Dark tea may help blood sugar, diabetes risk

The study was shown at the The European Association for the Study of Diabetes (EASD) annual meeting in Hamburg, Germany, which is running from October 2-6.

Data showed daily consumers of dark tea had a 53 per cent lower risk for prediabetes and 47 per cent reduced risk for type 2 diabetes, compared with people who don't drink tea at all. The findings stood up even after established risk factors known to drive the risk for diabetes, including age, gender, ethnicity, body mass index (BMI), average arterial blood pressure, fasting plasma glucose, cholesterol, alcohol intake, smoking status, family history of diabetes and regular exercise, were taken into account.

Researchers from the University of Adelaide in Australia and Southeast University in China collaboratively worked on the study.

Associate Professor Tongzhi Wu from the University of Adelaide and The Hospital Research Foundation Group Mid-Career Fellow is the study's co-lead author.

He said while the health benefits of tea in relation to cardiovascular disease and type 2 diabetes have been reported in recent years, the mechanisms underlying these benefits had been unclear.

"Our findings hint at the protective effects of habitual tea drinking on blood sugar management via increased glucose excretion in urine, improved insulin resistance and thus better control of blood sugar. These benefits were most pronounced among daily dark tea drinkers."

These beneficial effects on metabolic control may lie in the way dark tea is produced, which involves microbial fermentation, a process that may yield unique bioactive compounds (including alkaloids, free amino acids, polyphenols, polysaccharides, and their derivatives) to exhibit potent antioxidant and anti-inflammatory effects, improve both insulin sensitivity and the performance of beta cells in the pancreas, and change the composition of the bacteria in the gut.

This study included 1923 adults (562 men,1361 women aged 20-80 years) living in eight provinces in China. In total, 436 participants had diabetes, 352 had prediabetes, and 1135 had normal blood glucose levels.

Participants included both non-habitual tea drinkers and those with a history of drinking only a single type of tea. They were asked about the frequency (i.e. never, occasionally, often and every day) and type (i.e. green, black, dark, or other tea) of tea consumption.

The researchers examined the association between both the frequency and type of tea consumption and excretion of glucose in the urine (assessed by the morning spot urine glucose-to-creatine ratio [UGCR]), insulin resistance (measured using the triglyceride and glucose index [TyG] derived from fasting plasma glucose and fasting triglyceride levels), and glycaemic status (defined as a history of type 2 diabetes, current use of antidiabetic medications, or an abnormal 75g oral glucose tolerance test).

People with diabetes often have enhanced capacity for renal glucose reabsorption, so their kidneys retrieve more glucose, preventing it from being excreted in urine, which contributes to the higher blood sugar.

After accounting for differences in age, sex, and clinical and lifestyle factors, the analysis found that drinking tea every day was associated with an increase in urinary glucose excretion (UGCR by 0.11 mmol/mmol) and a reduction in insulin resistance (TyG by -0.23), as well as 15% lower risk for prediabetes and 28% reduced risk for type 2 diabetes, compared with never tea-drinkers.

These health effects were most robust for dark tea drinkers, with consumption of dark tea associated with an increase in UGCR by 0.16 mmol/mmol and a reduction in TyG by 0.31.

Associate Professor Wu said the findings suggested bioactive compounds in dark tea may directly or indirectly modulate glucose excretion in the kidneys, in a similar way to new anti-diabetic drug class sodium-glucose co-transporter-2 (SGLT2) inhibitors.

Co-lead author Professor Zilin Sun from Southeast University said the findings suggest drinking dark tea taily has the potential to lessen type 2 diabetes risk and progression through better blood sugar control.

"When you look at all the different biomarkers associated with habitual drinking of dark tea, it may be one simple step people can easily take to improve their diet and health."

Despite the promising findings, the authors caution that as with any observational study, the findings cannot prove that drinking tea every day improves blood sugar control by increasing urinary glucose excretion and reducing insulin resistance, but suggest that they are likely to contribute.

They are currently conducting a double-blind, randomised trial to investigate the benefits of dark tea on blood glucose control in people living with type 2 diabetes to validate their findings. Also, they cannot rule out the possibility that residual confounding by other lifestyle and physiological factors may have affected the results.