Oxidation of combined ingestion of glucose and fructose during exercise

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 O-2 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 O-2 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-C-13] fructose, and the ingested glucose was labeled with [U-C-14] glucose. Peak exogenous carbohydrate oxidation rates were similar to 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 similar to 1.3 g/min.