Energy utilisation efficiencies of digestible starch and non-starch polysaccharide in Nile tilapia

The current trend of incorporating diverse plant-based and novel ingredients in fish diets is leading to higher dietary non-starch polysaccharide (NSP) levels, altering the dietary starch-to-NSP ratio. While tilapia lack NSP-digesting enzymes, research indicates that tilapia can digest NSP, possibly by intestinal microbial fermentation, producing absorbable short-chain volatile fatty acids. This suggests that NSP may contribute to the growth of tilapia, necessitating a better understanding of the utilisation efficiency of the digested NSP for growth. This study assessed the utilisation efficiencies of digested NSP and starch for the growth of Nile tilapia, Oreochromis niloticus. Four experimental diets, including a control diet, with large contrasts in starch and NSP contents were formulated. A 1:1 blend of corn-derived distillers dried grains with solubles and de-oiled rice bran was used as the dietary NSP source. Each diet was administered at three feeding levels to fish (mean weight 45 g/fish) reared in a recirculating aquaculture system for 7 weeks at 30 °C. Dietary NSP supplementation negatively impacted weight gain (P < 0.001) and feed conversion ratio of the fish (P < 0.001). NSP-supplemented diets decreased the efficiency of digested energy for energy retention (RE) and increased the digestible energy requirements for maintenance compared to the control diet. RE (kJ/kg^0.8 per day) was related to digestible starch (dStarch) and digestible NSP (dNSP) intake (g/kg^0.8 per day), while the coefficients for digestible protein (dCP, 13.2 kJ/g) and digestible fat (dFat, 33.5 kJ/g) were fixed: RE = -30 + (13.2 × dCP) + (33.5 × dFat) + (12.5 × dStarch) + (0.9 × dNSP), where the intercept represents fasting heat production (kJ/kg^0.8 per day). The energy utilisation efficiencies of dStarch and dNSP were 73 and 5%, respectively, suggesting that dNSP marginally contributes to energy gain in Nile tilapia. These findings suggest that separate estimation of starch and NSP utilisation is essential, particularly when diets contain substantial amounts of NSP, to achieve precise energy evaluation in Nile tilapia feeds.