The results, which suggest a heart-healthy benefit for the tea, add to an ever-growing body of science linking consumption to a wide range of health benefits, including lower risk of certain cancers, weight loss, and protection against Alzheimer's.
Such benefits have mainly been put down to the polyphenol content of the tea. 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.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
The new study, led by Susana Coimbra from the University of Porto and in collaboration with researchers at the University of Beira Interior, evaluated the effect of drinking one litre of green tea every day for four weeks on the blood lipid profiles of 29 healthy volunteers (age range 22-63, average BMI 25 kg per sq. m, 22 women).
The volunteers were assigned to an initial wash-out period of three weeks when they were requested to drink one litre of water daily. They were then asked to drink one litre of green tea daily for the following four weeks. Fresh tea was prepared daily under the same conditions of temperature, time of infusion, and concentration.
Coimbra and her colleagues report that at the end of the intervention period significant beneficial improvements were observed in the lipid profile of the volunteers. A reduction in LDL-cholesterol was observed in 90 per cent of the subjects (average decrease of 8.9 per cent from baseline), and an increase in HDL-cholesterol was observed in 69 per cent of the subjects (average increase of 4 per cent from baseline).
No significant changes were documented for triacylglycerol and lipoprotein(a).
The ratio of total cholesterol to HDL cholesterol, reported to be the most specific lipid risk factor for CVD, decreased by six per cent after four weeks of tea drinking.
Cardiovascular disease (CVD) is the cause of almost 50 per cent of deaths in Europe, and reported to cost the EU economy an estimated ?169bn ($202bn) per year.
"The effect of green tea drinking in lipid profiles have been widely studied in… humans; however, controversial results are reported... This controversy may be related to differences in the study design, namely, in dietary and lifestyle habits, and/or in the experimental protocols," said the researchers. "Of the many reported methods for the preparation of tea, temperature, time of infusion, and concentration are important. In addition, the time of green tea consumption may also contribute to the controversy."
The researchers support the proposal that the protective effect of green tea for CVD is due to the high content of flavonoids, particularly the catechins. Previous studies have stated that these compounds can inhibit lipid peroxidation chain reactions that scavenge nitric and reactive oxygen species.
"Our data suggest that green tea drinking has beneficial effects, which protects against CVD by improving blood lipid profiles," said the researchers. "Further studies that would examine additional parameters of green tea consumption in humans are needed. It would be important to further clarify the effect of regular green tea consumption and the way it should be prepared to achieve a healthy effect."
This study is good news for both the tea market and the tea extract market. European demand for tea extracts is currently surging, having reached 500 metric tonnes by 2003.
This has seen companies such as DSM, with its Teavigo boasting 95 per cent purity of EGCG, and Taiyo International, with its Sunphenon claiming more than 90 per cent purity, position themselves firmly in specific catechin markets.
The global tea market is worth about ?790 (£540, $941) million. Green tea accounts for about 20 per cent of total global production, while black tea (green tea that has been oxidized by fermentation) accounts for about 78 per cent.
Source: Nutrition Research Volume 26, Issue 11 , November 2006, Pages 604-607 Green tea consumption improves plasma lipid profiles in adults Authors: S. Coimbra, A. Santos-Silva, P. Rocha-Pereira, S. Rocha, E. Castro