According to a meta-analysis of 19 cohort studies that included more than 1 million adults from eight different countries, moderate use of black, green, or oolong tea is linked to a lower risk of developing type 2 diabetes.

The new study shows that drinking at least four cups of tea daily is associated with a 17% lower risk of type 2 diabetes (T2D) over an average of ten years, and it will be presented at this year’s European Association for the Study of Diabetes (EASD) Annual Meeting in Stockholm, Sweden (19–23 September).

The study findings, as explained by the main author Xiaying Li, “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.”

Tea has long been thought to be beneficial to one’s health due to the antioxidants, anti-inflammatory, and anti-cancer substances found in it. Less is known about the relationship between consuming tea and the risk of T2D. The published cohort studies and meta-analyses have produced contradictory findings.

Researchers conducted a dose-response meta-analysis and a cohort study to better understand the relationship between tea consumption and the risk of developing type 2 diabetes in the future.

First, they looked at 5,199 participants from the China Health and Nutrition Survey (CHNS) who were enlisted in 1997 and followed up within 2009 and had no prior history of T2D. These participants were 2583 males and 2616 women, with an average age of 42. CHNS is a multicenter prospective study that analyzes socioeconomic factors and physical and mental health in nine provinces.

In the beginning, participants filled out a questionnaire about their food and drink intake and provided information about their lifestyle habits, such as how often they exercised, smoked, and drank. Overall, 2,379 (46%) participants reported drinking tea, and 522 (10%) participants had T2D by the trial’s end.

After controlling for established T2D risk factors such as age, gender, and physical inactivity, researchers discovered that tea drinkers had a similar chance of developing T2D as non-drinkers. Additionally, the results were not significantly altered when individuals who developed diabetes within the first three years of follow-up were excluded or the results were divided by age and sex.

Then, up until September 2021, the researchers carried out a systematic review of all cohort studies exploring the connection between tea drinking and the risk of T2D in individuals (aged 18 or older). 19 cohort studies comprising 1,076,311 people from eight different countries made up the dose-response meta-analysis.

They investigated how different types of tea (green tea, oolong tea, and black tea) might affect the risk of T2D, how frequently people drank tea (less than one cup per day, one to three cups per day, and four or more cups per day), whether they were male or female, and where the study was conducted (Europe and America or Asia).

The meta-analysis found a linear association between tea consumption and T2D risk, with each daily cup of tea reducing risk by roughly 1%.

Adults who drank 1-3 cups of tea per day had a 4% lower risk of T2D than those who didn’t, while those who drank at least 4 cups per day had a 17% lower risk.

The links were seen no matter what kind of tea the participants drank, whether they were male or female, or where they lived. This suggests that the amount of tea drunk may be more important than any other factor.

Our results imply that drinking tea is beneficial in lowering the risk of type 2 diabetes, but only at large levels (at least 4 cups a day), adds Li. However, more research is needed to pinpoint the precise dosage and processes underlying these observations.

“It is possible that particular components in tea, such as polyphenols,” add Li, “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.”

Similar to the plant used to produce green and black tea, oolong tea is a classic Chinese beverage. The way the tea is made is what makes the difference: green tea is not allowed to oxidize much, black tea is allowed to oxidize until it turns black, and oolong tea is only partially oxidized.

Despite the significant results, the authors point out that as the research was observational, it cannot conclusively show that tea consumption decreased the incidence of T2D but rather provides evidence that it probably does.

And the researchers say that there are some things to keep in mind. For example, they used subjective estimates of how much tea people drank, and they can’t say for sure that other lifestyle and physiological factors didn’t affect the results.

Source: EASD 2022 Stockholm, Abstract Number 281

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