Dietary trimethylamine N-oxide (TMAO) reduces hepatic lipid deposition and increases flesh hardness through enhancing L-carnitine synthesis and myofiber proliferation in largemouth bass (Micropterus salmoides)

The expansion of intensive aquaculture and shifts in feed ingredient supply and demand have driven the adoption of high-energy feeds, resulting in metabolic disorders and diminished flesh quality in farmed fish. The natural compound trimethylamine oxide (TMAO) shows great promise as a functional feed additive, such as dietary TMAO supplementation significantly reduces the abdominal fat ratio in bluntnose black bream (Megalobrama terminalis), and decreases crude lipid content in the liver of Nile tilapia (Oreochromis nilotica), yet its effects and mechanisms in carnivorous fish such as the largemouth bass (Micropterus nigricans) remain unclear. This study investigated the effects of TMAO at different supplementation levels on the growth performance, lipid metabolism, fish flesh quality, and targeted metabolomics of largemouth bass. A 56-day feeding trial was designed with three TMAO supplementation levels (0.1%, 0.2%, and 0.4%). A total of 360 largemouth bass (8.78 ± 2.9 g) were randomly assigned to four treatment groups, each with three replicates. TMAO supplementation at all levels had no effect on growth performance. TMAO reduced hepatic lipid deposition in largemouth bass with a concentration-dependent effect, upregulating fatty acid β-oxidation gene expression. Targeted metabolomics analysis further revealed that TMAO increased carnitine content and elevated methylation levels by increasing SAM content in the liver. Furthermore, TMAO increased fish flesh hardness, promoted muscle fiber growth and development, enhanced muscle fiber density, and upregulated gene expression in the mTOR pathway. This study elucidates that dietary TMAO supplementation promotes lipid metabolism by increasing carnitine levels in the liver, and enhances muscle hardness by promoting muscle fiber growth and development.