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Exogenous carbohydrate oxidation during ultraendurance exercise

Human Performance Laboratory, School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham, United Kingdom

Submitted 7 September 2004 ; accepted in final form 29 November 2005

The purposes of this study were: 1) to obtain a measure of exogenous carbohydrate (CHO_Exo) oxidation and plasma glucose kinetics during 5 h of exercise; and 2) to compare CHO_Exo following the ingestion of a glucose solution (Glu) or a glucose + fructose solution (2:1 ratio, Glu+Fru) during ultraendurance exercise. Eight well-trained subjects exercised three times for 5 h at 58% maximum O₂ consumption while ingesting either Glu or Glu+Fru (both delivering 1.5 g/min CHO) or water. The CHO used had a naturally high ¹³C enrichment, and five subjects received a primed continuous intravenous [6,6-²H₂]glucose infusion. CHO_Exo rates following the ingestion of Glu leveled off after 120 min and peaked at 1.24 ± 0.04 g/min. The ingestion of Glu+Fru resulted in a significantly higher peak rate of CHO_Exo (1.40 ± 0.08 g/min), a faster rate of increase in CHO_Exo, and an increase in the percentage of CHO_Exo oxidized (65–77%). However, the rate of appearance and disappearance of Glu continued to increase during exercise, with no differences between trials. These data suggest an important role for gluconeogenesis during the later stages of exercise. Following the ingestion of Glu+Fru, cadence (rpm) was maintained, and the perception of stomach fullness was reduced relative to Glu. The ingestion of Glu+Fru increases CHO_Exo compared with the ingestion of Glu alone, potentially through the oxidation of CHO_Exo in the liver or through the conversion to, and oxidation of, lactate.

stable isotopes; carbohydrate absorption; glucose; fructose; metabolism