Utilization of dietary carbohydrate by fish

The nutritional value of carbohydrates varies among fish with warmwater fish being able to utilize much higher levels of dietary carbohydrate than coldwater and marine fish. No dietary requirement for carbohydrate has been demonstrated in fish; however, if carbohydrates are not provided in the diet, other nutrients such as protein and lipids are catabolized for energy and to provide metabolic intermediates for the synthesis of other biologically important compounds. Thus, it is important to provide the appropriate level of carbohydrate in the diet of the fish species being cultured. The relative use of dietary carbohydrates by fish varies and appears to be associated with the complexity of the carbohydrate. For example, certain species have been shown to utilize simple sugars as well as, or better than, complex carbohydrates whereas other species do not utilize simple sugars as an energy source. Most species utilize cooked starch better than raw starch. Oral glucose tolerance tests result in persistent hyperglycemia in fish. This prolonged hyperglycemia and the relative inability of fish to utilize high levels of dietary carbohydrates has been assumed to be the result of low levels of endogenous insulin. However, recent studies have shown that insulin levels in fish are similar to or often higher than those observed in mammals, thus indicating that fish are not diabetic as previously thought.