Effects of dietary lipid and protein levels on metamorphosis, growth, metabolism and gut microbiota of tadpole (Lithobates catesbeianus)

Abstract

Tadpole cultivation is a crucial step in the artificial production of bullfrogs (Lithobates catesbeiana), but its feed nutrition is poorly understood. This study examined how dietary lipid and protein levels affect metamorphosis, growth and intestinal health of tadpole. Six groups of tadpoles were fed six diets with two lipid levels (7% and 10%) and three protein levels (36%, 40% and 44%) for 70 days. The results showed that at 7% lipid level, increasing protein enhanced metamorphosis rate and mean weight of tadpoles, reaching the optimal value at 40% protein (P < 0.05). At 10% lipid level, increasing protein also improved mean weight, peaking at 40% protein. Dietary nutrition also influenced liver amino acid and lipid metabolism directly. At 7% lipid level, increasing protein significantly increased the activities of hepatic enzymes GLS, PDH, ALT, AST, etc. (P < 0.05), and up-regulated the expression of hepatic genes fas, accb, fadd6 and cpt1a involved in lipid metabolism (P < 0.05). At 10% lipid level, increasing protein also up-regulated fadd6 and cpt1a genes (P < 0.05), but down-regulated acadl gene. Meanwhile, increasing dietary lipid level elevated TG and T-CHO levels in liver. Increasing dietary protein enhanced gut microbiota richness, while increasing lipid reduced it. Moreover, different gut microbiota groups had distinct biomarkers, such as Rikenellaceae, Desulfovibrionaceae, Burkholderiales and Bacteroidales, which were related to amino acid or lipid metabolism. In summary, under the test conditions, 7% lipid and 40% protein in diet are suitable levels, which can promote metamorphosis, growth and metabolism of tadpoles, and are more economical than high-lipid and high-protein diets.