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This Article Full Text Full Text (PDF) All Versions of this Article: 289/2/E206    most recent 00423.2004v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Moseley, L. Articles by Jeukendrup, A. E. Search for Related Content PubMed PubMed Citation Articles by Moseley, L. Articles by Jeukendrup, A. E.

Measurement of exogenous carbohydrate oxidation: a comparison of [U-¹⁴C]glucose and [U-¹³C]glucose tracers

Human Performance Laboratory, ¹Schools of Sport and Exercise Sciences and ²Biosciences, The University of Birmingham, Edgbaston, Birmingham, United Kingdom; and ³Physics and Nuclear Medicine Department, City Hospital, Birmingham, United Kingdom

Submitted 6 September 2004 ; accepted in final form 10 February 2005

The purpose of this study was to assess the level of agreement between two techniques commonly used to measure exogenous carbohydrate oxidation (CHO_EXO). To accomplish this, seven healthy male subjects (24 ± 3 yr, 74.8 ± 2.1 kg, _O2max 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-¹³C]glucose and [U-¹⁴C]glucose to allow CHO_EXO to be quantified simultaneously. The water trial was used to correct for background ¹³C enrichment. ¹³C appearance in the expired air was measured using isotope ratio mass spectrometry, whereas ¹⁴C appearance was quantified by trapping expired CO₂ in solution (using hyamine hydroxide) and adding a scintillator before counting radioactivity. CHO_EXO measured with [¹³C]glucose ([¹³C]CHO_EXO) was significantly greater than CHO_EXO measured with [¹⁴C]glucose ([¹⁴C]CHO_EXO) from 30 to 120 min. There was a 15 ± 4% difference between [¹³C]CHO_EXO and [¹⁴C]CHO_EXO such that the absolute difference increased with the magnitude of CHO_EXO. 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 Med89: 285–294, 1977). Therefore, the explanation for the discrepancy in calculated CHO_EXO remains to be fully understood.

exercise; isotopic fractionation; enrichment; specific activity