There is a belief that caffeinated drinks, such as tea, may adversely affect hydration. This was investigated in a randomised controlled trial.
Healthy resting males were recruited from the general population. Following 24 hours of abstention from caffeine, alcohol and vigorous physical activity, including a 10 hour overnight fast, all men underwent four separate test days in a counter-balanced order with a 5 day washout in between. The test beverages, provided at regular intervals, were 4 x 240 ml black (i.e. regular) tea and 6 x 240 ml black tea, providing 168 or 252 mg of caffeine. The controls were identical amounts of boiled water. The tea was prepared in a standardised way from tea bags and included 20ml of semi-skimmed milk. All food taken during the 12 hour intervention period was controlled, and subjects remained at rest. No other beverages were offered. Blood was sampled at 0, 1, 2, 4, 8 and 12 hour, and a 24 hour urine sample was collected.
Outcome variables were whole blood cell count, Na, K, bicarbonate, total protein, urea, creatinine and osmolality for blood; and total volume, colour, Na, K, creatinine and osmolality for urine. Although data for all twenty-one participants were included in the analysis (mean age 36 years and mean BMI 25·8 kg/m2), nineteen men completed all conditions. Statistical analysis, using a factorial ANOVA approach within PROCMIXED, revealed no significant differences between tea and water for any of the mean blood or urine measurements.
It was concluded that black tea, in the amounts studied, offered similar hydrating properties to water. EFSA guidelines are that to maintain healthy hydration levels at least 2.0L of water (from all sources) should be consumed. Tea, amongst other foods and beverages can contribute to maintaining healthy hydration. More research is needed.
You can access the journal paper here: https://www.ncbi.nlm.nih.gov/pubmed/21450118
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The Eatwell Guide, published by Public Health England, recommends intakes of 6-8 hydrating drinks per day. The guide states water, lower fat milk and sugar-free drinks including tea and coffee all count.
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