Optimized Solid-State Fermentation of Sugar Beet Pulp with Mixed Microbes Improves Its Nutritional Value and Promotes Growth, Health, and Intestinal Function in Yellow Catfish (Pelteobagrus fulvidraco)

The rising cost of conventional protein sources such as soybean meal has prompted the search for sustainable and economical alternatives in aquafeeds. Sugar beet pulp (SBP), an abundant by-product of the sugar industry, possesses nutritional potential but is limited by its high fiber and anti-nutritional factors. Solid-state fermentation (SSF) offers a promising approach to enhance its nutritive value and functional properties. This study evaluated the effects of dietary inclusion of mixed microbial solid-state fermented beet pulp (FBP) on the growth, systemic health and intestinal function of juvenile yellow catfish (<i>Pelteobagrus fulvidraco</i>). First, orthogonal optimization determined <i>Lactiplantibacillus plantarum</i>:<i>Saccharomycopsis fibuligera</i>:<i>Bacillus subtilis</i> = 1:3:3 as the optimal ratio, significantly improving the nutritional profile of FBP. Based on this optimized FBP, an 8-week feeding trial, five isonitrogenous and isolipidic diets were formulated by replacing 0-12% soybean meal with FBP. The results demonstrated that 9% FBP inclusion yielded optimal growth performance and significantly improved muscle texture. At the systemic level, FBP supplementation reduced serum lipid markers and liver enzyme activities while enhancing antioxidant capacity. At the intestinal level, FBP promoted intestinal health by increasing key digestive enzyme (lipase, trypsin, amylase) activities, stimulating villus development, and improving intestinal antioxidant status. Furthermore, gut microbiota analysis revealed that dietary FBP supplementation significantly modulated intestinal microbial composition, with notable enrichment of genera such as <i>Leucobacter</i>. In conclusion, FBP is a multi-functional ingredient that enhances growth, product quality, systemic physiology, and intestinal health in yellow catfish aquaculture. These findings provide a viable strategy for the sustainable utilization of agricultural by-products in aquafeeds.