"Drinking tea is actually better for you than drinking water. Water is essentially replacing fluid. Tea replaces fluids and contains antioxidants, so its got two things going for it,” reviewer Dr Carrie Ruxton from Nutrition Communications told the BBC.
Magic ingredients: polyphenols
The health benefits have been linked to the polyphenol content of the tea. Green tea contains between 30 and 40 percent of water-extractable polyphenols, while black tea (green tea that has been oxidised by fermentation) contains between 3 and 10 percent.
The four primary polyphenols found in fresh tea leaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
But Dr Ruxton and her co-authors from King's College London report that black tea also had positive effects on general health.
"We found some research showing that black and green tea contained similar amounts of antioxidants but different types. This can be expected as they come from the same plant but go through different processing," Dr Ruxton told NutraIngredients.com.
"Antioxidant activity in the blood is similar too, so one could assume that both types of tea confer heart health benefits," she said.
'Clear evidence found'
The review, sponsored by the Tea Council and published online in the European Journal of Clinical Nutrition (doi: 10.1038/sj.ejcn.1602489), searched databases for relevant epidemiological and clinical studies that were published between 1990 and 2004.
In terms of cardiovascular disease, the reviewers report that “clear evidence was found” to link drinking three or more cups per day and reduced risk.
Cancer risk did seem to be reduced in experimental (in vitro and animal studies) said the reviewers, but these results were not backed up by epidemiological evidence, they said.
A small note of caution, however, was noted by the observation that a non-effect or slight increase in risk of colorectal cancer occurred with consumption of black tea.
The dehydration issue
The researchers also tackled the old wives’ tale that drinking tea leads to dehydration, due to the diuretic caffeine content of the beverage. This was not backed up by the science, said the reviewers, with normal levels of hydration maintained when the tea contained less than 250mg per cup.
"Studies on caffeine have found that very high doses dehydrate and everyone assumes that caffeine-containing beverages dehydrate. But even if you had a really, really strong cup of tea or coffee, which is quite hard to make, you would still have a net gain of fluid,” Dr Ruxton told the BBC.
The reasons for these benefits were proposed to be related to the antioxidant action of tea polyphenols, wrote the reviewers.
“There was sufficient evidence to show risk reduction for CHD at intakes of more than three cups per day and for improved antioxidant status at intakes of one to six cups per day,” concluded the reviewers.
“A maximum intake of eight cups per day would minimise any risk relating to excess caffeine consumption. Black tea generally had a positive effect on health.” - (Decision News Media, August 2006)