Summary: Drinking four or more cups of black, green, or oolong tea a day was associated with a 17% lower risk of developing type 2 diabetes.
A systematic review and meta-analysis of 19 cohort studies involving over one million adults in eight countries found that moderate consumption of black tea, green tea or Oolong tea is associated with a lower risk of developing type 2 diabetes.
Results, presented at this year’s annual meeting of the European Diabetes Research Association (EASD) in Stockholm, Sweden (September 19-23), suggest that drinking at least four cups of tea a day is associated with a 17% lower risk of T2D over an average of 10 years.
“Our results are exciting as they suggest that people can do something as simple as drinking four cups of tea a day to potentially reduce the risk of developing type 2 diabetes,” says lead author Xiaying Li of Wuhan University of Science and Technology in China.
While it has long been known that drinking tea regularly can be beneficial to your health due to the various antioxidants, anti-inflammatory and anti-cancer compounds found in tea, the link between tea drinking and the risk of T2D is less clear-cut. So far published cohort studies and meta-analyzes report inconsistent findings.
To address this uncertainty, scientists conducted a cohort study and dose-response meta-analysis to better define the relationship between tea consumption and future T2DM risk.
They first tested 5,199 adults (2,583 men, 2,616 women) without a T2D history (mean age 42) of the China Health and Nutrition Survey (CHNS) who were recruited in 1997 and followed through to 2009. CHNS is a multi-center prospective study, research on the economics, sociology and health of the inhabitants of nine provinces.
Initially, participants completed a questionnaire on food and drink frequency and provided information on lifestyle factors such as regular exercise, smoking, and alcohol consumption. Overall, 2,379 (46%) participants reported drinking tea, and at the end of the study, 522 (10%) participants had developed T2D.
After taking into account factors known to be associated with an increased risk of T2D, such as age, gender and physical inactivity, the researchers found that tea drinkers had a similar risk of developing T2D compared to non-drinkers. The results did not change significantly when analyzed by age and sex, or when participants who developed diabetes within the first 3 years of follow-up were excluded.
In the next phase of the study, researchers systematically reviewed all cohort studies on tea drinking and the risk of T2D in adults (18 years of age or older) up to September 2021. A total of 19 cohort studies with 1,076,311 participants from eight countries  were included in the dose-response meta-analysis.
They examined the potential impact of different types of tea (green tea, oolong tea, and black tea), the frequency of tea drinking (less than 1 cup a day, 1-3 cups a day, and 4 or more cups a day), gender (male and female), and study location ( Europe and America or Asia) at T2D risk.
Overall, the meta-analysis showed a linear relationship between tea drinking and the risk of T2D, with each cup of tea consumed daily reducing the risk of developing T2D by approximately 1%.
Compared with adults who drank no tea, those who drank 1-3 cups a day reduced the risk of T2D by 4%, while those who consumed at least 4 cups a day reduced that risk by 17%.
The associations were observed with regard to the type of tea consumed by participants, whether they were male, female or where they lived, suggesting that the amount of tea consumed, and not any other factor, may play 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 in high doses (at least 4 cups a day),” says Li .
He adds: “It is possible that individual tea ingredients, such as polyphenols, can lower blood glucose levels, but you may need a sufficient amount of these bioactive compounds to function. This may also explain why we did not find a link between tea drinking and type 2 diabetes in our cohort study because we did not consider higher tea consumption. ‘
Oolong tea is a traditional Chinese tea made from the same plant that is used to make green and black teas. The difference is how the tea is processed – green tea is not allowed to oxidize too much, black tea is allowed to be oxidized until it turns black, and oolong tea is partially oxidized.
Despite important findings, the authors note that the study is observational and cannot prove that tea drinking reduces the risk of T2D, but suggest it may contribute.
The researchers note several caveats, including subjective judgments about the amount of tea consumed, and cannot rule out that the results may have been influenced by residual confusion due to different lifestyle and physiological factors.
About this diabetes research news
Author: Judy Naylor
Contact: Judy Naylor – Diabetology
Image: The image is in the public domain
Original research: The results will be presented at the annual meeting of the European Diabetes Research Association