Muscle Fiber Phenotype: A Culprit of Abnormal Metabolism and Function in Skeletal Muscle of Humans with Obesity

The proportion of the different types of fibers in a given skeletal muscle contributes to its overall metabolic and functional characteristics. Greater proportion of Type I muscle fibers is associated with favorable oxidative metabolism and function of the muscle. Humans with obesity have lower proportion of Type I muscle fibers. We discuss how lower proportion of Type I fibers in humans with obesity may explain metabolic and functional abnormalities reported in these individuals. These include lower glucose disposal rate, mitochondrial content, protein synthesis, and quality/contractile function at the muscle level, as well as increased risk for heart disease, lower levels of physical activity, and propensity for weight gain/resistance to weight loss. We delineate future research directions and the need to examine hybrid muscle fiber populations, which are indicative of a transitory state of fiber phenotype within skeletal muscle. We also describe methodologies for precisely characterizing muscle fibers and gene expression at the single muscle fiber level to enhance our understanding of the regulation of muscle fiber phenotype in obesity. By contextualizing research in the field of muscle fiber type in obesity, we lay a foundation for future advancements and pave the way for translation of this knowledge to address impaired metabolism and function in obesity.