Good news for devotees of post-fermented teas like Qingzhuan brick tea, Kangzhuan brick tea, Liubao and ripe Pu’er: Drinking these and other dark teas may decrease the risk of prediabetes and type 2 diabetes regardless of a person's weight, age, gender, high blood pressure, and family history, according to researchers from the University of Adelaide in Australia and Southeast University in China.
Their study of 1,923 Chinese adults found that the 1,000 habitual tea drinkers in the group had a 15% lower risk for prediabetes than their tea-teetotaling counterparts. Their risk for type 2 diabetes was 28% lower. Those consuming dark tea daily had a 53% lower risk for prediabetes and a 47% reduced risk for type 2 diabetes, likely because drinking tea induces greater excretion of glucose via urination. They also had reduced insulin resistance.
The study was described in a paper presented at the Annual Meeting of the European Association for the Study of Diabetes (EASD), held in October. The findings showed that tea drinkers benefited even if they had any among a long list of known risk factors, including fasting glucose levels, drinking alcohol, smoking, high cholesterol levels, and lack of regular exercise.
Associate Professor Tongzhi Wu, MD, Ph.D., co-authored the paper at the University of Adelaide Medical School. He writes, “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.”
“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,” he writes.
Black and dark teas are fermented using native enzymes (one example is polyphenol oxidase) that break down bioactive catechin, caffeine, thearubigins, and polyphenols. Dark tea is fully oxidized and undergoes additional processing until fully fermented, which may account for different effects on diabetes. Because of this extra process, it is sometimes called post-fermented tea. It is also sometimes referred to as "red" tea, indicating the color of the infusion it produces.
A news release summarizing the research explains that people with diabetes often have enhanced capacity for renal glucose reabsorption, so their kidneys retrieve more glucose, preventing it from being excreted in urine and contributing to higher blood sugar.
“These beneficial effects on metabolic control may lie in the unique way dark tea is produced, which involves microbial fermentation, which may yield unique bioactive compounds (including alkaloids, free amino acids, polyphenols, polysaccharides, and their derivatives).
"These compounds exhibit potent antioxidant and anti-inflammatory effects, improve insulin sensitivity and the performance of beta cells in the pancreas, and change the composition of the bacteria in the gut,” according to the summary.
The participants were 562 men and 1,361 women ranging in age from 20 to 80 years and living in eight provinces in China. Each person indicated the frequency and type of tea consumed. Blood and urine tests indicating high excretion of glucose and related favorable effects were most robust for dark tea drinkers.
According to Associate Professor Wu: “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."
SGLT2 inhibitors are a new class of anti-diabetic drugs that prevent and treat type 2 diabetes while also having protective effect on the heart and kidneys.
Worldwide, some 462 million people have been diagnosed with diabetes. In China, 10.9% percent of the population, approximately 388 million, suffer from prediabetes. In the United States, 37 million people are diagnosed as diabetics, and an estimated 96 million have prediabetes, according to Medical News Today.
The authors said drinking dark tea is a good blood sugar management tool, but overall diet also matters. Wu said that in 2024, the team will conduct double-blind, randomized trials to further investigate the clinical effect of microbial fermented tea and black tea in people having type 2 diabetes.