Type 2 diabetes mellitus and cardiovascular disease represent major causes of morbidity, which impact greatly on healthcare expenditure. Clinical studies suggest that ingestion of black tea, which contains a range of bioactive compounds, can inhibit oxidative damage and improve endothelial function. The objectives of this review are to: (1) evaluate observational evidence linking black tea consumption with the prevalence of cardiovascular diseases and type 2 diabetes; (2) consider the mechanisms by which black tea may have a protective effect; and (3) examine the potential role of tea drinking in relation to public health.
The findings from epidemiological studies suggested a significant association between regular black tea consumption and a reduced risk of coronary heart disease at around three or more cups per day. For diabetes risk, the data are restricted to a few large cohort studies that suggested a beneficial association at one to four cups daily. These findings need to be confirmed by intervention trials. While some studies suggest that drinking black tea may reduce the risk of stroke, likely mechanisms remain unclear, highlighting the need for more human intervention studies. Disparities found involving studies may have been influenced by variations in reported tea intakes, limited sample sizes in intervention trials and inadequate control of confounders. In conclusion, drinking black tea may have a role in lowering the risk of coronary heart disease and type 2 diabetes. Future research should focus on controlled trials and studies to elucidate likely mechanisms of action.