Replacing Soybean Meal with Ultrafine Pulverized Mulberry Leaf (Combined with Multi-Enzyme Preparations) Reduces Fat Deposition and Alleviates Oxidative Stress in Grass Carp via Gut–Liver Axis Regulation

Grass carp are prone to excessive fat deposition in intensive aquaculture, which impairs their health and reduces economic returns. Here, we investigated the effects of replacing soybean meal with ultrafine pulverized mulberry leaves combined with multi-enzyme preparations on lipid metabolism in grass carp using phenotypic and multi-omics approaches. A total of 480 grass carp (8.45 ± 0.17 g) were fed a basal diet and two experimental diets (6% and 12%) containing mulberry leaf for 60 d. Replacing soybean meal with ultrafine pulverized mulberry leaf with multi-enzyme preparations reduced serum triglyceride, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and dorsal muscle ether extract levels, downregulated hepatic glycolysis (gk, pfk, pk) and lipogenesis (fas and acc) genes and increased intestinal lipase activity. Intestinal omics identified the lysine degradation pathway as a key pathway that regulates lipid metabolism. Reduced levels of multiple differential metabolites and enhanced microbial metabolic functions are closely associated with inhibition of lipid metabolism in grass carp. The reduction in fat deposition, particularly polyunsaturated fatty acids, the downregulation of antioxidant capacity, as indicated by decreased glutathione peroxidase and superoxide dismutase activities and reduced total antioxidant capacity, along with a decrease in malondialdehyde levels, suggests that the lipid-lowering effects could alleviate oxidative stress. Replacing soybean meal with ultrafine pulverized mulberry leaves combined with multi-enzyme preparations reduced lipid deposition and alleviated oxidative stress in grass carp by regulating the gut–liver axis. When replacing soybean meal, a 12% proportion of ultrafine pulverized mulberry leaves combined with multi-enzyme preparations in the diet is recommended.