Green Tea Catechins and Blood Sugar: What 17 Randomized Trials Found

What is in green tea

Green tea is made from the leaves of Camellia sinensis, steamed or pan-fired soon after picking so that the leaf does not oxidize as black tea does. That keeps a high level of catechins, a family of polyphenols. The most abundant and most studied is epigallocatechin gallate, EGCG. A cup of brewed green tea also contains caffeine, typically less than a cup of coffee, plus the amino acid theanine.

Green tea extracts concentrate the catechins, sometimes with the caffeine removed. Supplements are often standardized to a percentage of total catechins or EGCG, and that figure, not the milligrams of extract, is what determines the dose ^[1].

How catechins might affect glucose

Laboratory studies suggest several routes. Catechins inhibit enzymes that break down starch and sugar in the gut, which could slow glucose absorption after a meal. In cell and animal studies they also affect insulin signaling and glucose uptake in muscle and fat. The seller's page summarizes this as improving cells' sensitivity to insulin and slowing the rate at which sugar enters the bloodstream.

Mechanisms seen in a dish do not always survive the trip through the human gut, where catechins are poorly absorbed and quickly broken down. Human trials are the test.

What 17 trials found

In 2013, researchers pooled 17 randomized controlled trials of green tea or green tea extract that measured glucose control ^[2]. Overall, green tea lowered fasting glucose and HbA1c by small but statistically significant amounts. It did not significantly change fasting insulin. The authors noted differences between the studies in dose, product and population.

Small is the important word. The average changes were a fraction of what diet, activity or glucose-lowering medicine achieve, and many of the trials were short. NIH summarizes the evidence on green tea for health conditions as limited or inconclusive for most uses ^[1].

Tea versus extract

The trials split into two kinds: people drinking several cups of green tea a day, and people taking concentrated extracts. Tea brings catechins together with water, caffeine and the rest of the leaf, in amounts people have consumed for centuries. Extracts concentrate catechins into a capsule or liquid and can deliver far more than tea drinkers ever take.

That difference matters most for safety, discussed next. For glucose, it means that drinking unsweetened green tea instead of sugary drinks is a cheap, well-tolerated way to get whatever benefit catechins offer, with the added effect of replacing sugar.

The liver warning

Concentrated green tea extract is one of the supplements most often linked to drug-induced liver injury. A 2017 review by U.S. liver injury researchers lists it among the most frequently implicated herbal products ^[3]. In 2018 the European Food Safety Authority reviewed green tea catechins and concluded that EGCG intakes of 800 mg a day or more from supplements were associated with signs of liver damage in trials, while catechins from brewed tea were generally considered safe ^[4].

NIH advises that anyone taking green tea extract should watch for signs of liver trouble, such as dark urine, abdominal pain or yellowing of the skin, and stop if they appear ^[1].

Caffeine, a complication

Most green tea extracts contain caffeine unless they are labeled decaffeinated, and caffeine affects glucose in its own right. In a small trial in people with type 2 diabetes, caffeine taken before a meal raised the after-meal glucose and insulin response ^[5]. For most people the caffeine in a cup of tea is not a concern, but it is a reminder that green tea is not a pure glucose-lowering agent. Honey Harmony also contains guarana, another caffeine source, though both are present in tiny amounts.

Green tea in Honey Harmony

Honey Harmony lists green tea leaf extract as a 20:1 extract, seventh in its 21-ingredient proprietary blend of 15.47 mg. The label gives no amount and no catechin or EGCG percentage.

Even if the whole blend were green tea extract, it would amount to about 15 mg, a small fraction of the catechins in a single cup of green tea. Split evenly with the other ingredients, it would be about 0.74 mg. At that level neither the modest glucose effect from the trials nor the liver risk of high-dose extracts is in play. A cup of green tea would deliver far more of the ingredient than a dropperful.

Practical takeaways

• If you like green tea, drinking a few cups a day, unsweetened, is the best-supported way to get its catechins, and it replaces sugary drinks.
• If you take a concentrated extract, keep EGCG well under 800 mg a day, take it with food, and stop if you notice signs of liver trouble.
• If you take glucose-lowering medicine, mention green tea extract to your prescriber; small additive effects are possible.
• Do not count on green tea in a small blend for anything.

Reading a green tea extract label

A useful green tea extract label states three things: the amount of extract, the percentage or milligrams of total catechins or EGCG, and whether it contains caffeine. A label that gives only the milligrams of extract, or hides the extract inside a blend, does not let you estimate your EGCG intake, which is the number that matters for both the possible benefit and the liver risk. If you take more than one product with green tea, add up the EGCG from each, and keep the total well under the level European regulators flagged ^[4].

Who should avoid concentrated extracts

People with liver disease, anyone who has had a reaction to a supplement before, and anyone taking several herbal products at once should be cautious with concentrated green tea extracts and discuss them with a clinician ^[3]. Pregnant women are usually advised to limit caffeine, which most extracts contain. People on glucose-lowering medicine should mention green tea extract, because small additive effects are possible. None of these concerns applies to an ordinary cup of tea.

Bottom line

Green tea has one of the larger bodies of evidence among glucose-related plants, and the result is a small effect at tea-level intakes, with a real liver risk for high-dose extracts. In Honey Harmony it is a name on a long list. If the sales page's description of green tea appeals to you, the kettle is a better source than the dropper.