Short-term effects of black tea on vascular function in healthy adults

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Population studies indicate that regular consumption of tea can reduce the risk of cardiovascular disease by between 5 and 15%. The pooled estimate of risk reduction for black tea is influenced by two studies which included populations from the United Kingdom where milk is traditionally added to tea. This has led to the suggestion that addition of milk may diminish or negate cardiovascular health benefits of tea. Results of population and intervention studies suggest that regular consumption of tea can reduce blood pressure by 1 to 10 mm Hg. At a population level, a 5 mm Hg lower blood pressure would be associated with a 10% lower risk of coronary artery disease, and a 20% lower risk of stroke. A meta-analysis of randomised controlled trials to have assessed the effects of tea on endothelial function was recently published. This analysis suggests that acute and chronic tea ingestion can improve endothelial function, assessed by measuring flow-mediated dilatation of the brachial artery, by 30 to 40% (~2% absolute improvement). If sustained, this would be associated with about a 25% reduction in risk of cardiovascular events, which would be consistent with estimates of risk reduction from population studies. The precise pathway linking flavonoids to vascular function is not clear. However, it is known that the majority of dietary flavonoids are not absorbed or bioactive without prior metabolism by specific groups of enzymes produced by the microbes residing in the gastrointestinal tract (the microbiome). Although increased flavonoid intake can improve measures of vascular function, there is substantial inter-individual variability in response. Current knowledge of flavonoid metabolism indicates that this is largely due to individual differences in the capacity of the microbiome to metabolise flavonoids. However, this hypothesis has yet to be explored. Initial research needs to describe changes to the microbiome that occur with tea consumption. Furthermore, despite studies showing that the effect of the addition of milk to tea on bioavailability of tea flavonoids is modest, questions remain regarding the effects of milk to black tea on its bioactivity. One study has suggested that addition of milk almost negates the acute improvement in endothelial function. However, it is possible that this may relate to the study design, where endothelial function was assessed at a single time point following a single dose of tea. It is possible that addition of milk may delay absorption of the active components of tea (the flavonoids), but the overall impact on endothelial function is similar. Thus, the primary aim of the proposed study will be to investigate the short-term effects of black tea on endothelial function. We will investigate the effect of the addition of milk to black tea on its bioactivity. We will also describe and compare the changes to the microbiome after short-term regular consumption of black tea with and without milk.

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