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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.
The health benefits, which have mainly focused on green tea, 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.
How the study was done
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 25kg 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 percent of the subjects (average decrease of 8,9 percent from baseline), and an increase in HDL-cholesterol was observed in 69 percent of the subjects (average increase of 4 percent 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 percent after four weeks of tea drinking.
Controversial reports
"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." - (Decision News Media, November 2006)
