The study, performed by Dr Nikolaos Alexopoulos and colleagues at the 1st Cardiology Department, Athens Medical School in Greece, was a randomized trial involving the diameter measurement (dilatation) of the brachial artery of healthy volunteers on three separate occasions -- after taking green tea, caffeine and hot water (for a placebo effect). The measurements were taken at 30, 90 and 120 minutes after consumption. Dilatation of the brachial artery as a result of increased blood flow (following a brief period of ischaemia of the upper limb) is related to endothelial function and is known to be an independent predictor of cardiovascular risk.
Results showed that endothelium-dependent brachial artery dilatation increased significantly after drinking green tea, with a peak increase of 3.9% 30 minutes after consumption. The effect of caffeine consumption (or hot water) was not significant.