Growth performance and metabolic responses of Nile tilapia fed diets with different protein to energy ratios

The effect of dietary digestible protein to digestible energy (DP/DE) ratio on the growth performance and metabolic responses of Nile tilapia juveniles was evaluated. Five experimental diets were formulated with increasing DP/DE ratio (13.2, 15.5, 18.4, 20.0, and 22.6 mg kJ(-1)) and fed to eight replicates of Nile tilapia (initial body weight of 9.3 g) for 110 days. The apparent digestibility coefficients of the diets were determined in a parallel trial. Digestibility coefficients of dry matter, crude protein, and crude energy linearly decreased with increasing dietary P/E ratios. A quadratic effect of dietary DP/DE ratio on growth performance was observed, and maximum weight gain (g kg(-1) day(-1)) or daily growth increment was attained with a dietary DP/DE ratio of 16.5 and 17.2 (mg kJ(-1)), respectively. Feed intake and energy retention (kJ ABW kg(-1) day(-1)) linearly decreased, and feed efficiency linearly increased with increased dietary DP/DE ratio. The increase of dietary DP/DE ratio decreased the whole body, muscle, hepatic, and visceral lipid content and increased the fillet yield. The decrease of dietary DP/DE linearly increased nitrogen retention (% N intake) and whole-body lipid deposition through the reduction of amino acid catabolism (GDH and ALAT) and gluconeogenesis (FBPase) related enzymes and the increase of glycolysis (GK and HK) and lipogenesis- (G6PDH and ME) related enzymes. In conclusion, a dietary DP/DE ratio of 16.5-17.2 mg kJ(-1) promoted the maximum growth performance of Nile tilapia juveniles. However, further studies are required to evaluate if a fine-tuning of the protein amino acid profile would allow a further reduction of the optimum dietary DP/DE ratio. Metabolic response to the decrease in dietary DP/DE levels suggested activation of glycolysis and lipogenesis and inhibition of amino acid catabolism and gluconeogenesis, highlighting the high capacity of Nile tilapia for metabolic utilization of dietary carbohydrates.

Automated summary

The study revealed that a dietary DP/DE ratio of 16.5-17.2 mg kJ(-1) promoted the maximum growth performance of Nile tilapia juveniles. With an increase in dietary DP/DE ratio, feed intake and energy retention decreased linearly, while feed efficiency increased linearly.