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Background: Vitamin A (VA) and its metabolite, retinoic acid (RA), are of great interest for their wide range of physiological functions. However, the regulatory contribution of VA to mitochondrial and muscle fiber composition in sheep has not been reported. Method: Lambs were injected with 0 (control) or 7,500 IU VA palmitate into the biceps femoris muscle on d 2 after birth. At the age of 3 and 32 weeks, longissimus dorsi (LD) muscle samples were obtained to explore the effect of VA on myofiber type composition. In vitro, we investigated the effects of RA on myofiber type composition and intrinsic mechanisms. Results: The proportion of type I myofiber was greatly increased in VA-treated sheep in LD muscle at harvest. VA greatly promoted mitochondrial biogenesis and function in LD muscle of sheep. Further exploration revealed that VA elevated PGC-1α mRNA and protein contents, and enhanced the level of p38 MAPK phosphorylation in LD muscle of sheep. In addition, the number of type I myofibers with RA treatment was significantly increased, and type IIx myofibers was significantly decreased in primary myoblasts. Consistent with in vivo experiment, RA significantly improved mitochondrial biogenesis and function in primary myoblasts of sheep. We then used si-PGC-1α to inhibit PGC-1α expression and found that si-PGC-1α significantly abrogated RA-induced the formation of type I myofibers, mitochondrial biogenesis, MitoTracker staining intensity, UQCRC1 and ATP5A1 expression, SDH activity, and enhanced the level of type IIx muscle fibers. These data suggested that RA improved mitochondrial biogenesis and function by promoting PGC-1α expression, and increased type I myofibers. In order to prove that the effect of RA on the level of PGC-1α is caused by p38 MAPK signaling, we inhibited the p38 MAPK signaling using a p38 MAPK inhibitor, which significantly reduced RA-induced PGC-1α and MyHC I levels. Conclusion: VA promoted PGC-1α expression through the p38 MAPK signaling pathway, improved mitochondrial biogenesis, and altered the composition of muscle fiber type. [ABSTRACT FROM AUTHOR]