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Oxidation of combined ingestion of glucose and fructose during exercise

¹Human Performance Laboratory, School of Sport and Exercise Sciences, and ²School of Biosciences, University of Birmingham, Birmingham B15 2TT; and ³Physics and Nuclear Medicine Department, City Hospital, Birmingham B18 7QH, United Kingdom

Submitted 9 September 2003 ; accepted in final form 3 November 2003

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O₂ consumption: 62 ± 3 ml·kg^-1·min^-1) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 ± 2% maximal O₂ consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-¹³C]fructose, and the ingested glucose was labeled with [U-¹⁴C]glucose. Peak exogenous carbohydrate oxidation rates were 55% higher (P < 0.001) in Fruc+Glu (1.26 ± 0.07 g/min) compared with Med-Glu and High-Glu (0.80 ± 0.04 and 0.83 ± 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher (P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 ± 0.06, 0.75 ± 0.04, and 0.75 ± 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance (P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of 1.3 g/min.

substrate utilization; carbohydrate absorption; isotopic tracers; metabolism

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