The Respiratory Exchange Ratio Is Higher in Older Subjects, but Is Reduced by Aerobic Exercise Training

Background: Previous studies have suggested reduced fat metabolism in older subjects. However, corrections for their reduced maximal oxygen consumption and the effects of training and substrate availability have not been fully examined. Objectives: Fat metabolism (FM) in older subjects (n = 14, 75 ± 7 yrs), and the effects of exercise training were compared with FM in younger subjects (n = 16, 22 ± 3 yrs). Materials and Methods:All subjects completed a maximal exercise test and a sustained submaximal run at 70% of their maximal capacity. The respiratory exchange ratio (RER) and blood substrate levels were determined. Older subjects were re-tested after training. Results: Young subjects had higher oxygen consumption (VO2) peak (36.3 ± 6.7 vs. 23.7 ± 6.2 ml/kg/min) and lower slope of RER vs. VO2 than older subjects. However, the slope of the RER vs. VO2 relationship was not different between younger and older subjects, after correction for their VO2 peaks. Younger subjects had longer sustained exercise times (45.5 ± 17.6 min) than the elderly (30.2 ± 14.0 min), pre-training. Post- training, there was a significant increase in VO2 peak (25%) in older subjects (P = 0.001) and submaximal exercise time (30.2 ± 14.0 vs. 58.3 ± 27.3min, P = 0.020). Respiratory exchange ratio was reduced during both exercises after training (0.90 ± 0.03 vs. 1.00 ± 0.03, P = 0.04). Conclusions: The RER of older subjects was not different from that of younger subjects, after correction for the VO2 peak. The VO2 peak, sustained exercise time, and RER decreased after training in older subjects, indicating increased fat metabolism.