Data from 1,536 participants indicated that green tea consumption was associated with an average systolic blood pressure 1.94 mmHg lower than placebo, but no benefits on diastolic blood pressure were observed.
In addition, green tea consumption was also associated with lower total and LDL cholesterol levels, compared to placebo, report scientists from the University of Oxford in the UK and the University of Washington in Seattle in Nutrition, Metabolism and Cardiovascular Diseases.
“The available evidence from RCTs suggests that dietary supplementation with green tea generates significant reductions on systolic blood pressure, total and LDL cholesterol,” wrote the researchers. “The effect size on systolic blood pressure is small, but the sizes of the effects on total and LDL cholesterol appear moderate.
“There is some evidence that daily consumption of 5-6 cups of green tea could result in reductions in systolic blood pressure, total cholesterol, and LDL cholesterol,” they added. “However, at this time green tea should not be recommended as a substitute for current management of patients with established hypertension or dyslipidaemia.”
The review appears in line with the ever-growing body of science supporting the potential benefits of green tea and its constituents, most notably EGCG (epigallocatechin gallate). To date green tea has been linked to a reduced risk of Alzheimer's and certain cancers, improved cardiovascular and oral health, as well as benefits in weight management.
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).
In addition to finding an association between green tea consumption and improved blood pressure and cholesterol levels, the reviewers also identified a significant correlation between the doses of EGCG and a reduction in systolic blood pressure in trials lasting at least 12 weeks. However, no additional benefits were observed for EGCG doses above 200mg (about 5-6 cups of tea).
The source of funding was also noted by the researchers, with manufacturer-funded studies reporting greater benefits than government-funded trials.
“Because of subtle differences in the effects seen on blood pressure and lipid profile between manufacturer- and government-funded studies, more independent clinical trials which are adequately powered to detect such effects are required,” they stated.
Commenting on the potential mechanism of action, the researchers said that there are several different potential mechanisms, including decreasing the action of ACE (angiotensin-converting-enzyme inhibitor), which leads to a relaxation of blood vessels and lowering of blood pressure. Other studies have suggested that EGCG may decrease levels of kallikrein and prostaglandin E2 which can lower blood pressure.
“Green tea also contains gamma-aminobutyric acid (GABA) which may decrease blood pressure by modulating neurotransmitter release,” they added. “Indeed, consumption of food enriched with GABA resulted in significant reductions in blood pressure in mildly hypertensive patients.”
Finally, they noted that flavan-3-ols from green tea may impact endothelial function.
Source: Nutrition, Metabolism and Cardiovascular Diseases
Published online ahead of print, doi: 10.1016/j.numecd.2014.01.016
“The effect of green tea on blood pressure and lipid profile: a systematic review and meta-analysis of randomized clinical trials”
Authors: I. Onakpoya, E. Spencer, C. Heneghan, M. Thompson