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Breath [¹³CO₂] recovery from an oral glucose load during exercise: comparison between [U-¹³C] and [1,2-¹³C]glucose

¹Département de kinésiologie, Université de Montréal, CP 6128 Centre Ville, Montréal H3C 3J7; ²Département de kinanthropologie, Université du Québec à Montréal, Montréal H3C 3P8; and ³Département des Sciences de l'activité physique, Université du Québec à Trois Rivières, Trois Rivières, Québec, Canada G9A 5H7

Submitted 30 January 2003 ; accepted in final form 31 March 2003

The purpose of the present experiment was to compare ¹³CO₂ recovery at the mouth, and the corresponding exogenous glucose oxidation computed, during a 100-min exercise at 63 ± 3% maximal O₂ uptake with ingestion of glucose (1.75 g/kg) in six active male subjects, by use of [U-¹³C] and [1,2-¹³C]glucose. We hypothesized that ¹³C recovery and exogenous glucose oxidation could be lower with [1,2-¹³C] than [U-¹³C]glucose because both tracers provide ^[13C]acetate, with possible loss of ¹³C in the tricarboxylic acid (TCA) cycle, but decarboxylation of pyruvate from [U-¹³C]glucose also provides ¹³CO₂, which is entirely recovered at the mouth during exercise. The recovery of ¹³C (25.8 ± 2.3 and 27.4 ± 1.2% over the exercise period) and the amounts of exogenous glucose oxidized computed were not significantly different with [1,2-¹³C] and [U-¹³C]glucose (28.9 ± 2.6 and 30.7 ± 1.3 g, between minutes 40 and 100), suggesting that no significant loss of ¹³C occurred in the TCA cycle. This stems from the fact that, during exercise, the rate of exogenous glucose oxidation is probably much larger than the flux of the metabolic pathways fueled from TCA cycle intermediates. It is thus unlikely that a significant portion of the ¹³C entering the TCA cycle could be diverted to these pathways. From a methodological standpoint, this result indicates that when a large amount of [¹³C]glucose is ingested and oxidized during exercise, ¹³CO₂ production at the mouth accurately reflects the rate of glucose entry in the TCA cycle and that no correction factor is needed to compute the oxidative flux of exogenous glucose.

stable isotopes; indirect respiratory calorimetry; energy metabolism; substrate oxidation

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