Porphyra Polysaccharides Alleviated High-Carbohydrate Diet-Induced Growth Retardation, Lipid Deposition, Impairment of Immune and Antioxidant Functions, and Intestinal Morphology in Rabbitfish (Siganus canaliculatus)

Porphyra polysaccharide (PPS), derived from marine red seaweeds of the Porphyra genus, has been reported to improve growth performance, lipid metabolism, and antioxidant capability in animals. The present study investigated the effects of PPS supplementation to a high-carbohydrate diet on growth performance, lipid metabolism, immunity, antioxidant capacity, intestinal morphology, and microbial composition in rabbitfish (Siganus canaliculatus). Rabbitfish were fed a basal starch diet (BSD, 15% starch) and high-starch diets (HSD, 25% starch) supplemented with varying levels of PPS (0%, 1.25%, 2.5%, and 5%) for 8 weeks. The results showed that HSD intake significantly decreased body weight and increased hepatosomatic index compared to rabbitfish fed BSD. But all dietary levels of PPS reversed these two indicators of fish fed HSD. In addition, the supplementation of 2.5% and 5% PPS significantly reduced the higher lipid contents in whole fish and abdominal muscle of fish fed HSD. Notably, 2.5% PPS reduced lipid droplets in the liver, possibly through the downregulation of genes associated with lipid synthesis and the upregulation of genes associated with lipid catabolism. Moreover, all levels of PPS supplementation decreased the higher serum alanine aminotransferase activity in fish fed HSD, indicating the alleviation of HSD-induced liver impairment. Additionally, PPS inclusion significantly increased the activity of serum lysozyme, superoxide dismutase, and catalase while decreasing serum malondialdehyde in fish fed HSD, suggesting improvements in immunity and antioxidant capacity. Furthermore, PPS inclusion mitigated damage to intestinal villi induced by HSD. Interestingly, 2.5% PPS increased the abundance of Bacteroidetes and Tenericutes while reducing the abundance of Proteobacteria and Spirochetes, indicating the reshaping of gut microbiota. In summary, dietary PPS alleviated the negative effects of HSD on growth performance, lipid metabolism, immunity, antioxidant capacity, and intestinal morphology and altered microbial composition in rabbitfish. This highlighted the beneficial effects of dietary PPS in fish and suggested it could contribute to the new strategies for treating metabolic syndrome and health impacts in aquatic animals.