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

Barstow, Thomas J. and Cooper, Dan M. and Epstein, Samuel and Wasserman, Karlman (1989) Changes in breath ^(13)CO_2/^(12)CO_2 consequent to exercise and hypoxia. Journal of Applied Physiology, 66 (2). pp. 936-942. ISSN 8750-7587.

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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.

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Additional Information:© 1989 American Physiological Society. This information is current as of May 16, 2013. Received 9 February 1988; accepted in final form 16 September 1988. The authors thank Joe Ruth and Eleanor Dent for excellent technical assistance in the analysis of the exhaled gas samples for ^(13)CO_2. This study was supported in part by National Heart, Lung, and Blood Institute Grant HL-11907. D. M. Cooper is the recipient of the Clinician Scientist Award, American Heart Association, Greater Los Angeles Affiliate. S. Epstein was supported by National Science Foundation Grant EAR-8504096 and by auxiliary funds from the Weingart Foundation. Address for reprint requests: T. J. Barstow, Division of Respiratory and Critical Care Physiology and Medicine, Harbor-UCLA Medical Center, 1000 West Carson St., A-15 Annex, Torrance, CA 90509.
Funding AgencyGrant Number
National Heart, Lung, and Blood Institute GrantHL-11907
Clinician Scientist Award, American Heart Association, Greater Los Angeles AffiliateUNSPECIFIED
Weingart FoundationUNSPECIFIED
Subject Keywords:natural enrichment; carbon isotopes; respiratory exchange ratio; fuel mix
Issue or Number:2
Record Number:CaltechAUTHORS:20140414-140805972
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Official Citation: Changes in breath 13CO2/12CO2 consequent to exercise and hypoxia T. J. Barstow, D. M. Cooper, S. Epstein, K. Wasserman Journal of Applied Physiology Feb 1989,66(2)936-942;
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44935
Deposited By: Ruth Sustaita
Deposited On:14 Apr 2014 21:52
Last Modified:03 Oct 2019 06:24

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