Effects of Glycine Supplementation on Growth Performance, Antioxidant Activity, Immunity, and Muscle Tissue Structure of Whiteleg Shrimp (Litopenaeus vannamei) Under Fermented Soybean Meal Substitution

Exogenous amino acid supplementation has become a nutritional strategy to improve the tolerance of whiteleg shrimp (Litopenaeus vannamei) to a high proportion of vegetable protein diets. In the present study, the effects of glycine in high-proportion fermented soybean meal (FSBM) feed for shrimp were verified. SBM fermented by Bacillus subtilis was used to replace 50%, 75%, and 100% of fish meal (FM), respectively. Concomitantly, 1% glycine was added to the diets containing FSBM. After an 8-week feeding trial, FSBM substitution significantly inhibited the growth parameters of shrimp, including weight gain rate (WGR) and specific growth rate (SGR). Glycine supplementation significantly alleviated the inhibition of growth performance induced by 50% FSBM substitution but not in the 75% and 100% FSBM substitution groups. Glycine significantly increased the level of glycine in the muscle. In addition, glycine supplementation improved the structure of hepatopancreas and increased the length of sarcomeres and myofiber density in muscle tissue. RT-qPCR results revealed that glycine inhibited the mRNA expression of smyhc1 and smyhc2. Further investigation revealed that glycine enhanced the antioxidant capacity in muscle tissue and inhibited the mRNA expression of immune genes, including traf6, toll, and lgbp, caused by FSBM substitution. In summary, the results indicated that appropriate glycine supplementation could ensure that 50% FSBM substitution of FM did not affect the growth performance of shrimp. Moreover, glycine may improve the structure of muscle tissue by enhancing antioxidant capacity and immunity. This study further emphasized the crucial role of glycine in the development of low FM feed for shrimp.