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Changes in breath 13CO2/12CO2 consequent to exercise and hypoxia
T J Barstow1, D M Cooper, S Epstein
1Department of Medicine, Harbor-UCLA Medical Center, School of Medicine, Torrance 90509.
Journal of Applied Physiology (Bethesda, Md. : 1985)
|February 1, 1989
Summary
The natural enrichment of exhaled carbon dioxide with 13C (EN) can indicate substrate utilization at rest. However, EN is not a reliable indicator of fuel mix during short-term exercise due to delays in 13CO2 appearance.
Area of Science:
- Human physiology
- Metabolic research
- Isotope tracing
Background:
- Carbohydrate naturally has a higher 13C enrichment than lipid.
- This difference suggests exhaled CO2's 13C enrichment (EN) could reflect endogenous substrate utilization.
Purpose of the Study:
- To investigate if EN can gauge endogenous substrate utilization in exercising humans.
- To compare EN with the respiratory exchange ratio (R) under various conditions.
Main Methods:
- Measured EN and R in seven subjects during rest and exercise.
- Utilized room air and hypoxic (15% O2) breathing conditions.
- Included exercise below and above the lactate threshold (LT).
Main Results:
- EN increased significantly with exercise duration under all conditions.
- Exercise above LT showed greater EN increases than below LT.
- Hypoxia did not independently affect EN; no correlation was found between R and EN during exercise.
Conclusions:
- EN reflects long-term substrate utilization changes but is not suitable for short-term exercise fuel mix assessment.
- Delays in 13CO2 transport and potential isotopic fractionation limit EN's utility during exercise.