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Determining exogenous glucose oxidation during moderate exercise Bozac, Anna Elena


The purpose of this study was to determine the quantity of a glucose drink oxidized during cycle ergometer exercise at 60% VO₂max for 75 minutes. A second purpose was to determine if the glucose drink improved sprint time to exhaustion at 90% VO₂max after 75 minutes of exercise. Six trained male cyclists (VO₂max > 60 ml•kg⁻¹•min•¹) exercised on three occasions during which they ingested either water ad lib (W), ¹³C-cornsyrup (100 g, 2.02 M) + water ad lib (CS), or NaH¹²CO₃/NaH¹³CO₃ mixture (5 mg•kg⁻¹, 1% ¹³C-enriched) + water ad lib (B). Treatments B and CS were ingested after 5 minutes of cycling at 60% VO₂max. During exercise, there was no difference between treatments in plasma lactate response, changes in plasma volume, sprint time to exhaustion, or in respiratory exchange ratio (RER), VO₂, or VCO₂. RER showed a significant decline (p< .01) from 5 minutes (1.00±0.05, X±SD) to 75 minutes (0.96±0.05), and VO₂ showed a significant positive shift (p< .01) from 3.15(±0.29) to 3.52(±0.45) l•min⁻¹. A transient rise in plasma glucose was observed with CS. Changes from rest in ¹³C/¹²C ratio (∂13C) showed a significant increase (p< .01) following CS. Peak glucose oxidation rate was 7.26 g•15 min⁻¹ which occurred after 75 minutes. Total dose of exogenous ¹³C-glucose recovered as ¹³CO₂ (above baseline) was 22%. These observations suggest that (1) during moderate exercise of 75 minutes duration, oxidation of exogenous glucose occurs within 15 minutes but contributes marginally to total carbohydrate utilization as RER continued to fall with or without CS, and (2) sprint time to exhaustion after 75 minutes of cycling is not improved with glucose ingestion.

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