Profile of the digestive and absorptive functions in the intestine of fish with different dietary habits and in response to increased dietary carbohydrate or protein.

Global aquaculture has grown significantly, but to maintain current consumption, supply must double over the next 20 years. Here, we characterised the digestive and absorptive capacities in the intestines of three commercially important, farmed fish species with varying natural diets: Nile tilapia (Oreochromis niloticus), common carp (Cyprinus carpio ), and rainbow trout (Oncorhynchus mykiss). Disaccharidase activity (maltase, sucrase) and sodium-dependent D-glucose absorption were highest in Nile tilapia, followed by common carp, and lowest in rainbow trout. We found regional intestinal differences in disaccharidase activity and SGLT1 expression in the three species. Aminopeptidase A (APA) and aminopeptidase N (APN) activities were typically highest in Nile tilapia. Intestinal expression of the neutral amino acid transporter B0AT1 was approx. double in tilapia and carp compared to trout. We also assessed common carp, zebrafish and Rainbow trout's digestion profile when fed low-carbohydrate/high protein and high-carbohydrate/low-protein diets. Common carp and zebrafish adapted to high-carbohydrate diets by up-regulating both digestive enzymes and SGLT1. Common carp increased SGLT1 protein abundance 2.7-fold in the foregut and 4.6-fold in the hindgut. Rainbow trout increased disaccharidase activity but not SGLT1 expression in response to a high-carbohydrate diet. Both Rainbow trout and common carp down-regulated their APN activity but upregulated their B0AT1 expression 2-fold in response to increased dietary protein. These physiological limitations are critical considerations for formulating sustainable plant-based aquafeeds.