A review of previous cohort studies suggests that drinking at least four cups of black, green or oolong tea a day is associated with a 17% lower risk of type 2 diabetes over an average period of 10 years.
Li et al. found that moderate consumption of black, green or oolong tea is linked to a lower risk of developing type 2 diabetes. Image credit: Sci.News.
Since its initial use in China over 4,000 years ago, tea — produced from the evergreen Camellia sinensis — has become one of the most commonly consumed beverages worldwide, second only to water.
Tea leaves contain polyphenols belonging to the catechin family: epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin-3-gallate.
The various types of tea are distinguished by the processing technique: unoxidized tea as green tea, half-oxidized tea as oolong tea, and fully oxidized tea as black tea.
While it’s long been known that regularly drinking tea may be beneficial for health, less clear has been the relationship between tea drinking and the risk of type 2 diabetes. So far, published cohort studies and meta-analyses have reported inconsistent findings.
To address this uncertainty, Dr. Xiaying Li from the Wuhan University of Science and Technology and colleagues conducted a cohort study and a dose-response meta-analysis to better define the relationship between tea consumption and future type 2 diabetes risk.
First, they studied 5,199 adults (2,583 men, 2,616 women, average age 42) with no history of type 2 diabetes from the China Health and Nutrition Survey (CHNS), who were recruited in 1997 and followed until 2009. The 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 type 2 diabetes.
After adjusting for factors that are known to be linked with increased risk of type 2 diabetes, like age, sex, and physical inactivity, researchers found that tea drinkers had a similar risk of developing type 2 diabetes compared to non-drinkers. 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 step, the researchers did a systematic review of all cohort studies investigating tea drinking and the risk of type 2 diabetes 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 type 2 diabetes.
Overall, the meta-analysis found a linear association between tea drinking and type 2 diabetes risk, with each cup of tea consumed per day reducing the risk of developing type 2 diabetes by around 1%.
When compared with adults who didn’t drink tea, those who drank 1-3 cups daily lowered their risk of type 2 diabetes 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),” Dr. Li said.
“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.”
The authors will present their findings on September 21 at EASD2022, the Annual Meeting of the European Association for the Study of Diabetes in Stockholm, Sweden.
X. Li et al. Tea consumption and the risk of type 2 diabetes: a cohort study and updated systematic review and dose-response meta-analysis. EASD2022, abstract # 281