Changes in breath ^(13)CO_2/^(12)CO_2 consequent to exercise and hypoxia

Abstract

Because the natural enrichment of carbohydrate with ^(13)C is greater than that of lipid, we hypothesized that the natural enrichment of exhaled CO_2 with ^(13)C (E_N) could be used to gauge endogenous substrate utilization in exercising human subjects. To test this, E_N 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 O_2 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 E_N reflecting greater utilization of glucose. There was significant intersubject (P < 0.05) but not intrasubject variability in resting E_N. By 40 min of exercise, E_N increased significantly (P < 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 E_N during the below-LT work, even though there were significant increases in O_2 uptake (VO_2). In contrast, above-LT work resulted in significantly greater increases in E_N by 20 and 40 min of exercise (P < 0.05). Contrary to our expectations, we observed no separate effect by hypoxia on the E_N during exercise. Both E_N and R tended to increase from rest to exercise, but during exercise there was no overall correlation between R and the E_N. E_N reflects changes in endogenous substrate utilization over relatively long periods of time such as at rest, but delays in the appearance of ^(13)CO_2 at the mouth due to dilution in body CO_2 pools, and possibly isotopic fractionation, preclude the usefulness of E_N as an indicator of endogenous fuel mix during short-term exercise.