Effect of dietary bile acids on growth, body composition, lipid metabolism and microbiota in grass carp (Ctenopharyngodon idella)

Abstract

To investigate the effects of dietary bile acids (BA) on growth and metabolism of lipid in grass carp (Ctenopharyngodon idella, C. idella) at high dietary lipid level, a basal diet (50 g kg–1 lipid, 5L group) was supplemented with 20 g kg–1 soybean oil (70 g kg–1 lipid, 7L group); then, 0.06 g/kg BA was added in 7L diet to form the third diet (7L+BA group). The 96 C. idella (69.86 ± 6.24 g) were divided into three groups (duplicate per group) and fed three diets, respectively, for 8 weeks, and then, growth and lipid metabolism were determined. Results showed that growth of fish in 7L+BA group was significantly higher than 5L and 7L groups. The lipid level in whole body, hepatopancreas and muscle of grass carp in 7L+BA group were significantly lower than 7L group. Relative expression of lipid catabolism genes in hepatopancreas and muscle of 7L+BA group was significantly higher than 5L group. The amount of microbiota in intestine of fish in 7L+BA group was significantly higher than the other two groups. The present results indicated that BA in 7L diet improved growth of fish by increasing protein synthesizing, decreasing lipid content in fish body and by regulating amount of microbiota in intestine of fish. KEYWORDS adipose triglyceride lipase, fatty acid synthase, growth performance, intestinal microbiota, lipid accumulation, protein deposition

Lipids are important energy-dense macronutrients, which are well utilized by most fish species, but also provide essential fatty acids, fat soluble vitamins, phospholipids and cholesterol required by fish for normal growth, development and maintaining their health condition (NRC 2011). The inclusion of non-protein energy sources such as lipids and carbohydrates maximizes the utilization of dietary protein in fish (Bowyer, Qin & Stone, 2014; Nankervis, Matthews, & Appleford, 2000; NRC 2011). It has also been reported that many fish species have the ability to utilize the non-protein energy and effectively spare protein for growth purposes (Gao et al., 2011; Pavlidis & Mylonas, 2011). In general terms, non-protein energy sources also have been shown to modulate growth performance (Han et al., 2014), voluntary feed intake (Saravanan et al., 2012), feed utilization (Han et al., 2014), lipid metabolism (Kamalam, Médale, Larroquet, Corraze, & Panserat, 2013), oxidative enzymes activity (Rueda-Jasso et al., 2004) and immune responses (Jin et al., 2013; Li, Lim, Klesius, & Cai, 2012) in different fish species. However, the current trend of high-lipid diet use has been shown to induce an undesirable increase in fat deposition (Du et al., 2006, 2008), as well as physiological symptoms, such as susceptibility to autoxidation and tissue lipid peroxidation, which they might also lead to a decrease in feed consumption and growth reduction (Du et al., 2006, 2008; Rueda-Jasso et al., 2004), and ultimately??

affecting flesh quality (Dias et al., 1998) and the commercial value of fish (Hanley, 1991). Bile acids (BA), which are defined as amphipathic sterol compounds (Kortner, Gu, Krogdahl, & Bakke, 2013; Romański, 2007), have important roles in digestion and absorption of dietary lipids in the intestine (Hofmann & Hagey, 2008; Li, Jadhav, & Zhang, 2013) by contributing to the emulsification of fats in the chime and increasing the activity of bile salt-activated lipase, as it has been described in Japanese flounder (Paralichthys olivaceus; Alam, Teshima, Ishikawa, & Koshio, 2002), Japanese eel (Anguilla japonica; Maita, Tachiki, Kaibara, Itawaki, & Ikeda, 1996) and in humans (Bauer, Jakob, & Mosenthin, 2005; Romański, 2007). In addition, dietary BA have been reported to improve growth performance in yellowtail (Seriola quinqueradiata; Deshimaru, Kuroki, & Yone, 1982), A. japonica (Maita et al., 1996), P. olivaceus (Alam et al., 2002), Prussian carp (Carassius auratus gibelio; Tan, Wei, & Zen, 2008) and cobia (Rachycentron canadum; Zhou et al., 2010). Bile acids have been reported to decrease lipid deposition in the liver of turbot (Scophthalmus maximus; Sun et al., 2014), giant prawn (Macrobrachium rosenbergii; Ma et al., 2008) as well as in higher vertebrates like rats (Pieters, Schouten, & Bakkerren, 1991), whereas they have also been reported to decrease body lipid deposition in R. canadum (Zhou et al., 2010) and in the cyprinid Schizothorax prenanti (Zheng et al., 2016). The herbivorous grass carp (Ctenopharyngodon idellus) is one of the most important freshwater fish species farmed in China. The lipid requirements of C. idellus are relatively low (ca. 50g/kg) (Du et al., 2006, 2008), although diets with higher lipid levels are usually used to improve and boost the growth. However, the use of high dietary lipid levels in C. idellus results in excessive fat deposits in body, especially in the viscera and liver (Guo et al., 2013). In this species, the dietary addition of BA was reported to enhance the antioxidant capacity of the serum, as well as in the hepatopancreas, whereas there is no available information about the effect of dietary BA in C. idellus fed high-lipid levels and their impact on growth, body composition, lipid metabolism and intestinal microbiota. Thus, this experiment was designed to provide insight into the effect of dietary BA administration in C. idellus and their on growth performance, feed efficiency parameters, body composition, activity of selected pancreatic digestive enzymes, expression of selected gene markers of lipid metabolism and intestinal microbiota.

Three isonitrogenous experimental diets were prepared to evaluate the effects of dietary inclusion in BA in C. idellus, a basal diet (5L diet) containing just 50 g kg–1 crude lipids (Jin et al., 2013), a diet containing 70 g kg–1 crude lipids (7L diet), in which lipid levels were achieved by the addition of 20 g kg–1 soybean oil, and a third diet containing 70 g kg–1 crude lipids and 0.06 g/kg BA (7L+BA diet). The formulation and proximate composition of diets are given in Table 1. Feed ingredients were purchased from Huaqin Husbandry and Technology Co., Ltd. (Yangling, Shaanxi, China), whereas bile acids (>950 g k purity) were obtained from Longchang Animal Health Products Co., Ltd. (Jinan, Shandong Province, China). Diets were prepared by mixing all dry ingredients for 30 min. Then, fish oil and sufficient distilled water, as well as BA, were added to form soft dough that was mechanically extruded to obtain pellets of ca. 2 mm in size. Pellets were dried in a convection oven at 25°C and stored in resealable plastic bags at ?20°C until their use