Research on more than one million people in eight countries has found that moderate consumption of black, green or Oolong tea is linked to a lower risk of contracting type 2 diabetes.
The findings, presented this year at the European Association for the Study of Diabetes (EASD) annual meeting in Stockholm, Sweden (September 19-23), suggest that drinking at least four cups of tea per day is associated with a 17% lower risk of T2DM over an average of 10 years.
“Our findings are exciting because they suggest that people can do something as simple as drinking four cups of tea a day to potentially reduce their risk of developing type 2 diabetes,” says lead author Xiaying Li of Wuhan University of Science and Technology in China.
Although it has long been known that regular consumption of tea can be beneficial to health due to the various antioxidant, anti-inflammatory and anti-carcinogenic compounds it contains, the relationship between tea consumption and the risk of T2D is less clear. So far, published cohort studies and meta-analyses have reported inconsistent results.
To address this uncertainty, the researchers conducted a cohort study and dose-response meta-analysis to better define the relationship between tea consumption and future risk of T2D.
First, they studied 5,199 adults (2,583 men, 2,616 women) with no history of T2D (mean age 42 years) from the China Health and Nutrition Survey (CHNS), recruited in 1997 and followed up until in 2009. The CHNS is a prospective, multicenter study examining the economics, sociological issues, and health of residents of nine provinces.
At baseline, participants completed food and drink frequency questionnaires 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 known to be linked to an increased risk of T2DM, such as age, gender and physical inactivity, the researchers found that tea drinkers had a similar risk of developing T2DM compared to non-drinkers. -drinkers. And the results did not change significantly when analyzed by age and sex, or when participants who developed diabetes during the first 3 years of follow-up were excluded.
In the next stage of the study, the researchers carried out a systematic review of all cohort studies looking at tea consumption and the risk of T2D in adults (aged 18 or older) up to September 2021. A total of 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 consumption (less than 1 cup/day, 1-3 cups/day and 4 or more cups/ day), sex (male and female) and location of the study (Europe and America, or Asia), on the risk of T2DM.
Overall, the meta-analysis found a linear association between tea consumption and the risk of T2D, with each cup of tea consumed per day reducing the risk of developing T2D by around 1%.
Compared to adults who did not drink tea, those who drank 1-3 cups a day reduced their risk of T2D by 4%, while those who drank at least 4 cups a day reduced their risk by 17%.
The associations were seen regardless of what type of tea the participants drank, whether male or female, or where they lived, suggesting it may be the amount of tea consumed, rather than any another factor, which plays a major role.
“Although further research is needed to determine the exact dosage and the mechanisms behind these observations, our results suggest that drinking tea is beneficial in reducing the risk of type 2 diabetes, but only at high doses (at minus 4 cups a day),” Li said.
She adds: “It is possible that particular components of tea, such as polyphenols, lower blood sugar, but sufficient amounts of these bioactive compounds may be required to be effective. This may also explain why we did not find an association between tea consumption and type 2 diabetes in our cohort study, as we did not examine higher tea consumption.”
Oolong tea is a traditional Chinese tea made from the same plant used to make green and black teas. The difference is in how the tea is processed – green tea cannot oxidize much, black tea can oxidize until it turns black, and oolong tea is partially oxidized.
Despite the important findings, the authors note that the study is observational and cannot prove that drinking tea reduces the risk of T2D, but suggest it may help.
And the researchers point out several caveats, including that they relied on subjective assessments of the amounts of tea consumed and that they cannot rule out the possibility that residual confusion due to other physiological and lifestyle may have affected the results.
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