Does regular tea consumption reduce the risk of diabetes?

Even though most people do not think about it in this way, tea is a herbal remedy. We know that it is pleasant, but is it also effective?

This study explored the associations between tea drinking and the incident risk of type 2 diabetes mellitus(T2 DM). A dynamic prospective cohort study among a total of 27 841 diabetes-free permanent adult residents randomly selected from 2, 6, and 7 rural communities between 2006-2008, 2011-2012, and 2013-2014, respectively. Questionnaire survey, physical examination, and laboratory test were carried out among the participants. In 2018, the researchers conducted a follow-up through the electronic health records of residents. Cox regression models were applied to explore the association between tea drinking and the incident risk of T2 DM and estimate the hazard ratio(HR), and its 95%CI.

Among the 27 841 rural community residents in Deqing County, 10 726(39%) were tea drinkers, 8215 (77%) of which were green tea drinkers. A total of 883 new T2 DM incidents were identified until December 31, 2018, and the incidence density was 4.43 per 1000 person-years (PYs). The incidence density was 4.07/1000 PYs in those with tea drinking habits and 4.71/1000 PYs in those without tea drinking habits. The incidence density was 3.79/1000 PYs in those with green tea drinking habits. After controlling for sex, age, education, farming, smoking, alcohol consumption, dietary preference, body mass index, hypertension, impaired fasting glucose, and family history of diabetes, the risk of T2 DM among rural residents with tea drinking habits was 0.79 times higher than that among residents without tea drinking habits(HR=0.79, 95%CI 0.65-0.96), and the risk of T2 DM among residents with green tea drinking habits was 0.72 times higher than that among residents without tea drinking habits(HR=0.72, 95%CI 0.58-0.89). No significant associations were found between other kinds of tea and the risk of T2 DM, nor the amount of green tea-drinking.

The authors concluded that drinking green tea may reduce the risk of T2 DM among adult population in rural China.

Epidemiological studies of this nature resemble big fishing expeditions that can bring up all sorts of rubbish and – if lucky – also some fish. The question thus is whether this study identified an interesting association or just some odd rubbish.

A quick look into Medline seems to suggest great caution. Here are the conclusions from a few further case-control studies:

• In Chinese adults, daily green tea consumption was associated with a lower risk of incident T2D and a lower risk of all-cause mortality in patients with diabetes, but the associations for other types of tea were less clear. In addition, daily tea consumption was associated with a lower risk of diabetic microvascular complications, but not macrovascular complications.
• Green tea drinking was associated with an increased risk of T2D in Chinese adults. The mechanisms underlying the association need to be elucidated.
• Tea consumers had reduced risks of all-cause mortality and partial cause-specific mortality, but not for the risk of death from cancer. On the contrary, daily tea drinkers with smoking habits and excessive alcohol drinking had an increased risk of death from cancer.

Thus the question of whether tea drinking might prevent diabetes remains open, in my view.

Yet, the paper might teach us two important lessons:

1. Case-control studies must be taken with a pinch of salt.
2. Correlation is not the same as causation.