Drinking green tea daily helps prevent T2D, diabetic complications, says study

A study of Chinese adults has found a protective benefit with daily green tea consumption against incident type 2 diabetes (T2D), as well as all-cause mortality in patients with T2D. However, the associations for other types of tea are unclear.

Additionally, “daily tea consumption was associated with a lower risk of diabetic microvascular complications, but not macrovascular complications,” the researchers said.

In total, 482,425 diabetes-free participants (mean age 51.2±10.5 years, 41 percent male) and 30,300 diabetic patients (mean age 58.2±9.6 years, 39 percent male) at study enrolment from the China Kadoorie Biobank (CKB) were included.

The researchers obtained tea consumption data at baseline by interviewer-administered questionnaires and identified incidences of diabetes, diabetic complications, and death by linkages to the National Health Insurance system, disease registries, and death registries. Hazard ratios (HRs) and 95 percent confidence intervals (CIs) were estimated using Cox proportional hazard regression models.

Of the daily tea consumers, the greater majority (85.8 percent) preferred drinking green tea. In diabetes-free participants, 17,434 developed incident T2D during 11.1 years of follow-up. Daily consumers of green tea had a lower risk of incident T2D (HR, 0.92, 95 percent CI, 0.88–0.97) than those who never consumed tea in the past year. [Am J Clin Nutr 2021;114:194-202]

Among diabetic patients, a total of 6,572 deaths occurred during follow-up; in addition, 12,677 diabetic macrovascular cases and 2,441 diabetic microvascular cases had been identified. Daily green tea drinkers in this population had a reduced risk of all-cause mortality (HR, 0.90, 95 percent CI, 0.83–0.97) and microvascular complications (HR, 0.88, 95 percent CI, 0.78–1.00), respectively, when compared with patients who did not consume tea in the past year.

Moreover, tea consumption did not increase the risk of macrovascular complications among diabetic patients. Of note, the inverse associations between daily tea consumption and risks of T2D and all-cause mortality in patients with diabetes were seen only among daily green tea drinkers.

“Our findings are in line with a meta-analysis that included 15 prospective observational studies, although we did not observe a dose–response relation among daily tea consumers, which was found in mainly European cohort studies,” the researchers said. [Br J Nutr 2014;111:1329-1339]

Other studies from the CKB focusing on tea and cardiovascular outcomes also did not find a dose–response relation. Such outcomes could be attributed to differences in baseline characteristics between individuals who consumed higher amounts of tea leaves and other consumption groups. [Heart 2017;103:783-789; Am J Clin Nutr 2020;111:197-206]

“For example, those who consumed more tea leaves were more likely to be urban residents, smokers, and have a larger waist circumference. The expected lower risk of T2D in participants who consumed more tea leaves was offset by an increase in other risk factors of T2D. Although we carefully adjusted for those covariates, residual confounding may still mask the dose–response relation,” the researchers said.

The current findings that tea consumption correlated with lower diabetes and complications risks are biologically possible based on earlier studies using surrogate endpoints as outcome measures. In a meta-analysis of 27 trials, green tea consumptions led to lower concentrations of fasting blood glucose, which was mostly observed in Asian studies. [Nutrients 2018;11:48]

Other meta-analyses of trials among T2D patients showed that interventions with tea or tea extraction reduced circulation concentrations of C-reactive protein, maintained a stable fasting blood insulin, and reduced waist circumference. [Complement Ther Med 2019;46:210-216; Diabetes Metab Res Rev 2016;32:2-10]

Furthermore, “[t]he level of pesticides in Chinese tea leaves and its dose–response effect on health need to be confirmed by further studies,” the researchers said. “Our findings show that there is not sufficient evidence to avoid tea drinking due to concern of pesticides.”