Dietary protein-to-carbohydrate ratios affect metabolism and growth of juvenile surubim cachara (Pseudoplatystoma reticulatum)

Abstract

This study was conducted to evaluate the effects of dietary carbohydrates (CHO) and protein on growth performance, nutrient retention, blood metabolites, tissue energy reserves (hepatic glycogen and lipids, mesenteric fat index, and hepatosomatic index), and key metabolic enzyme activities (hexokinase, glucokinase, glucose-6-phosphate dehydrogenase, and aspartate aminotransferase) of surubim cachara (Pseudoplatystoma reticulatum). Six experimental diets containing one of two CHO levels (170 or 300 g kg−1) and three crude protein (CP) levels (360, 400, or 440 g kg−1) were fed to four replicate groups of P. reticulatum (69.2 ± 4.4 g) for 60 days. The results showed that 300 g kg−1 CHO resulted in a worse final weight (FW, 208.4 ± 26.9 g), weight gain (WG, 139.4 ± 23.9 g), specific growth rate (SGR, 1.8 ± 0.2% day−1), apparent feed conversion (AFC, 1.5 ± 0.3), and protein efficiency ratio (PER, 163.7 ± 23.4%) than fish fed 170 g kg−1 CHO (FW, 241.4 ± 37.3 g; WG, 171.9 ± 36.6 g; SGR, 2.1 ± 0.2% day−1; AFC, 1.2 ± 0.2; PER, 203.7 ± 36.9%). Fish fed 170 g kg−1 CHO showed greater retention of CP (REcp, 36.3 ± 6.9%) and energy (REge, 23.9 ± 5.6%) in their carcass than fish fed 300 g kg−1 CHO (REcp, 28.9 ± 3.9%, REge, 18.5 ± 3.7%). The highest blood glucose levels (97.7 ± 32.7 mg dL−1) were observed in fish fed the 300CHO/400CP diet, and they had a significantly lower mesenteric fat index (0.85 ± 0.26%) than fish fed with the 170CHO/400CP diet (1.25 ± 0.33%). Hepatic hexokinase activity increased in fish fed 300 g kg−1 CHO, resulting in an activity of 5.0 μmol mg−1 of protein, whereas fish fed 170 g kg−1 CHO had an activity of 3.2 μmol mg−1 of protein. These results demonstrate that dietary CHO directly affect the intermediary metabolism of P. reticulatum and feeding 300 g kg−1 CHO and 440 g kg−1 CP should be considered excessive as it results in decreased nutrient retention in the carcass and decreased growth performance despite metabolic adjustments.