Healthy male adults were recruited through bulletins at local universities
and public places. We recruited only men to avoid confounding interactions
of caffeine metabolism with reproductive hormones that vary
across the female cycle [33,34]. Seventeen normal weight (body mass
index [BMI]: mean = 23.8, SD = 3) men aged 19–40 years (mean =
28.5, SD = 4.4) completed the study. Participants reported to be
healthy. Their quotidian consumption of caffeine and sugary drinks
was assessed based on the daily intake of coffee, tea, coke, and energy
drinks, and the intake of juice, soda and other sweet drinks, respectively
(Table 1). Importantly, participants were allowed to consume their
regular amounts of caffeine or glucose up to 2 h prior to testing thereby
maintaining their habitual intake behavior and thus avoiding withdrawal
symptoms. Then, participants were instructed to refrain from
eating and drinking anything but water.