| Dietary lipids play a predominant role in nutrition, it can provide energy and essential fatty acids(FAs) that are required for normal growth and development. Adequate dietary lipids had positive effects on growth performance, such as protein sparing effects. With the rapid development of intensive aquaculture, high-lipid diet was widely used currently.However, excessive dietary lipid intake would result in increasing body and tissues lipid deposition, seriously affecting the health status, flesh quality and commercial value. Bile acids are amphipathic sterol compounds and play an important role in the digestion and absorption of dietary lipids, and are widely used as dietary additives in livestock and poultry breeding,and aquaculture.To investigate the effects of bile acids on the growth performance, lipid metabolism and the intestinal microbiota of grass carp, three isonitrogenous experimental diets, including control group(CT, 35% protein, 5% lipid), high-lipid group(HL, 35% protein, 7% lipid) and bile-acids group(BA, 35% protein, 7% lipid, 60 mg bile acids /kg feed), were fed to 96 grass carp(initial weight: 69.86±6.24 g) for 8 weeks. Such data were of theoretical significance and practical values, they will enrich the application of bile acids in fresh-water fish nutrition, and provide reference information for the regulation of lipid accumulation and protein deposition in cultured fish.Trial 1 Effects of bile acids on the growth performance, lipid and protein metabolism of juvenile grass carp(Ctenopharyngodon idellus). The growth performance, biological indices,apparent digestibility coefficient(ADC), intestinal digestive enzymatic activities, proximate composition, serum biochemical indices, expression of lipid/protein-metabolism-related genes and hepatic antioxidation status of grass carp were measured and analyzed after the feeding trial. The results showed:(1) BA group showed significantly higher specific growth rate(SGR)than CT and HL groups(P<0.05). Compared with CT group, BA group showed lower feed conversion ratio(FCR) and higher protein ef?ciency ratio(PER)(P<0.05). BA group also showed higher protein productive value(PPV) than HL group(P<0.05). Dietary bile acids decreased intraperitoneal fat index(IPFI) and increased gallbladder somatic index(GBSI) inthe experimental fish(P<0.05).(2) There was no significant difference observed in the ADCs of protein and lipid(P>0.05), but bile acids increased the intestinal lipase activities of the fish(P<0.05).(3) Compared with HL group, BA group showed lower lipid contents in body and hepatopancreas, and higher protein content in body(P<0.05).(4) Addition of bile acids reduced the contents of serum total cholesterol(T-chol) and high-density lipoprotein cholesterol(HDL-c)(P<0.05).(5) In the hepatopancreas and muscle, BA group showed higher ATGL m RNA expression, while in the intraperitoneal fat, the HSL m RNA expression was higher(P<0.05). BA group showed significantly higher PPARα m RNA expression levels in intraperitoneal fat and muscle(P<0.05). In the muscle, the up-regulated IGF-1 and down-regulated GLDH m RNA expression were observed in BA group, compared with those in HL group(P<0.05).(6) In the hepatopancreas, HL and BA accumulated the higher concentration of malondialdehyde(MDA) than CT group(P<0.05). However, the activities of superoxide dismutase(SOD) were not different from each other(P>0.05).Trial 2 Effects of bile acids on the tissue FA profile of juvenile grass carp(Ctenopharyngodon idellus). The relative and absolute values of the tissues FA profile, and the FA correlation between tissues and diets were measured and analyzed. The results suggested:(1) HL and BA groups showed higher n-6 polyunsaturated fatty acid(PUFA) and linoleic acid(LA, 18:2n-6) percentages than those of CT group in all tissues(P<0.05). BA group had lower absolute values of total FAs, n-6 PUFA and LA in the intraperitoneal fat(IPF), compared with HL(P<0.05), with an inverse trend in the hepatopancreas. The percentages and absolute values of EPA(20:5n-3) and DHA(22:6n-3) in the IPF were lower in HL and BA groups(P<0.05). In the hepatopancreas, BA group increased the percentages of n-3 PUFA, but decreased in the muscle, compared with HL(P<0.05). Besides, HL and BA groups recorded lower saturated fatty acid(SFA) percentages than CT in all tissues(P<0.05).However, in the IPF, the absolute value of SFA in BA group was lower than that in HL(P<0.05), whereas those in the hepatopancreas were similar among all groups(P>0.05).(2)The FA correlations between tissues and diets were high, but with some differences. The muscle was the highest, followed by the IPF, and the hepatopancreas was lowest(P<0.05).BA group showed the highest correlation, followed by the HL, and CT group was the lowest(P<0.05).Trial 3 Effects of bile acids on the intestinal microbiota of juvenile grass carp(Ctenopharyngodon idellus). The intestinal microbiota were analyzed using bacterial 16 S r RNA pyrosequencing. The results investigated that:(1) bile acids decreased the intestinal microbiota diversity and increased the species richness in grass carp.(2) Fish in BA groupcontained more Bacteroidetes and Fusobacteria, and fewer Firmicutes at phylum level, and expanded Bacteroides and Cetobacterium at genera level.Overall, dietary bile acids inclusion effectively suppressed lipid accumulation, promoted protein deposition, influenced the expression of lipid/protein-metabolism-related genes, and thereby improved the growth performance of grass carp. On the other hand, bile acids might regulate the transportation, accumulation and utilization of FAs in the tissues, and increase the FA correlation between tissues and diets. Besides, bile acids decreased the intestinal microbiota diversity, and might promote the growth performance of fish and inhibit lipid deposition by regulating the intestinal microbiota. |
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