Carbohydrate Utilization in Fish

Carbohydrate utilization in fish describes the limited but species-variable capacity of finfish to digest and metabolize starch and sugars, far below that of warm-blooded livestock. Carnivorous fish such as salmon and seabass digest gelatinized starch only at 30-60% efficiency and tolerate about 10-20% digestible carbohydrate in the diet, while omnivorous carp and tilapia handle 30-45%. The constraint is metabolic rather than purely digestive: fish show low hepatic glycogen storage regulation and persistent gluconeogenesis, and excess carbohydrate causes glycogen and lipid accumulation in the liver (hepatosteatosis), reducing growth and liver function. Carbohydrate still plays essential technological roles as a binder, pellet matrix, and energy sparer: each unit of energy supplied by starch displaces expensive protein and lipid. Processing strongly affects value: extrusion gelatinizes starch, raising its digestibility from raw values of 20-40% to 50-80%, whereas native grain starch in cold pellets is poorly used. For the feed producer, the practical targets are species-appropriate starch ceilings, preference for highly gelatinized extruded products, and monitoring of liver condition in broodstock and fast-growing fish. Over-stepping the carbohydrate ceiling shows first as soft feces, fat deposits, and a creeping FCR penalty.