Metabolic effects of changes in the dietary protein: carbohydrate ratio in eel (Angilla anguilla) and trout (Oncorhynchus mykiss)

The metabolic consequences of the isoenergetic replacement of dietaryprotein by carbohydrates (CHO) were compared in studies of European eel andrainbow trout. Diets with 45/20, 37/30, 29/40 and 21/50 percentprotein/carbohydrate were assessed during a 12-week experiment. The CHO sourcewas pre-gelatinized corn starch. Fish of initial average weight 45gwere fed to satiation twice daily. Weight-gain and feed-efficiency data weregenerally better in trout than in eel, presumably because trout digestibilitycoefficients for protein and energy were higher than in eel. In both species,regardless of physiological differences in digestion and absorption, feedintakeand nutrient digestibility were lower when dietary CHO was higher, but theinter-species differences decreased with increasing dietary CHO.Pyruvate kinase activity (PK) in trout was not affected by dietary CHO content,while blood-glucose and liver-glycogen levels significantly rose withincreasingdietary CHO. In contrast, blood-glucose levels appeared to be regulated ineels,perhaps by an accelerated glycolysis rate, revealed by changes in PK.Gluconeogenic activity was inhibited in trout fed a diet containing20–30%CHO, while in eels this activity was not inhibited by dietary replacement ofprotein by carbohydrate. Liver glucose 6-phosphate dehydrogenase activity(G6PDH) was inhibited in trout fed increasing dietary CHO, whereas in eelsG6PDHoperated at a high rate regardless of dietary composition. These results mayindicate that eels have a better capacity for metabolising high-CHO/low-proteindiets than do trout.