Skeletal muscle of females and males with constitutional thinness: a low intramuscular lipid content and oxidative profile

Constitutional thinness (CT) is a non-pathological state of underweight. The present study aimed to explore skeletal muscle energy storage in individuals with constitutional thinness and to further characterize muscle phenotype at baseline and in response to overfeeding. Thirty subjects with CT (15 females, 15 males) and 31 normal-weight control subjects (16 females, 15 males) participated in the study. Histological and enzymological analyses were performed on muscle biopsies before and after overfeeding. In skeletal muscle of CT participants compared to controls, it was observed a lower content in intramuscular triglycerides for type I (p<0.01, -17%) and type IIA (p<0.05, -14%) muscle fibers, a lower glycogen content for type I fibers (p<0.01, -6%) and type IIA fibers (p<0.05, -5%), a specific fiber type distribution, a marked muscle hypotrophy (p<0.001, -20%), a low capillary-to-fiber ratio (p<0.001, -19%), and a low citrate synthase activity (p<0.05, -18%). In response to overfeeding, CT participants increased their intramuscular triglycerides content in type I (p<0.01, +10%) and type IIA (p<0.01, +9%) muscle fibers. CT individuals seem to present an unusual muscle phenotype and different adaptations to overfeeding compared to normal-weight participants, suggesting a specific energy metabolism and muscle adaptations. NCT02004821 Novelty ●Low intramuscular triglycerides and glycogen content in skeletal muscle of constitutionally thin individuals ●Low oxidative capacity, low capillary supply, and fiber hypotrophy in skeletal muscle of constitutionally thin participants ●Increase in intramuscular triglycerides in constitutional thinness in response to overfeedin