Skeletal muscle morphology and exercise response in congenital generalized lipodystrophy

OBJECTIVE - Congenital generalized lipodystrophy (CGL) is an autosomal recessive genetic disorder characterized by almost complete absence of adipose tissue, muscular appearance, and severe insulin resistance since birth. We investigated whether insulin resistance in CGL patients is associated with abnormal muscle morphology and whether increased muscularity imparts increased muscle strength and exercise capacity. RESEARCH DESIGN AND METHODS - We obtained quadriceps muscle biopsies to study muscle fiber types and capillary density in three African-American women (aged 17-20 years) with CGL. We also assessed quadriceps muscle strength, muscle metabolism, and maximal O₂ consumption in the patients. RESULTS - Quadriceps muscle biopsies revealed a markedly higher percentage of type II (fast-twitch glycolytic) muscle fibers in patients with CGL versus sedentary young women (75-78 vs. 47-57%, respectively). The capillary-to-fiber ratio (2.7-3.0), however, was normal. Cross-sectional areas of type I (slow-twitch oxidative) (1,262-2,685 μm²) and type II (2,304-3,594 μm²) fibers were far below the normal values (3,811-4,310 and 3,115-4,193 μm², respectively), suggesting muscle hyperplasia but not hypertrophy The quadriceps muscle strength, as measured by Cybex, was below average; the maximal O₂ consumption (23-32 ml · kg^-1 · min^-1) was also below average. ³¹P nuclear magnetic resonance spectroscopy of the forearm muscles revealed normal pH and metabolic responses to static and dynamic exercises. CONCLUSIONS - We conclude that insulin resistance in patients with CGL is associated with an increased proportion of type II muscle fibers but not reduced capillary density. Increased muscularity in CGL is due to muscle hyperplasia and is not associated with increased muscle strength.