Substrate utilization during treadmill running in prepubertal girls and women

Ten prepubertal girls (9.1 yr) and 10 women (24.4 yr) were studied during 30-min runs at the same relative (70% VO2max) (RI) and absolute (7.2 km.h-1) (AI) intensities. Oxygen consumption and respiratory exchange ratio (RER) were monitored continuously during the exercise test. Venous blood samples were obtained before and immediately after exercise and were analyzed for glucose, lactate, FFA, and glycerol. During exercise at the same RI, RER was significantly (P less than 0.05) lower in the girls compared with the women. In addition, RER decreased significantly during exercise in the girls while the change did not reach significance in the women. Lactate concentration increased significantly during the exercise tests, this increase being greater in the women than in the girls during exercise at the same RI, but greater in the girls than in the women during exercise at the same AI. FFA and glycerol concentrations were significantly higher after the exercise tests in both groups. Glucose concentration did not change significantly during the tests in either group. Prepubertal girls rely more on fat utilization and less on carbohydrate metabolism than women during exercise of moderately heavy intensity.