Because the natural enrichment of carbohydrate with 13C is greater than that of lipid, we hypothesized that the natural enrichment of exhaled CO2 with 13C (EN) could be used to gauge endogenous substrate utilization in exercising human subjects. To test this, EN and the respiratory exchange ratio (R) which equals the respiratory quotient (RQ) in the steady state, were measured simultaneously in seven subjects. Rest and exercise protocols, performed under conditions of room air (sea level) and hypoxic (inspired O2 fraction = 0.15) breathing, were chosen to cause a variety of patterns of oxidative substrate utilization. Work rates were performed both below and above the subject's lactate threshold (LT). Work above the LT was expected to cause the greatest increase in EN reflecting greater utilization of glucose. There was significant intersubject (P less than 0.05) but not intrasubject variability in resting EN. By 40 min of exercise, EN increased significantly (P less than 0.05) over resting values in all exercise protocols during both room air and hypoxia conditions. In the room air studies, we found no difference in EN during the below-LT work, even though there were significant increases in O2 uptake (VO2). In contrast, above-LT work resulted in significantly greater increases in EN by 20 and 40 min of exercise (P less than 0.05). Contrary to our expectations, we observed no separate effect by hypoxia on the EN during exercise. Both EN and R tended to increase from rest to exercise, but during exercise there was no overall correlation between R and the EN. EN reflects changes in endogenous substrate utilization over relatively long periods of time such as at rest, but delays in the appearance of 13CO2 at the mouth due to dilution in body CO2 pools, and possibly isotopic fractionation, preclude the usefulness of EN as an indicator of endogenous fuel mix during short-term exercise.

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.

The effect of changes in metabolic rate on the dynamics of CO2 exchange among its various compartments in the human body is not well understood. We examined CO2 dynamics in six healthy male subjects using an intravenous bolus of [13C]bicarbonate. Subjects were studied while resting, during light exercise [50% of the lactate threshold (LT), 3-4 times resting O2 uptake (VO2)], and during moderate exercise (95% of the LT, 6 times resting VO2). The sum of three exponential terms well described the washout of 13CO2 in exhaled breath both at rest and during each exercise level despite substantial increases in metabolic rate accompanying the exercise studies. Average recovery of 13C label rose from 67% during rest to 80% during light and moderate exercise (P less than 0.01). The estimate of CO2 elimination (VCO2) calculated from the washout parameters and corrected for recovery was in very good agreement with the VCO2 directly measured simultaneously breath by breath (r = 0.993, SE for VCO2 = 0.079 l/min). By use of a three-compartment mammillary model, the quantity of CO2 in the central pool (Q1) doubled from rest to light exercise (233 +/- 60 to 479 +/- 76 mmol, P less than 0.01) but did not change further with moderate exercise (458 +/- 74 mmol). Rate constants for exchange between pools and for irreversible loss from the system tended to increase with metabolic rate, but there was large variation in the responses. We conclude that the compartmental dynamics of CO2 transport and storage are very sensitive to changes in metabolic rate induced by exercise.

During exercise, less additional CO2 is stored per kilogram body weight in children than in adults, suggesting that children have a smaller capacity to store metabolically produced CO2. To examine this, tracer doses of [13C]bicarbonate were administered orally to 10 children (8-12 yr) and 12 adults (25-40 yr) at rest. Washout of 13CO2 in breath was analyzed to estimate recovery of tracer, mean residence time (MRT), and size of CO2 stores. CO2 production (VCO2) was also measured breath by breath using gas exchange techniques. Recovery did not differ significantly between children [73 +/- 13% (SD)] and adults (71 +/- 9%). MRT was shorter in children (42 +/- 7 min) compared with adults (66 +/- 15 min, P less than 0.001). VCO2 per kilogram was higher in the children (5.4 +/- 0.9 ml.min-1.kg-1) compared with adults (3.1 +/- 0.5, P less than 0.0001). Tracer estimate of CO2 production was correlated to VCO2 (r = 0.86, P less than 0.0001) and when corrected for mean recovery accurately predicted the VCO2 to within 3 +/- 14%. There was no difference in the estimate of resting CO2 stores between children (222 +/- 52 ml CO2/kg) and adults (203 +/- 42 ml CO2/kg). We conclude that orally administered [13C]bicarbonate can be used to assess CO2 transport dynamics. The data do not support the hypothesis of lower CO2 stores under resting conditions in children.