The physiological effects of caffeine in women during treadmill walking

Although the effects of caffeine ingestion on athletic performance in men have been studied extensively, there is limited previous research examining caffeine's effects on women of average fitness levels participating in common modes of physical activity. The purpose of this study was to determine the effect of 2 levels of caffeine dosage on the metabolic and cardiorespiratory responses to treadmill walking in women. Subjects were 20 women (19-28 years of age) of average fitness, not habituated to caffeine. Each subject was assigned randomly a 3-mg x kg(-1) dose of caffeine, 6-mg x kg(-1) dose of caffeine, and placebo for 3 trials of moderate steady-state treadmill walking at 94 m x min(-1) (3.5 mph). Steady-state rating of perceived exertion (RPE), heart rate (HR), respiratory exchange ratio (RER), weight-relative VO2, %VO2max reserve (%VO2R), and rate of energy expenditure (REE) were measured during each trial. Repeated measures analysis of variance revealed that a 6-mg x kg(-1), but not a 3-mg x kg(-1) dose of caffeine increased VO2 (p = 0.04), REE (p = 0.03), and %VO2R (p = 0.03), when compared to the placebo. Caffeine had no effect on RPE, HR, or RER. No significant differences were observed between the placebo trials and the 3-mg x kg(-1) dose trials. Although a 6-mg x kg(-1) dose of caffeine significantly increased REE during exercise, the observed increase (approximately 0.23 kcal x min(-1)) would not noticeably affect weight loss. Because caffeine had no effect on RPE, it would not be prudent for a trainer to recommend caffeine in order to increase a woman's energy expenditure or to decrease perception of effort during mild exercise. These data also demonstrate that caffeine intake should not interfere with monitoring walking intensity by tracking exercise heart rate in women.