Dietary L-carnitine supplementation affects flesh quality through modifying the nutritional value and myofibers morphological characteristics in largemouth bass (Micropterus salmoides)

The increase of dietary fat content is becoming prevalent in aquaculture practices, however, the increased dietary fat content is associated with undesirable impacts such as growth inhibition, increased feed conversion ratio, and deteriorated flesh quality. L-carnitine has been used as a feed additive to alleviate severe fat accumulation-induced metabolic disorders, however, its effects on flesh quality remain largely unexplored. In the present study, the effects of L-carnitine on flesh quality were examined by feeding triplicate groups of largemouth bass juveniles (10.75 ± 0.1 g) with three experimental diets including a control diet (CD), a high-fat low-protein diet (HFLPD), and HFLPD supplemented with 0.5 g kg^{− 1} of L-carnitine for 8 weeks. The results showed that L-carnitine supplementation does not significantly influence final body weight, body length, specific growth rate and carcass ratio. Whole body lipid and muscle triglycerides (TG) contents significantly increased in fish fed the HFLPD compared to the CD group, and L-carnitine supplementation decreased their values. In addition, expression of lipid catabolism related genes such as CPT and ACOX3 were significantly upregulated by L-carnitine application. The fish fed L-carnitine containing diet exhibited markedly higher monounsaturated fatty acids (MUFA) and eicosapentaenoic acid (EPA) contents, and lower saturated fatty acids (SFA) content than the HFLPD-fed fish. Moreover, the texture qualities such as hardness, gumminess and chewiness remarkably decreased in the HFLPD + Car group compared to the HFLPD group. Most specifically, the percentage of medium-diameter fibers (60–120 µm diameter) was significantly decreased in the HFLPD group compared to the CD group, while this was notably reversed by L-carnitine addition. In summary, the present study demonstrated that dietary L-carnitine supplementation enhances nutritional value, alleviates HFLPD-induced metabolic disorders and flesh quality changes in largemouth bass.