen losses were not different between the two dietary treatments, suggesting that, in juvenile Siberian sturgeon, dietary lipids can be more efficient in sparing protein compared with dietary carbohydrates. Lipids appeared to be used as energy sources more efficiently than carbohydrates at the metabolic level.

Postprandial Regulation of Hepatic MicroRNAs Predicted to Target the Insulin Pathway in Rainbow Trout

Rainbow trout are carnivorous fish and poor metabolizers of carbohydrates, which established this species as a model organism to study the comparative physiology of insulin. Following the recent characterisation of key roles of several miRNAs in the insulin action on hepatic intermediary metabolism in mammalian models, we investigated the hypothesis that hepatic miRNA expression is postprandially regulated in the rainbow trout and temporally coordinated in the context of insulin-mediated regulation of metabolic gene expression in the liver. To address this hypothesis, we used a time-course experiment in which rainbow trout were fed a commercial diet after short-term fasting. We investigated hepatic miRNA expression, activation of the insulin pathway, and insulin regulated metabolic target genes at several time points. Several miRNAs which negatively regulate hepatic insulin signaling in mammalian model organisms were transiently increased 4 h after the meal, consistent with a potential role in acute postprandial negative feed-back regulation of the insulin pathway and attenuation of gluconeogenic gene expression. We equally observed a transient increase in omy- miRNA-33 and omy-miRNA-122b 4 h after feeding, whose homologues have potent lipogenic roles in the liver of mammalian model systems. A concurrent increase in the activity of the hepatic insulin signaling pathway and the expression of lipogenic genes (srebp1c, fas, acly) was equally observed, while lipolytic gene expression (cpt1a and cpt1b) decreased significantly 4 h after the meal. This suggests lipogenic roles of omy-miRNA-33 and omy-miRNA-122b may be conserved between rainbow trout and mammals and that these miRNAs may furthermore contribute to acute postprandial regulation of de novo hepatic lipid synthesis in rainbow trout. These findings provide a framework for future research of miRNA regulation of hepatic metabolism in trout and will help to further elucidate the metabolic phenotype of rainbow trout.