MYH4 and obesity due to melanocortin 4 receptor deficiency: Yan et al. (2016) reported that obese pig microbiota transplantation upregulated MYH7 (slow myosin) while downregulating MYH4 (fast myosin) in GF mice, promoting slow-twitch fiber formation. This aligns with findings that Lactobacillus plantarum supplementation improved exercise capacity concomitant with increased type I fibers in gastrocnemius (Chen et al., 2016). Such microbiota-induced fiber-type shifts may involve mitochondrial biogenesis (Kootte et al., 2012), with transmissible characteristics including obesity-associated phenotypes (Yan et al., 2016; Nicholson et al., 2012).