In this six-month randomized trial of black tea consumption among adults at higher cardiovascular risk, we found no significant effects of black tea on any biomarkers of advanced cardiovascular risk.
Several possible explanations for our findings should be considered. First, black tea may truly have no important effects on the outcomes we studied, at least over a sustained period of time. Indeed, clinical trials have generally not supported strong effects of black tea on most biomarkers.4–6, 12, 15, 18, 25, 26
If this explanation is correct, then the observed associations of tea with lower risk of cardiovascular disease in cohort studies may either reflect uncontrolled confounding or effects of black tea on other plausible pathways. For example, we did not measure flow-mediated vasodilatation or any other functional measure of vascular function; several clinical trials have shown benefits of tea on such functional parameters.14, 21, 27, 28
Second, the dose of black tea may have been insufficient to produce meaningful effects on these biomarkers. While the dose of three cups/day that we used corresponds well to the levels of intake found to be associated with lower risk in observational studies,3
it is smaller than the doses typically used in those clinical trials that have found benefits of black tea consumption on biomarkers.4, 11
Moreover, because the dose was somewhat smaller than in previous trials, any lack of adherence may have biased our results toward the null findings we observed. Although we undertook extensive efforts to promote adherence, including providing prepackaged containers of a readily solubilized powder and frequent contact with participants, the exact magnitude of adherence is difficult to ascertain in the absence of a long-term biomarker uniformly specific for tea exposure; 4-O-methylgallic acid does not fit this criterion.29, 30
Third, the black tea preparation we chose may not share the benefits of others previously tested, although we have no strong reason to suspect this. We chose a commercially available, representative black tea with ample polyphenol content. While substantial attention has been given to green tea, all forms of tea contain antioxidant polyphenols, and the positive findings in observational studies from the U.S. and Europe clearly reflect the predominately black tea consumption in these populations.
Fourth, the inherent variability in many of these markers – and particularly those related to inflammation and endothelial function – may have obscured any true, modest effect of tea, especially in a population of older adults with cardiovascular risk factors. We intentionally selected this higher risk group based on observational studies,31–33
which have generally supported stronger beneficial effects among higher risk individuals, but it may have introduced additional variability that would overshadow modest effects of tea in a small study such as this. Trials of this duration in younger, healthier subjects will be an important future step.
Fifth, our study was of limited size, and although powered to detect differences similar to those seen in other trials of dietary supplements,34
the confidence limits around our estimates indicate that differences of a potentially important magnitude, at least at the population level, could have been missed. Meta-analyses of small trials like ours may pose the best opportunity to clarify effects of tea with precision.
Although we did not find large benefits of tea, our study provides some reassurance about the safety of long-term tea intake in a clinical trial setting. Despite the inhibitory effect of tea on iron absorption,35
we did not find major changes in hemoglobin levels through the trial, consistent with recent reviews that identified this interaction as being chiefly relevant for individuals with borderline iron status.36, 37
Although some trials have found that tea consumption acutely raises blood pressure14
, we found no such effect over 6-months of follow-up, and a recent meta-analysis of five supplementation studies of four-week duration found no significant effect of tea on blood pressure.38
Heart rate tended to rise somewhat more among the tea group than the water group at 3 months, but it also fell more steeply at 6 months in the tea group.
In summary, we found no consistent effects of six months of black tea intake on biomarkers of cardiovascular risk in this study. Our results do not preclude an important effect of tea intake on risk of cardiovascular disease, but they do suggest that alternate mechanisms are apt to be most important, and that long-term randomized trials of tea intake are needed to determine its true effects on cardiovascular risk with certainty.