| Recently,with the improvement of living standards,the flesh quality is gradually attached to consumers,which makes how to improve the flesh quality has become a research hotspot in the aquaculture.However,high-fat feed(HFD),a common feed in aquaculture,has some adverse effects on farmed fish,especially in fat deposition and flesh quality.Therefore,how to improve the flesh quality of farmed fish fed with HFD has become a top priority for the whole aquaculture.In aquaculture,the traditional method is to mitigate the damage caused by HFD to farmed fish by adding feed additives,changing the breeding environment or feeding methods.Enhancing lipid catabolism in farmed fish significantly reduces the long-term stress of HFD.Dietary supplementation with L-carnitine and short-term starvation treatment significantly enhanced lipid catabolism.Therefore,we hypothesized that enhancing lipid catabolism could improve the flesh quality in HFD-fed fish.In this study,we investigated the effects of two measures to improve lipid catabolism on the growth and flesh quality of HFD-fed Nile tilapia,and further explored the mechanisms to improve the flesh quality of farmed fish.Our study offers a new research idea for improving the flesh quality of farmed fish.(1)Effect of L-carnitine on growth,nutritional composition and fillet quality of HFD-fed Nile tilapiaIn aquaculture,L-carnitine,as a commonly used lipid-lowering additive,can enhance mitochondrial fatty acid β-oxidation in farmed fish and weaken the damage of HFD to farmed fish.This experiment was conducted to investigate whether the dietary L-carnitine supplementation could improve the flesh quality of HFD-fed Nile tilapia.In the present study,Nile tilapias with approximate average weight(3.2 ± 0.3 g)were fed with three different feeds separately: medium-fat diet group(MFD,6 % dietary lipid),high-fat diet group(HFD,12 % dietary lipid)and high-fat + L-carnitine group(HFCD,12 % dietary lipid;400 mg/kg L-carnitine supplementation)for 10 weeks.After the feeding trail,the fish fed with HFD significantly increased the final body weight,whole-body fat,liver fat content,p H value,the percentage of ∑n-6,PUFA and DAA,significantly decreased lactic acid content,the percentage of SFA,FAA and ∑n-3/∑n-6,but similar levels of the body length,crude protein,crude lipid and glycogen in muscle than those in MFD group.Compared with the HFD group,HFCD group had higher value of SFA and FAA,lower values of the final body weight,whole-body fat and liver fat content,p H value,the percentage of ∑n-6,EPA+DHA and ∑n-3,but similar levels of the body length,crude protein,crude lipid and glycogen in muscle.Moreover,the HFD-fed fish had higher myofiber number,the proportion of small myofiber and lower myofiber diameter,the proportion of large myofiber than those in MFD group,whereas the fish fed with high-fat + L-carnitine diet(HFCD)had higher the myofiber diameter,the proportion of large myofiber and lower the number of myofiber,the proportion of small myofiber than those in HFD group.In the gene expressions,genes related to myofiber hypertrophy,myofiber hyperplasia,autophagy and oxidative stress had not significant difference among three groups.But compared with the MFD group,the HFD feeding increased caspase-3 and genes related to endoplasmic reticulum stress and muscle inflammation significantly increased,while the expressions of these genes in the HFCD group were significantly lower than HFD group.Taken together,this research demonstrates that the practical usage of endogenous L-carnitine in farmed fish could alleviate the adverse effects of p H value,hardness and myofiber size caused by HFD in flesh quality.(2)Effects of starvation on growth,nutritional composition and meat quality of HFD-fed Nile tilapiaIn addition to nutritional factors,the management of culture techniques is also one of the important factors affecting lipid catabolism in cultured fish.In aquaculture,shortterm starvation of farmed fish is a relatively simple and common measure to enhance lipid catabolism and thus weaken the damage of HFD to farmed fish.The aim of this experiment was to investigate the effect of short-term starvation treatment on the flesh quality of HFD-fed Nile tilapia.Male Nile tilapia weighed 5.2±0.3 g were screened and randomly divided into 8 groups(3 breeding tanks per group,25 tilapias per tank).The experiment was divided into two stages: in the first stage,24 tanks were fed with medium-fat and high-fat diets for 10 weeks,the first 4 groups were fed with mediumfat diets(6% fat content,MFD)and the last 4 groups were fed with high-fat diets(12%fat content,HFD)to construct the MFD tilapia model and the HFD tilapia model;in the second stage,starvation treatments were performed from the 10 th week onwards,and the starvation period was 2 weeks.Samples were taken at four starvation time points of0 day of starvation,3 days of starvation,7 days of starvation and 14 days of starvation,respectively,and the experiment lasted for 12 weeks.The results showed that with increasing starvation time,Nile tilapia fed with HFD significantly decreased body weight,condition factor,Viscerosomatic index,Hepatosmatic index,while the crude lipid content in muscle first increased and then decreased,moisture in muscle first decreased and then increased.These results indicate that short-term starvation treatments significantly reduced fat deposition and significantly altered muscle composition in Nile tilapia fed with HFD.With the increase of starvation time,in MFDfed Nile tilapia muscle,the percentage of PUFA,EPA+DHA,∑n-3 and ∑n-6 increased significantly,∑n-3/∑n-6 ratio decreased significantly,while the percentage of MUFA decreased first and then increased.With increasing starvation time,in Nile tilapia fed with MFD and HFD,the p H value,viscosity,elasticity,and chewiness showed a trend of decreasing and then increasing,with a significant decrease in muscle viscosity and the number of myofiber while a significant increase in myofiber area.In both MFD and HFD groups,Nile tilapia myofiber diameter was lowest at 0 days of starvation,highest steaming loss and centrifugal loss at 3 days of starvation,and highest muscle stiffness and gelatinization at 14 days of starvation.In addition,the expression of Mu RF,Atrogin-1,Myo G and Myo D increased significantly with increasing starvation,while the expression of ATF4 decreased significantly.In HFD group,the expression of CPT1αwas lowest on 0 day of starvation,the expression of CPT1β on 7 days and 14 days of starvation was significantly higher than the other two groups,and in the 7 days of starvation,the expression of PPARα had higher value than the other three groups.These data showed that,on the one hand,short-term starvation treatment significantly alleviated fat deposition and altered the muscle composition of HFD-fed Nile tilapia.On the other hand,short-term starvation treatment significantly altered the textural characteristics,fatty acid composition and myofiber tissue structure of Nile tilapia muscle,and significantly enhanced the expression of mitochondrial β-oxidation,muscle atrophy and myofiber proliferation-related genes in muscle,which in turn improved the fish quality of Nile tilapia under MFD and HFD feeding.In summary,this study investigated the improvement of flesh quality of farmed fish under HFD feeding by using exogenous L-carnitine and short-term starvation treatment,two measures to enhance catabolism metabolism and reduce the negative effects of HFD.It was found that enhancing catabolism metabolism of farmed fish not only effectively alleviated the negative effects of HFD on the growth of farmed fish,but also significantly improved the deposition of nutrients in fish fillets and enhanced the nutritional value and taste and flavor of fillet,thus systematically improving the flesh quality of farmed fish fed with HFD.Therefore,this study provides an important theoretical reference for farmers and farming enterprises to regulate the flesh quality of farmed fish in the breeding stage,and provides a new research idea for further improving the flesh quality of farmed fish. |
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