The measurement of exogenous carboydrate oxidation: A comparisons of [U-13C]-glucose and [U14C]-glucose tracers

The purpose of this study was to assess the level of agreement between two techniques commonly used to measure exogenous carbohydrate oxidation (CHOEXO). To accomplish this, seven healthy male subjects (24 +/- 3 yr, 74.8 +/- 2.1 kg, (V) over dotO(2max) 62 +/- 4 ml.kg(-1).min(-1)) exercised at 50% of their peak power for 120 min on two occasions. During these exercise bouts, subjects ingested a solution containing either 144 g glucose (8.7% wt/vol glucose) or water. The glucose solution contained trace amounts of both [U-C-13]glucose and [U-C-14]glucose to allow CHOEXO to be quantified simultaneously. The water trial was used to correct for background C-13 enrichment. C-13 appearance in the expired air was measured using isotope ratio mass spectrometry, whereas C-14 appearance was quantified by trapping expired CO2 in solution (using hyamine hydroxide) and adding a scintillator before counting radioactivity. CHOEXO measured with [C-13]glucose ([C-13]CHOEXO) was significantly greater than CHOEXO measured with [C-14] glucose ([C-14]CHOEXO) from 30 to 120 min. There was a 15 +/- 4% difference between [C-13]CHOEXO and [C-14]CHOEXO such that the absolute difference increased with the magnitude of CHOEXO. Further investigations suggest that the difference is not because of losses of CO2 from the trapping solution before counting or an underestimation of the "strength" of the trapping solution. Previous research suggests that the degree of isotopic fractionation is small (S. C. Kalhan, S. M. Savin, and P. A. Adam. J Lab Clin Med 89: 285-294, 1977). Therefore, the explanation for the discrepancy in calculated CHOEXO remains to be fully understood.