Gender Specific Changes In Substrate Utilization Following 20 Weeks Of Aerobic Training

PURPOSE To evaluate gender specific differences in changes in macronutrient utilization at submaximal power outputs (PO) as a result of a supervised 20-wk aerobic exercise program in 733 sedentary individuals (312 males and 411 females) aged 17–65 yr participating in the HERITAGE Family Study. METHODS Cycle ergometer training was performed at four participating clinical centers, three times a week for 20 weeks. Intensity was initially set at a training heart rate (THR) equivalent to 55% VO2max for 30 min per session and progressed to a THR corresponding to 75% pre-training VO max for 50 min for the final 6 weeks. Metabolic data were collected after 3 minutes of relative steady state at a PO of 50W and at 60% VO2max. LIMITATIONS The HERITAGE exercise tests were not designed to address metabolic issues. Subjects undertook the exercise tests under variable conditions: No effort was made to standardize the diet or meal time prior to the tests and the exercise tests varied slightly to adjust for individual fitness. The pre-exercise conditions were heterogeneous among subjects and possibly varied from the pre- to post-training tests. Metabolic data were collected after 3 minutes of relative steady state at a PO of 50W and at 60% VO2max. RESULTS Overall, fat utilization, as a percentage of the total energy substrate, increased post-training at the fixed value of 50W (21.8 ±16% to 36.5 ±13%, P <0.01), and at the relative value of 60% of VO2max (11.3 ±13.6% to 21.9 ± 12.2%, P <0.01) even though watts at 60% of VO2max increased from 97.6 ± 27.5 to 121.9 ± 34.4W. Females increased fat utilization, as a percent of the total, more at 50Wthan did males (A=17.2% ±13% vs. A=11.5 ±13%, P <0.01) but were not different at 60% VO2max. CONCLUSIONS Aerobic exercise training in previously sedentary individuals increases fat as a substrate at fixed intensities of training as well as at moderate intensities relative to VO2max. Women increased fat oxidation at fixed intensities, as a percentage of total macronutrient utilization, more than did men. The large sample size and consistency of the data suggest that these results are valuable information for previously sedentary individuals in spite of the numerous limitations. These data show the benefits of aerobic training for improving fat oxidation due to exercise training. Sponsored by multiple grants by NHLBI.