Journal of Applied Physiology, Vol 74, Issue 1 133-138, Copyright © 1993 by American Physiological Society
Endogenous substrate oxidation during exercise and variations in breath 13CO2/12CO2
J. F. Gautier, F. Pirnay, B. Jandrain, M. Lacroix, F. Mosora, A. J. Scheen and P. J. Lefebvre
Department of Medicine, University of Liege, Belgium.
This study attempted to induce a major shift in the utilization of
endogenous substrates during exercise in men by the use of a potent
inhibitor of adipose tissue lipolysis, Acipimox, and to see to what extent
this affects the 13C/12C ratio in expired air CO2. Six healthy volunteers
exercised for 3 h on a treadmill at approximately 45% of their maximum O2
uptake, 75 min after having ingested either a placebo or 250 mg Acipimox.
The rise in plasma free fatty acids and glycerol was almost totally
prevented by Acipimox, and no significant rise in the utilization of
lipids, evaluated by indirect calorimetry, was observed. Total carbohydrate
oxidation averaged 128 +/- 17 (placebo) and 182 +/- 21 g/3 h (Acipimox).
Conversely, total lipid oxidation was 84 +/- 5 (placebo) and 57 +/- 6 g/3 h
(Acipimox; P < 0.01). Under placebo, changes in expired air CO2 delta
13C were minimal, with only a 0.49/1000 significant rise at 30 min. In
contrast, under Acipimox, the rise in expired air CO2 delta 13C averaged
1/1000 and was significant throughout the 3-h exercise bout; in these
conditions calculation of a "pseudooxidation" of an exogenous sugar
naturally or artificially enriched in 13C, but not ingested, would have
given an erroneous value of 19.8 +/- 2.6 g/3 h. Thus under conditions of
extreme changes in endogenous substrate utilization, an appropriate control
experiment is mandatory when studying exogenous substrate oxidation by
13C-labeled substrates and isotope-ratio mass spectrometry measurements on
expired air CO2.
