A lot has been said about the health benefits of green tea, but a new study published in the online journal BMJ Open suggests that black is, well, the new black.
Researchers conducted a mathematical analysis of data from 50 countries across six continents and found that the prevalence of type 2 diabetes is low in countries where consumption of black tea is high.
The fermentation process that turns green tea black induces a range of complex flavonoids, including theaflavins and thearubigins, to which several potential health benefits have been attributed.
The researchers analysed information on black tea consumption based on 2009 sales data collected by an independent specialist market research company.
They also analysed World Health Organisation data for those same countries on the prevalence of respiratory, infectious and cardiovascular diseases, as well as cancer and diabetes.
Ireland topped the league table for black tea drinkers, at more than 2kg per year per person, closely followed by Britain and Turkey. At the bottom of the table were South Korea, Brazil, China, Morocco and Mexico, with very low consumption.
A statistical approach was used to tease out the key contribution of black tea to each of the health indicators selected at the population level. A link was found between black tea and rates of diabetes, but not for any of the other health indicators studied. Further analysis confirmed a strong linear association between low rates of diabetes in countries and high black tea consumption. The global prevalence of type 2 diabetes has increased sixfold over the past few decades, and the International Diabetes Federation calculates that the number of those with the disease will soar from 285 million in 2010 to 438 million in 2030.
In Hong Kong, type 2 diabetes affects about one in 10 people, with prevalence ranging from 2 per cent in people aged under 35 to more than 20 per cent in those older than 65. The incidence of diabetes is increasing, with more than half of them being undiagnosed. The disease is the leading cause of kidney failure, blindness, leg amputations, cardiovascular diseases and stroke.
The study authors acknowledge several caveats to their findings. They caution that the quality and consistency of data among the countries are likely to vary, as will the criteria used to diagnose diabetes. And what may seem positive at the population level may not work as well on an individual level.
They also point out that various factors are likely to have contributed to the dramatic rise in diabetes prevalence, and that a link between black tea consumption and the prevalence of the disease does not imply that one is caused by the other.
But their findings do back those of previous research, they say. "These original study results are consistent with previous biological, physiological, and ecological studies conducted on the potential of [black tea] on diabetes and obesity, and they provide valuable additional scientific information at the global level," the report says.