Protein-sparing effect of dietary lipid in practical diets for blunt snout bream (Megalobrama amblycephala) fingerlings: effects on digestive and metabolic responses

This study aimed to evaluate the protein-sparing effect of dietary lipid on digestive and metabolic responses of fingerling Megalobrama amblycephala. Fish were fed nine practical diets with three protein levels (270, 310 and 350 g kg−1) and three lipid levels (40, 70 and 100 g kg−1) for 8 weeks. Weight gain was significantly affected only by dietary lipid levels with the highest found in fish fed 70 g kg−1 lipid. Relative feed intake and whole-body protein content showed little difference among all the treatments. Activities of intestine lipase and amylase increased significantly as dietary lipid levels increased, whereas little difference was observed in protease activities. Liver lipid content was significantly affected only by protein levels with the lowest found in fish fed 310 g kg−1 protein. Liver aspartate aminotransferase (GOT) activities increased significantly with decreasing lipid levels, whereas the highest GOT activity was obtained in fish fed 310 g kg−1 protein in terms of dietary protein levels. Activities of liver lipoprotein lipase, total lipase and plasma cholesterol concentration of fish fed 350 g kg−1 protein were significantly lower than that of the other groups, whereas the same was true for plasma 3, 5, 3′-triiodothyronine level of fish fed 270 g kg−1 protein. The results indicated that an increase of dietary lipid content from 40 to 70 g kg−1 can enhance the growth and digestive enzyme activities of this species and reduce the proportion of dietary protein catabolized for energy without inducing hepatic steatosis; meanwhile, decreasing protein level from 350 to 310 g kg−1 leads to the increase of lipase activities both in intestine and liver coupled with the reduced liver lipid content.