Regular Tea Consumption, Particularly Dark Tea, May Help Reduce Diabetes Risk

Tea, a beverage consumed extensively worldwide, has been reported to be associated with substantial health benefits, including a reduced risk of cardiovascular disease and type 2 diabetes. However, the mechanism underlying these benefits has been uncertain. In a new cross-sectional study, researchers from the University of Adelaide and China’s Southeast University examined the association of both the frequency and type of tea consumption with urinary glucose excretion and insulin sensitivity, as well as glycemic status, in a Chinese community-dwelling adult population. They found that compared with never tea drinkers, daily consumers of dark tea had 53% lower risk for prediabetes and 47% reduced risk for type 2 diabetes.

Tea is a popular beverage made from the leaves of the plant Camellia sinensis.

Amongst the six major tea categories in China, dark tea is the only one that involves microbial fermentation in the manufacturing process.

The popular dark teas include ripen Pu-erh tea, Liubao tea, Fuzhuan brick tea, Qingzhuan brick tea and Kangzhuan brick tea, which are respectively from Yunnan, Guangxi, Hunan or Shaanxi, Hubei, and Sichuan in China.

The fermentation of dark tea, which is also known as fermented or post-fermented tea, is different from that of black tea (full-fermented tea) and oolong tea (semi-fermented tea).

The so-called fermentation in black tea and oolong tea refers to the catalysis by endogenous enzymes — such as polyphenol oxidases and peroxidases — when the tea leaves are crashed.

However, the activity of these endogenous oxidases is passivated by heat before microbial fermentation of dark tea. It is the microbial enzymes and metabolites that play essential role in the formation of unique flavors and functions in dark tea.

The microbial fermentation also yields unique bioactive compounds (including alkaloids, free amino acids, polyphenols, polysaccharides, and their derivatives) that have potent antioxidant and anti-inflammatory activity.

“The substantial health benefits of tea, including a reduced risk of cardiovascular disease and type 2 diabetes, have been reported in several studies over recent years, but the mechanisms underlying these benefits have been unclear,” said University of Adelaide researcher Tongzhi Wu.

“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 unique way dark tea is produced, which involves microbial fermentation.”

The new cross-sectional study involved 1,923 adults (562 men,1,361 women aged 20-80 years) living in the community across 8 provinces in China.

In total, 436 participants were living with diabetes and 352 with prediabetes, and 1,135 had normal blood glucose levels.

The 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 glycemic 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 scientists 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 favorable 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.

“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,” Dr. Wu said.

“Our findings suggest that drinking dark tea every day has the potential to lessen type 2 diabetes risk and progression through better blood sugar control,” said Southeast University’s Professor Zilin Sun.

“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.”

The authors will present their findings this week at the EASD Annual Meeting 2023 in Hamburg, Germany.

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T. Li et al. Tea consumption is associated with increased urinary glucose excretion, improved insulin resistance and reduced risk of dysglycaemia, in Chinese community-dwelling adults. EASD Annual Meeting 2023