Abstract: We identified 11 RCTs with a total of 821 participants, two trials awaiting classification and one ongoing trial. Seven trials examined a green tea intervention and four examined a black tea intervention. Dosage and form of both green and black tea differed between trials. The ongoing trial is examining the effects of green tea powder capsules. No studies reported cardiovascular events. Black tea was found to produce statistically significant reductions in low-density lipoprotein (LDL) cholesterol (mean difference (MD) -0.43 mmol/L, 95% confidence interval (CI) -0.56 to -0.31) and blood pressure (systolic blood pressure (SBP): MD -1.85 mmHg, 95% CI -3.21 to -0.48. Diastolic blood pressure (DBP): MD -1.27 mmHg, 95% CI -3.06 to 0.53) over six months, stable to sensitivity analysis, but only a small number of trials contributed to each analysis and studies were at risk of bias. Green tea was also found to produce statistically significant reductions in total cholesterol (MD -0.62 mmol/L, 95% CI -0.77 to -0.46), LDL cholesterol (MD -0.64 mmol/L, 95% CI -0.77 to -0.52) and blood pressure (SBP: MD -3.18 mmHg, 95% CI -5.25 to -1.11; DBP: MD -3.42, 95% CI -4.54 to -2.30), but only a small number of studies contributed to each analysis, and results were not stable to sensitivity analysis. When both tea types were analysed together they showed favourable effects on LDL cholesterol (MD -0.48 mmol/L, 95% CI -0.61 to -0.35) and blood pressure (SBP: MD -2.25 mmHg, 95% CI -3.39 to -1.11; DBP: MD -2.81 mmHg, 95% CI -3.77 to -1.86). Adverse events were measured in five trials and included a diagnosis of prostate cancer, hospitalisation for influenza, appendicitis and retinal detachment but these are unlikely to be directly attributable to the intervention. There are very few long-term studies to date examining green or black tea for the primary prevention of CVD. The limited evidence suggests that tea has favourable effects on CVD risk factors, but due to the small number of trials contributing to each analysis the results should be treated with some caution and further high quality trials with longer-term follow-up are needed to confirm this.

Plain-language summary: This review assessed the effectiveness of green tea, black tea or black/green tea extracts in healthy adults and those at high risk of CVD. We found 11 randomised controlled trials, four of which examined black tea interventions and seven examined green tea interventions. There were variations in the dosage and form (drink, tablets or capsules) of the black and green tea interventions, and the duration of the interventions ranged from three months to six months. Adverse events were reported in five of the included trials. These included a diagnosis of prostate cancer, hospitalisation for influenza, appendicitis and retinal detachment; these are unlikely to be associated with the intervention. The results showed black and green tea to have a beneficial effect on lipid levels and blood pressure, but these results were based on only a small number of trials that were at risk of bias. Analysis conducted over both tea types showed beneficial effects of tea on LDL-cholesterol and blood pressure but again this was based on only a few trials that were at risk of bias. To date the small number of studies included suggest some benefits of green and black tea on blood pressure and lipid levels but more longer-term trials at low risk of bias are needed to confirm this.