Acetate and combinations of short-chain fatty acids increase oxidative phenotype and contribute to muscle fiber type shift in myotubes

Introduction Intestinal microbial fermentation produces short-chain fatty acids (SCFAs) that signal from the large intestine to skeletal muscle. Skeletal muscle exhibits phenotypic plasticity, with fiber-type composition shifting between oxidative (slow-twitch) and glycolytic (fast-twitch) states in response to metabolic and environmental cues. While SCFAs have been implicated in modulating metabolism, their role in skeletal muscle fiber-type regulation remains poorly defined. This study aimed to investigate whether acetate and SCFA mixtures (acetate, propionate, and butyrate) can promote a shift towards an oxidative skeletal fiber type in skeletal muscle cells. Methods Cultured C2C12 myotubes were exposed to SCFAs, and fiber type-specific gene and protein expression was assessed. Exposure: Exposure to ≥0.5 mM of the SCFA mixture for 8 h increased Pgc1α and Tfam gene expression compared to 10 mM acetate. This effect persisted after 24h exclusively in the SCFA mixture. Pparα gene expression at 8h was increased by ≥5 mM acetate and 10 mM SCFA mixture. Myh7 gene expression increased after 24h with ≥0.5 mM SCFA mixture and ≥5 mM acetate. After 48h, ≥0.5 mM acetate increased myosin heavy chain (MyHC) I staining, whereas 10 mM SCFA mixture reduced MyHC II. By 72h, the mixture further enhanced MyHC I and sustained MyHC II reduction. Conclusion This study shows that both acetate and SCFA combinations shift muscle myotubes towards oxidative fiber phenotype, with the mixture demonstrating a more pronounced effect at lower concentrations. This supports a role for gut-derived metabolites in muscle adaptation and demonstrates that SCFAs promote a shift toward an oxidative fiber type.