A systematic review and meta-analysis of 19 cohort studies involving more than 1 million adults from eight countries finds that moderate consumption of black, green or Oolong tea is linked to a lower risk of developing type 2 diabetes.
The findings, being presented at this year’s European Association for the Study of Diabetes (EASD) Annual Meeting in Stockholm, Sweden (19-23 Sept), suggest that drinking at least four cups of tea a day is associated with a 17% lower risk of T2D over an average period of 10 years.
“Our results are exciting because they suggest that people can do something as simple as drinking four cups of tea a day to potentially lessen their risk of developing type 2 diabetes”, says lead author Xiaying Li from Wuhan University of Science and Technology in China.
While it’s long been known that regularly drinking tea may be beneficial for health because of the various antioxidant, anti-inflammatory and anticarcinogenic compounds tea contains, less clear has been the relationship between tea drinking and the risk of T2D. So far, published cohort studies and meta-analyses have reported inconsistent findings.
To address this uncertainty, researchers conducted a cohort study and a dose-response meta-analysis to better define the relationship between tea consumption and future T2DM risk.
First, they studied 5,199 adults (2583 men, 2616 women) with no history of T2D (average age 42) from the China Health and Nutrition Survey (CHNS), who were recruited in 1997 and followed until 2009. The CHNS is a multicentre prospective study looking at the economics, sociological issues and health of residents from nine provinces.
At the outset, participants filled in a food and drink frequency questionnaire and provided information on lifestyle factors such as regular exercise, smoking and alcohol consumption. Overall, 2,379 (46%) participants reported drinking tea, and by the end of the study, 522 (10%) participants had developed T2D.
After adjusting for factors that are known to be linked with increased risk of T2D, like age, sex, and physical inactivity, researchers found that tea drinkers had a similar risk of developing T2D compared to non-drinkers. And the results did not change significantly when analysed by age and sex, or when participants who developed diabetes during the first 3 years of follow-up were excluded.
In the next step of the study, the researchers did a systematic review of all cohort studies investigating tea drinking and the risk of T2D in adults (aged 18 or older) up to September 2021. Overall, 19 cohort studies involving 1,076,311 participants from eight countries  were included in the dose-response meta-analysis.
They explored the potential impact of different types of tea (green tea, oolong tea, and black tea), frequency of tea drinking (less than 1 cup/day, 1-3 cups/day, and 4 or more cups/day), sex (male and female), and the location of the study (Europe and America, or Asia), on the risk of T2D.
Overall, the meta-analysis found a linear association between tea drinking and T2D risk, with each cup of tea consumed per day reducing the risk of developing T2D by around 1%.
When compared with adults who didn’t drink tea, those who drank 1-3 cups daily lowered their risk of T2D by 4%, while those who consumed at least 4 cups every day reduced their risk by 17%.
The associations were observed regardless of the type of tea participants drank, whether they were male or female, or where they lived, suggesting that it may be the amount of tea consumed, rather than any other factor, that plays a major role.
“While more research needs to be done to determine the exact dosage and mechanisms behind these observations, our findings suggest that drinking tea is beneficial in reducing the risk of type 2 diabetes, but only at high doses (at least 4 cups a day)”, says Li.
She adds, “It is possible that particular components in tea, such as polyphenols, may reduce blood glucose levels, but a sufficient amount of these bioactive compounds may be needed to be effective. It may also explain why we did not find an association between tea drinking and type 2 diabetes in our cohort study, because we did not look at higher tea consumption.”
Oolong tea is a traditional Chinese tea that’s made from the same plant used to make green and black teas. The difference is how the tea is processed—green tea is not allowed to oxidise much, black tea is allowed to oxidise until it turns black, and oolong tea is partially oxidised.
Despite the important findings, the authors note that the study is observational and cannot prove that drinking tea reduced the risk of T2D, but suggests that it is likely to contribute.
And the researchers point to several caveats, including that they relied on subjective assessments of the quantities of tea consumed and they cannot rule out the possibility that residual confounding by other lifestyle and physiological factors may have affected the results.
 China, the USA, Japan, Finland, the UK, Singapore, the Netherlands and France.
The study was funded by the Young Talents Project of Hubei Provincial Health Commission, China; Science and Technology Research Key Project of Education Department of Hubei Province, China; Sanuo Diabetes Charity Foundation, China; and Xiangyang Science and Technology Plan Project, China.