Effects of diets with distinct protein-to-carbohydrate ratios on nutrient digestibility, growth performance, body composition and liver intermediary enzyme activities in gilthead sea bream (Sparus aurata, L.) fingerlings

The effect of replacing dietary protein with gelatinized cornstarch (GCS) on apparent digestibility coefficient (ADC), body composition, growth performance and liver enzyme activities involved in control of intermediary metabolism, was studied in Sparus aurata L. Fingerlings of S. aurata were fed 93 days three diets containing 63% protein and 5% gelatinized cornstarch (LC diet), 54% protein and 18% GCS (MC diet) or 47% protein and 26% GCS (HC diet). Diet HC gave ADC values for carbon, nitrogen and dry matter that were significantly below the corresponding values of the other diets. Fish on MC diet registered higher fresh weight than fish on LC and HC, and higher specific growth rate (SGR) than fish on HC. The lipid body content ranked in the order HC > MC > LC. High correlations between carbohydrate level and liver enzyme activity were found for pyruvate kinase, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and alanine aminotransferase. For cultures of S. aurata, we conclude that carbohydrates like GCS could replace dietary protein, enhance growth rate and reduce nitrogen losses to the ambient waters when used at levels below 20%.