Effects of dietary citric acid on growth performance, mineral status, body and muscle composition, muscle growth and mTOR signaling in yellow catfish Pelteobagrus fulvidraco fed with low-manganese diets

The positive impacts of dietary citric acid (CA) on aquaculture species have been studied. However, the wide application of CA in aquatic-feed requires a comprehensive understanding of its nutritional functions. Here, an 8week feeding trial was performed to determine whether dietary CA supplementation could improve manganese (Mn) utilization and thus reduce Mn excretion by investigating the effect of dietary CA on growth performance, mineral statues, body and muscle composition, muscle growth and mTOR signaling of yellow catfish Pelteobagrus fulvidraco fed with low-manganese (L-Mn) diets. Four isonitrogenous and isolipidic diets were formulated and fed to triplicated groups of fish. A suitable Mn diet containing 5.7 mg kg(-1) Mn was set as the control, while the diets without extra Mn addition was regarded as L-Mn group. The other two diets were supplemented with 30 g kg(- 1) CA into the control and L-Mn diet, respectively. The results showed that the weight gain, specific growth rate, feed efficiency, protein content in muscle, iron concentration in serum, and Mn content in whole body and muscle were significantly decreased after L-Mn diet and resumed by dietary CA addition (P < 0.05). Dietary lowMn markedly decreased the diameters of muscle fibers, which was alleviated by CA supplementation. Dietary Mn and CA interacted to affect the density of the muscle fiber density, the mRNA expression of muscle development-, myocyte myogenic and myocyte enhancer factors-related genes (P-interaction <0.05). Besides, expression of genes in muscle involved in mTOR signaling including mtor, s6 and eif4e were significantly down-regulated in fish fed L-Mn diet compared to the control. CA supplementation reversed the L-Mn diet induced decrease of their mRNA levels. Furthermore, the protein expression of p-mTOR, S6 and p-S6 were significantly decreased after LMn diet, and were upregulated by CA supplementation in the muscle (P < 0.05). Taken together, this study revealed that CA as a feed additive could improve growth performance, mineral bioavailability, muscle growth and protein synthesis in yellow catfish under the condition of Mn deficiency. These positive effects may be partially attributed to dietary CA supplementation improving Mn absorption and bioavailability. This study also has a significance for the development of environmentally friendly feeds for the aquatic species.