The small study with 14 healthy adults found that the benefits of the beverage was not related to the caffeine, indicating the possible role of green tea polyphenols, previously linked to protection against certain kinds of cancers and Alzheimer's.
The study is published in the European Journal of Cardiovascular Prevention and Rehabilitation .
"These findings have important clinical implications," said study co-author Charalambos Vlachopoulos from Athens Medical School in Greece.
"Tea consumption has been associated with reduced cardiovascular morbidity and mortality in several studies.
Green tea is consumed less in the Western world than black tea, but it could be more beneficial because of the way it seems to improve endothelial function.
"In this same context, recent studies have also shown potent anticarcinogenic effects of green tea, attributed to its antioxidant properties," added Dr Vlachopoulos.
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).
Study details The Greek researchers randomly assigned the volunteers (average age 30) to one of three groups.
The first receive six grams of green tea, the second received 125 mg of caffeine (equivalent to the caffeine dose from six grams of tea), and the third group received hot water.
The volunteers consumed each of the interventions on three separate occasions, and the effects on flow-mediated dilation (FMD), the measure of a blood vessel's healthy ability to relax, measured 30, 90 and 120 minutes after consumption.
Green tea increased FMD by 3.9 per cent 30 minutes after consumption, while no changes in FMD were observed following consumption of caffeine or the hot water placebo.
"Green tea consumption has an acute beneficial effect on endothelial function, assessed with FMD of the brachial artery, in healthy individuals," wrote the researchers.
"This may be involved in the beneficial effect of tea on cardiovascular risk," they concluded.
Source: European Journal of Cardiovascular Prevention and Rehabilitation 2008, Volume 15, Pages 300-305 "The acute effect of green tea consumption on endothelial function in healthy individuals" Authors: N. Alexopoulos, C. Vlachopoulos, K. Aznaouridis, K. Baou, C. Vasiliadou, P. Pietri, P. Xaplanteris, E. Stefanadi, C. Stefanadis