Dark tales of glucose and confounding factors

4 minute read

Give me a fermented cup of mystery fluid over an insulin pen any day.

Scene: Exterior, Reykjavik. It is 11am but the sky is dark and glowering, the howling wind obliterating the sound of the water lapping on the black stone beach.

Interior. Baldur Jonsdottir is wrapped in a blanket as he nurses a hot cup of tea. He takes a sip as the windows rattle in their frames and the log fire crackles. The tea hits his system and suddenly Baldur finds himself out on the beach, his shoes crunching on the pebbles as he stumbles forward. He looks down at his bloodied hands, and the dripping knife slides from his fingers.

Bloody hell, thinks Baldur. That tea’s a bit dark.

Apparently “dark tea” is a thing. No, it’s not black tea. It is tea manufactured via a microbial fermentation process, which contributes unique flavours and functions.

One of those functions appears to be being protective against prediabetes and type 2 diabetes, according to new research out of the University of Adelaide and Southeast University in China.

The study 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, even after taking into account established factors known to drive 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.

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

They 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).

Turns out that microbial fermentation process 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.


“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 the study’s co-lead author Associate Professor Tongzhi Wu from the University of Adelaide.

“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 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.”

As with all Scandi-noir however, there is a cautionary tale.

The authors said that as with any observational study, the findings could not prove that drinking tea every day improves blood sugar control by increasing urinary glucose excretion and reducing insulin resistance, but suggested that they are likely to contribute.

The researchers 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. In addition, they cannot rule out the possibility that residual confounding by other lifestyle and physiological factors may have affected the results.

Booooooo! Blame it on the tea, Baldur! They can’t prove otherwise. (Yet.)

Send story tips in saga form to penny@medicalrepublic.com.au.

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