Acute effects of caffeine on heart rate variability in habitual caffeine consumers.

During the last years, heart rate variability (HRV) has become a promising risk factor for cardiovascular events. However, the effect of caffeine on HRV in habitual caffeine consumers has barely been investigated. Therefore, we treated 30 male habitual caffeine users in a randomized double-blinded crossover study design with either placebo, 100 or 200 mg caffeine orally and determined parameters of HRV under resting conditions and metronomic breathing.