A bird's eye view of several mugs of various types of dark tea filled with a tea bag each. Share on Pinterest
Drinking dark tea may help in controlling blood sugar levels. Harald Walker/Stocksy
  • Type 2 diabetes is a major health concern, with prevalence increasing worldwide.
  • Type 2 diabetes is largely a result of unhealthy diets and lifestyles and is strongly associated with aging, overweight, and obesity.
  • Lifestyle changes, such as increased exercise and a healthful diet can reduce the risk of developing type 2 diabetes.
  • A new study suggests that regular tea drinking may also help to control blood sugar and reduce diabetes risk.

Type 2 diabetes occurs when the body is unable to regulate blood sugar levels, usually because the body stops responding correctly to insulin, the hormone that controls blood glucose. If uncontrolled, type 2 diabetes can lead to high blood pressure, cardiovascular disease, nerve damage, vision loss, and kidney damage.

The condition currently affects around 462 million people worldwide and numbers are rising. In the United States alone, more than 37 million people have diabetes, and 96 million have prediabetes, a condition where blood glucose levels are higher than they should be, which often leads to type 2 diabetes.

Lifestyle changes are the best way to reduce the risk of developing type 2 diabetes, and, alongside medication, to control symptoms of the condition. Doctors recommend a healthful diet, including plenty of fresh vegetables, whole grains, protein, and heart-healthy fats, together with regular exercise and maintaining a healthy body mass index (BMI).

Now, an observational study from China suggests that people who drink tea regularly, particularly dark tea, may improve their blood glucose levels and decrease insulin resistance, reducing the risk of type 2 diabetes.

The study, by the University of Adelaide, Australia, and SouthEast University, China, was presented at this year’s Annual Meeting of The European Association for the Study of Diabetes (EASD), Hamburg (Oct. 2–6 Oct).

“Given that this is a cross-sectional study with only a small-ish amount of participants (1,923) in China, it is hard to apply the results to a broader population, and we cannot establish a cause-effect relationship between dark tea and its role in glucose regulation in our bodies.”
Dr. Sue Inonog, internal medicine, primary care doctor at Harbor Health in Austin, Texas, who was not involved in the study.

A total of 562 men and 1,361 women ages 20–80 years from eight provinces in China took part in the study. Of them, 436 had diabetes, 352 had prediabetes, and 1,135 had healthy blood glucose levels.

Of the 1,923 people, 1,000 were habitual tea drinkers. They drank different types of tea — 300 reported drinking green tea, 125 black tea, 521 dark tea, and 54 people drank other types of tea. All drank their tea without milk or sugar.

The researchers looked for any association between the frequency and type of tea consumption and the excretion of glucose in the urine, which they assessed using the morning spot urine glucose-to-creatine ratio (UGCR). They also measured insulin resistance and recorded glycaemic status (a history of type 2 diabetes, current use of antidiabetic medications, or an abnormal 75g oral glucose tolerance test).

They found that people who drank tea every day excreted more glucose in their urine and had reduced insulin resistance. They also had a 15% lower risk for prediabetes, and a 28% reduced risk for type 2 diabetes compared with those who never drank tea.

The effects were greater in people who drank dark tea, a type of tea that uniquely has a fermentation process involving microorganisms.

Dark teas include Qingzhuan brick tea, Kangzhuan brick tea, Liubao tea, and Ripen Pu-erh tea.

“We believe habitual tea consumption, particularly dark tea, is beneficial for reducing the risk of prediabetes and diabetes. Our study suggested that these benefits may be related to increased urinary glucose excretion and improved insulin sensitivity.”
Associate Professor Tongzhi Wu from The Adelaide Medical School, University of Adelaide, and The Hospital Research Foundation Group Mid-Career Fellow, co-lead author of the study, speaking to Medical News Today.

Dr. Inonog agreed that the findings were interesting but had reservations:

“The study hints at the possibility that dark tea may increase urinary excretion of glucose (which could correlate to decreased reabsorption of glucose from the urine into the blood) and may improve our body’s sensitivity to insulin, but […] this is a correlation and not an established cause-effect relationship.”

The authors agree that, as this is an observational study, their findings cannot prove that drinking tea improves blood sugar control. However, Dr. Wu did suggest why it might have this effect:

“These findings suggest that the actions of bioactive compounds in dark tea may directly or indirectly modulate glucose excretion in the kidneys, an effect, to some extent, mimicking that of sodium-glucose co-transporter-2 (SGLT2) inhibitors, a new anti-diabetic drug class that is not only effective at preventing and treating type 2 diabetes but also has a substantial protective effects on the heart and kidneys.”

The research group is planning further investigations, as Dr. Wu told MNT:

“Our group is conducting a double-blind, randomized trial to further investigate the clinical effects of regular consumption of microbial fermented tea vs. black tea on glycemic control in people with type 2 diabetes, anticipating outcomes to be released in 2024.”

This could be a useful step to try and verify their findings, as Dr. Inonog told MNT:

“It would be interesting to see if these results could be reproduced in larger populations around the world, and if the association still holds, to then study any potential mechanisms by which dark tea impacts glucose regulation.”

“If a mechanism is discovered, [dark tea] would be a wonderful natural supplement to consider in a treatment plan for those at risk or who have established type 2 diabetes.”
— Dr. Sue Inonog

For some time, tea has been thought beneficial to health, and scientific research is starting to back up those claims. Polyphenols, which are found in black and green tea, have been shown to have anti-aging properties, cardiovascular benefits, and may even help protect against some cancers.

This latest study suggests that lowering diabetes risk could be added to that list.

There is little evidence that drinking tea has any negative health effects, unless you drink huge amounts. The current study suggests that adding a daily cup of tea to your diet might help keep blood glucose levels in the healthy range.