Monday March 28 2011
Tea's diuretic effect is evident at higher doses of caffeine
"Drinking four to six mugs of tea a day is as good for keeping you hydrated as a litre of water," reported the Daily Mail. It said the finding disproves "the idea that regular tea drinking can dehydrate the body because of its caffeine content".
The newspaper report is of a trial of 21 volunteers, which compared hydration levels in people drinking four mugs of tea with their levels when drinking the same amount of water on two different days. The study, sponsored by the UK Tea Advisory Panel, which in turn is sponsored by the UK tea industry, found no difference in hydration levels when tea or water was drunk.
It is perhaps unsurprising that in terms of hydration there was little difference in the two experimental conditions. The important question is whether caffeine is a diuretic, but this study did not measure the caffeine in the tea. It also cannot comment on which of the two drinks – tea or water – are healthier overall.
Where did the story come from?
The study was carried out by researchers from Fife and Reading. It is available on the website of the Tea Advisory Panel, which also funded the research. The Daily Mail says the research was or is due to be published by a journal, but this publication was not available at time of writing.
There is limited information about the methods used in the study. The newspaper appears to have based its conclusion that tea is as good as water on the finding that they both led to similar levels of hydration. It also stated that the study is ‘a high-quality clinical trial’. This may be true, although there is limited methodological detail on which to base this conclusion and the study was small, including only 21 men.
What kind of research was this?
The Tea Advisory Panel’s website reports that this is a randomised crossover trial. There is limited further detail provided about the study’s methods. Twenty-one healthy males aged 20 to 55 years old were enrolled. The study excluded people taking medication that may impact on markers of hydration, those who may be allergic to test ingredients, smokers and those with excessive caffeine intake (more than 10 cups of coffee per day). The same group of men were assessed during a 24-hour period of tea drinking, then a 24-hour period of water drinking. Measures of hydration were compared between the two experimental conditions.
What did the research involve?
Before the study began, baseline measures of hydration were taken. This included a blood and a urine sample to measure total urine volume and other markers of hydration, including creatinine concentration, osmolality of urine and the concentration of certain electrolytes. The blood sample was used to measure levels of electrolytes along with total protein concentration, urea, creatinine and osmolality.
In a crossover trial, the expected methodology is to randomise the participants to their starting condition, i.e. some will start with the tea drinking while the others start with the water drinking. The groups are then swapped over with a period in between to negate the effects of the initial treatment. There isn’t enough detail in the publication of this study to determine whether or how this crossover was done. However, the website describes that on the first study day, volunteers were presented with four cups of tea four hours apart, drinking a total of 960ml of black tea. Blood tests were taken at 0, 1, 2, 4, 8 and 12 hours, and urine was collected at 24 hours. Standard meals were provided.
After day one, the participants fasted for 10 hours, although they were allowed water. They also abstained from caffeine, alcohol and vigorous exercise for 24 hours. They were then offered four cups of hot water on day two in a set up similar to the previous day’s experimental conditions. Presumably, those who started with tea then crossed over to the water condition, although this aspect of the methods is not discussed.
At the end of the study, hydration levels were compared between test days one and two to determine whether there was a difference between tea and water drinking.
What were the basic results?
Nineteen of the 21 participants completed the study and were available for analysis. There was no difference in measures of hydration between the two study days in terms of 24-hour urine volume, blood osmolality or other measures.
How did the researchers interpret the results?
The researchers conclude that their study agrees with others which have found that moderate consumption of caffeine from caffeinated drinks has no adverse effect on hydration. They note that they did not measure caffeine levels in this current study but expect them to be about 200mg/day. They say that they are going to examine the effects of six cups of tea on hydration in a separate study.
This is a small study and it has not been possible to appraise the research fully given the limited details presented in the Tea Advisory Panels website. It suggests that, at least in terms of hydration, consumption of equivalent amounts of water and tea over the course of a day deliver the same level of hydration. Of note:
- The report says that the level of caffeine in the tea was not measured, so it is not possible to say whether all participants received the same dose or what the exact dose was.
- The authors say they used black, i.e. regular tea. The results may not apply to other teas.
- It is possible that the diuretic effect of tea is evident at higher doses of caffeine. The researchers aim to conduct further studies of people drinking more tea. If these are conducted, it seems reasonable to include women in the trial too.
The finding that water and moderate tea consumption are equally hydrating are perhaps unsurprising.
The important question here is whether caffeine is a diuretic. However, this study did not measure the caffeine in the tea that was given. It also cannot comment on which of the two drinks – tea or water – is healthier overall.