Dietary calcium β-hydroxy-β-methylbutyrate supplementation improved the flesh quality, but did not promote the growth performance of largemouth bass (Micropterus salmoides)

β-hydroxy-β-methylbutyric acid (HMB) is an intermediate product of leucine metabolism, which can promote protein synthesis. The study explored the dietary effects of HMB on the growth, muscle composition, flesh textural characteristics and flesh quality-related genes expression of largemouth bass (Micropterus salmoides). Calcium β-hydroxy-β-methylbutyrate (HMB-Ca) was supplemented to the control diet at levels of 0.5 (HMB-0.5) and 1 g/kg (HMB-1), to form three iso-nitrogenous diets. Then, 108 largemouth bass with average body weight of (110.0 ± 0.1) g were divided into 3 groups with three replicates per group and 12 fish per replicate to feed the above three diets for 12 weeks. No significant difference was observed in weight gain, feed conversion ratio as well as viscerosomatic index, condition factor and intraperitoneal fat ratio among the three treatments (P < 0.05). Compared with the control group, muscle crude lipid, n-3 PUFA content and n-3/n-6 ratio were raised in both HBM-Ca groups, and crude protein, collagen content, non-essential and total amino acids in flesh were promoted only in HMB-0.5 group. The addition of 0.5 and 1 g/kg HMB-Ca in diets enhanced flesh yellowness and decreased lightness (P < 0.05). In terms of flesh textural properties, the hardness, springiness, chewiness and shear force in the HMB-0.5 group were higher than those of the control group (P < 0.05). In addition, the supplementation of 0.5 g/kg HMB-Ca significantly lowered muscle fiber diameter and elevated muscle fiber density (P < 0.05). In gene expression, the mRNA levels of 4E-BP1, FoxO3a, MuRF-1, Atrogin-1 and MSTN were significantly lower, and S6K1, MyoD, MyOG, Myf5 and Myf6 were higher in the HMB-0.5 group than those of the control group (P < 0.05). Moreover, the expression levels of PI3K, AKT and TOR mRNA were also up-regulated in both HMB-Ca groups (P < 0.05). Overall, dietary HMB-Ca supplementation did not promote the growth of large-size largemouth bass, but improved muscle nutritional composition and flesh texture, partially attributing to the combined effect of AKT/TOR-FoxO3a signaling pathway and myogenic regulatory factors. The optimal content of HMB-Ca supplementation for largemouth bass was recommended to be 0.5 g/kg.