Respiratory and metabolic responses during exercise and skeletal muscle morphology in obesity

Obese subjects need additional energy to move larger bodies and a greater amount of O2 to accomplish an equal external work-load than lean subjects. Moreover, obesity is characterized by a marked insulin resistance, which may modify metabolic responses to physical exercise. We measured the respiratory and metabolic responses to a progressive ergometric test in a group of 10 men obese and in 10 lean controls. Muscle biopsies of the left quadriceps were performed to study correlations between metabolic responses and muscle morphology. Oxygen consumption, carbon dioxide production and the respiratory exchange ratio (RER) were assessed, as were plasma concentrations of K+ and catecholamines in the basal state (rest) and during the last 30 s of each work load at exercise. Lactic acid was also measured. Obese subjects achieved a lower maximal power output (129 vs. 140 W) and a significantly lower anaerobic threshold (82 vs. 114 W, p<0.05). RER was significantly lower in the obese group, and catecholamines rose earlier and with a lower peak in correspondence of maximal power outputs. K+ and lactic acid levels increased in both groups, although significantly less in obese subjects. The main feature of muscle specimens from obese subjects was hypertrophy of the fibers, present in 9 of 10 cases, and increased amounts of neutral lipids (Oil Red O staining), present in 7 of 10 individuals. Low RER in obese subjects indicated a condition of insulin resistance, which persisted during physical activity. We conclude that fats are the fuel preferred by obese individuals during strenuous exercise.