In terms of microbiota functional predictions, “lipid metabolism” pathway was significantly enriched in the BET group, which may align with the known relationship between betaine supplementation and anti-obesity effects.18 Here, our results identify betaine as a novel microbiota-derived key metabolite that orchestrates fiber-type remodeling via the betaine-Mettl3-m6A-Myh7 axis. Here, METTL3 is linked to obesity due to melanocortin 4 receptor deficiency.