Effect of increased metabolic rate on oxygen isotopic fractionation.
16O16O is preferentially used over 18O16O (a stable isotope of oxygen comprising about 0.2% of atmospheric O2) as oxygen is consumed during respiration in humans (Epstein and Zeiri, 1988, Proc. Natl. Acad. Sci. USA 85: 1727-1731). To test the hypothesis that oxygen isotopic fractionation is related to the metabolic rate, 8 healthy adults performed 5 min of constant work rate cycle ergometer exercise below and above their anaerobic threshold. Moreover, 3 subjects performed an incremental exercise to the limit of tolerance. Oxygen uptake (VO2) was measured breath by breath. Samples of the exhaled breath for oxygen isotope measurement were obtained at rest and at various times during exercise and recovery. Oxygen isotopic fractionation was determined by isotope ratio mass spectrometry and calculated as the ratio of the degree of fractionation to the oxygen consumed in the breath sample (Z value). For the constant work rate protocol, both low and high intensity exercise resulted in a significant decrease in Z compared to the rest values (P less than 0.01). However, for the high intensity exercise the reduction in fractionation was greater compared to the low intensity protocol (P less than 0.05). For the incremental test, there was a significant negative correlation between oxygen isotopic fractionation and VO2 expressed as percent of the maximal oxygen uptake (r = -0.91, P less than 0.0001). These data suggest that during exercise low-fractionating processes become more important as limiting steps for O2 transport.