Caffeine increases exogenous carbohydrate oxidation during exercise

Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized crossover design, eight male cyclists (age 27 2 yr, body mass 71.2 2.3 kg, maximal oxygen uptake 65.7 2.2 ml kg1 min1) exercised at 64 3 % of maximal oxygen uptake for 120 min on three occasions. During exercise subjects ingested either a 5.8 % glucose solution (Glu; 48 g/h), glucose with caffeine (GluCaf, 48 g/h 5 mg kg1 h1), or plain water (Wat). The glucose solution contained trace amounts of [U-13C]glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and 13C appearance in the expired gases was measured by continuous-flow IRMS. Averag