An average cup of tea contains around 40 mg of caffeine, depending on the blend of tea and the brewing procedure,4 and black tea and coffee are the main sources of caffeine in the Western diet. It takes around 30 minutes for the effects of caffeine to be detected, with peak plasma concentrations of caffeine being reached between 30 and 120 minutes after consumption.5 Caffeine easily crosses the blood-brain barrier and its effects on the brain include a general increase in neurotransmitter activity by blocking the inhibitory action of adenosine, a neuromodulator.
There is a relatively large body of literature reporting the positive effects of caffeine per se and at doses equivalent to one to two cups of tea.6,–8 Caffeine is typically associated with improved performance of tasks that require sustained effort and attention.9,–14 Other studies have shown some specific effects of caffeine on speed of response,15,16 and on feelings of well-being, energy, motivation, self-confidence, alertness, and concentration.17 However, the direct effects of caffeinated tea are less well known.
There follows a summary of studies that have compared the effects of tea and coffee containing varying doses of caffeine (Table 1), many of which represented the typical intake levels for these beverages. Both immediate and day-long statistically significant effects have been reported. For the purpose of this review, the studies were divided into those that describe psychological well-being, mood, and alertness, and those that describe changes in cognitive performance.
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