Tea and fluid balance: what the controlled trials found about caffeine
The review collects controlled trials of moderate caffeine intake and fluid balance and reports that intakes up to 400 mg of caffeine a day — six to eight servings of tea — are consistent with normal hydration. The concern the review answers is a lay one: that a caffeinated drink cannot count towards fluid intake.
The review sets out the scale of the question first: after water, tea is the most consumed beverage in the world, and in the United Kingdom more than four adults in five drink it. Whether it counts towards daily fluid intake is therefore not a marginal question for those populations.
Several controlled trials examined the effect of moderate caffeine consumption on fluid balance, taken from tea and from other sources. The review reports their common conclusion: up to 400 mg of caffeine a day, which corresponds to six to eight servings of tea, does not conflict with normal hydration.
The review also names what distinguishes tea from water and from other caffeinated drinks — its flavonoid content — and describes a growing body of work associating regular tea drinking with heart, cognitive and dental outcomes and with bodyweight management. Those are associations reported in the literature, and the review discusses them against typical intakes in the United Kingdom rather than as a general recommendation.
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