Altered skeletal muscle fiber composition and size precede whole-body insulin resistance in young men with low birth weight

Context: Low birth weight (LBW), a surrogate marker of an adverse fetal milieu, is linked to muscle insulin resistance, impaired insulin-stimulated glycolysis, and future risk of type 2 diabetes. Skeletal muscle mass, fiber composition, and capillary density are important determinants of muscle function and metabolism, and alterations have been implicated in the pathogenesis of insulin resistance. Objective: The aim of this study was to investigate whether an adverse fetal environment (LBW) induces permanent changes in skeletal muscle morphology, which may contribute to the dysmetabolic phenotype associated with LBW. Design and Subjects: Vastus lateralis muscle was obtained by percutaneous biopsy from 20 healthy 19-yr-old men with birth weights at 10th percentile or lower for gestational age (LBW) and 20 normal birth weight controls, matched for body fat, physical fitness, and whole-body glucose disposal. Myofibrillar ATPase staining was used to classify muscle fibers as type I, IIa, and IIx (formerly type IIb), and double immunostaining was performed to stain capillaries (LBW, n = 8; normal birth weight, n = 12). Results: LBW was associated with increased proportion of type IIx fibers (+ 66%; P = 0.03), at the expense of decreased type IIa fibers (- 22%; P = 0.003). No significant change was observed in proportion of type I fibers (+ 16%; P = 0.11). In addition, mean area of type IIa fibers was increased (+ 29%; P = 0.01) and tended to be increased for type I fibers as well (+ 17%; P = 0.08). Capillary density was not significantly different between groups. Conclusion: Alterations in fiber composition and size may contribute to development of type 2 diabetes in individuals with LBW.