And the more you drink, the greater the cuts in stroke risk, according to a meta-analysis of nine studies involving 4,378 strokes among 194,965 individuals.
"What we saw was that there was a consistency of effect of appreciable magnitude," said lead author Lenore Arab. "By drinking three cups of tea a day, the risk of a stroke was reduced by 21 per cent. It didn't matter if it was green or black tea."
The study, funded by the Unilever Lipton Institute of Tea, is published online ahead of print in the journal Stroke, and the findings were also presented at the American Heart Association's annual International Stroke Conference in San Diego, California.
Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent. Oolong tea is semi-fermented tea and is somewhere between green and black tea.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC).
Arab and her co-workers pooled data from the nine studies, all of which reported stroke occurrence and tea consumption in humans. Compared to people who drank less than cup of tea a day, drinking three cups a day was associated with a 21 per cent reduction in the risk of stroke.
The researchers also report a linear association with another three cups reducing the risk by further 21 per cent.
The potentially beneficial effects were limited to tea made from the plant Camellia sinensis, with herbal teas providing no benefits, added the UCLA team.
No one is certain which compounds in tea are responsible for these potential effects, said Arab, some researchers have suggested that EGCG or the amino acid theanine may be responsible.
A new study published in this month’s Nutrition (February 2009, Vol. 25, pp. 147-154) by US scientists reported that daily supplements of green tea extracts could reduce systolic and diastolic blood pressures by 5 and 4 mmHg, respectively, while total cholesterol levels were reduced by 10 mg/dL. These were the findings of a randomised, double-blind, placebo-controlled, parallel study.
Commenting on theanine, Arab said that studies have already shown that theanine is nearly 100 per cent absorbed. "It gets across the blood-brain barrier and it looks a lot like a molecule that's very similar to glutamate, and glutamate release is associated with stroke.
"It could be that theanine and glutamate compete for the glutamate receptor in the brain," she added.
The researchers added that a randomised clinical trial is needed to confirm this effect.
Source: Stroke Published online ahead of print, February 2009, doi:10.1161/STROKEAHA.108.538470“Green and Black Tea Consumption and Risk of Stroke. A Meta-Analysis” Authors: L. Arab, W. Liu, D. Elashoff