The acute physiological and mood effects of tea and coffee: The role of caffeine level

The objective of this study was to determine the effect of caffeine level i n tea and coffee on acute physiological responses and mood. Randomised full crossover design in subjects after overnight caffeine abstention was studi ed. In study 1 (n = 17) the caffeine level was manipulated naturalistically by preparing tea and coffee at different strengths (1 or 2 cups equivalent ). Caffeine levels were 37.5 and 75 mg in tea, 75 and 150 mg in coffee, wit h water and no-drink controls. In study 2 (n = 15) caffeine level alone was manipulated (water, decaffeinated tea, plus 0, 25, 50, 100, and 200 mg caf feine). Beverage volume and temperature (55 degrees C) were constant. SBP, DBP, heart rate, skin temperature, skin conductance, and mood were monitore d over each 3-h study session. In study 1, tea and coffee produced mild aut onomic stimulation and an elevation in mood. There were no effects of tea v s. coffee or caffeine dose, despite a fourfold variation in the latter. Inc reasing beverage strength was associated with greater increases in DBP and energetic arousal. In study 2, caffeinated beverages increased SBP, DBP, an d skin conductance and lowered heart rate and skin temperature compared to water. Significant dose-response relationships to caffeine were seen only f or SBP, heart rate, and skin temperature. There were significant effects of caffeine on energetic arousal but no consistent dose-response effects. Caf feinated beverages acutely stimulate the autonomic nervous system and incre ase alertness. Although caffeine can exert dose-dependent effects on a numb er of acute autonomic responses, caffeine level is not an important factor. Factors besides caffeine may contribute to these acute effects. (C) 2000 E lsevier Science Inc.