Drinking four or more cups of tea per day could lower the risk of type 2 diabetes, research suggests.

A study found that drinking black, green, or oolong tea every day was linked to a 17% lower risk of diabetes over an average of 10 years.

Drinking between one and three cups a day cut the risk by 4%.

The findings, presented at the European Association for the Study of Diabetes annual meeting in Stockholm, are based on a review of 19 studies involving more than one million people.

They have not yet been peer-reviewed or published in a journal.

Lead author Xiaying Li, from Wuhan University of Science and Technology in China, said: “Our results 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.”

Previous research has found that tea could be beneficial for health, partly because it contains antioxidants and polyphenols, which may protect against disease.

However, to reduce the risk of type 2 diabetes, experts agree that people should primarily keep their weight in check.

Obesity is a major driver of type 2 diabetes, accounting for 80 to 85% of the risk of developing the condition.

Obese people are thought to be up to 80 times more likely to develop type 2 diabetes than those with a body mass index of under 22.

In the new study in China, researchers first looked at data from 5,199 adults in the China Health and Nutrition Survey who did not have diabetes, who were recruited in 1997 and followed until 2009.

People filled in a food and drink frequency questionnaire and provided information on lifestyle factors such as regular exercise, smoking and alcohol consumption.

This study found no benefit from drinking tea on the risk of diabetes.

But when researchers did a systematic review of existing studies up to September 2021 from eight countries, the findings were different.

This analysis suggested that each cup of tea per day reduced the risk of developing type 2 diabetes by around 1%.

The findings held true regardless of what type of tea people drank, whether they were male or female and regardless of where they lived.

Xiaying Li said tea was shown to reduce risk, but only when drunk in fairly large quantities.

She added: “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.”

When it comes to whether the reduced risks held true if people added milk to their tea, the authors said they had reviewed previously published literature on this issue.

This showed that “dairy and dairy products were associated with a reduced risk of diabetes”, they wrote.

Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said: “Given the nature of this study, it cannot prove tea prevents diabetes per se.

“Rather it could be that people who drink more tea avoid or less often drink more harmful sugary drinks or equivalent or that they have other health behaviours that leads them to have lower risks of type 2 diabetes.

“There is no good trial evidence whatsoever that the chemicals in tea prevent diabetes, so I suspect it’s more about tea being healthier (less calorific) than many alternative drinks or tea drinkers leading healthier lives more generally.”

Matt Sydes, professor of clinical trials and methodology at the Medical Research Council’s clinical trials unit, said: “This is large, observational data. It’s not a randomised controlled trial so there’s plenty of room for data to be misunderstood.

“Importantly, everyone drinks fluids. If there is an effect here (and that’s a big if), it might be not about the tea they drink, but about what they don’t drink because they are drinking tea at those moments.

“One can’t tell at the moment.”