Endogenous substrate oxidation during exercise and variations in breath 13CO2/12CO2

Abstract

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.

Novel diagnostics of metabolic dysfunction detected in breath and plasma by selective isotope-assisted labeling

“…Breath stable isotopes have been previously applied to metabolic [31-32] and nutritional statuses [33]. Breath measurements by CRDS are particularly useful to determine the total amount of glucose flux through both PC and EM pathways because of its high sensitivity measuring 13 CO 2 / 12 CO 2 ratios [24].…”

Effect of a 20-day ski trek on fuel selection during prolonged exercise at low workload with ingestion of 13C-glucose

“…The glucose (Biopharm, Laval, QC, Canada) derived from corn ( 13 C/ 12 C = -11.03% d 13 C PDB 1 ) was artificially enriched with U 13 C-glucose ( 13 C/C [ 99%; Isotec, Miamisburg, OH, USA) in order to achieve a final isotopic composition close to 35% d 13 C PDB 1 (actual values measured by mass spectrometry = 34.1% d 13 C PDB 1 ). This high 13 Cenrichment of exogenous glucose provides a strong signal in plasma glucose and expired CO 2 , and allows the neglect of changes in background enrichment of expired CO 2 observed from rest to exercise (Peronnet et al 1990) which remains very small at low workload (Gautier et al 1993).…”

Substrate utilization during prolonged exercise with ingestion of 13C-glucose in acute hypobaric hypoxia (4,300 m)

“…Normapur, Prolabo, Fontenaysous-Bois, France) derived from corn ( 13 C/ 12 C=À11.03& d 13 C PDB 1 ) was artificially enriched with U 13 C-glucose ( 13 C/C>99%, Isotec, Miamisburg, OH) in order to achieve a final isotopic composition close to 60& d 13 C PDB 1 : the actual value measured by mass spectrometry (see below) was 59.2& d 13 C PDB 1 . This high 13 C-enrichment of exogenous glucose provides a strong signal in expired CO 2 , and allows the neglect of the comparatively small changes in background enrichment of expired CO 2 observed from rest to exercise in subjects ingesting a standard Western European diet (Gautier et al 1993). …”