Green Tea May Help Control Blood Sugar

Green Tea May Help Control Blood Sugar
The long list of health benefits from this wondrous brew grows longer still
By Will Block

he British are an odd lot. Granted, they’re ever so cultured—they gave the world the Magna Carta and John Locke, and Shakespeare and Milton, and Newton and Hawking, and the British Museum and the Royal Society, and . . . well, it’s a long list, and very impressive to us colonials. And they do speak English quite well, for foreigners.

But look what those same people put in their mouths: warm beer, for God’s sake, and strange “foods” that could be charitably described as almost edible, and—are you ready for this?—they put milk in their tea! Gaah! Now, everyone knows you’re supposed to put lemon in your tea, as all patriotic Americans do. (Of course, the Japanese, who have elevated the preparation, serving, and drinking of tea to an art form, might disagree with that—they probably regard all of us as a bunch of barbarians when it comes to tea.)

Tea Gives Insulin Activity a Major Boost

But does it really matter whether you put milk or lemon in your tea? Yes, it does—if you’re using that tea in laboratory experiments, as scientists at the U.S. Department of Agriculture did recently to determine whether tea has insulin-enhancing activity. The researchers found that tea does have such activity, at least in fat cells taken from rats.1 Ordinary tea, prepared just as humans normally consume it, was shown, in fact, to increase the insulin activity in these cells by more than 15-fold—a dramatic effect by any standard.

But here’s the kicker: adding milk (whole or skim) to the tea nearly wiped out the beneficial effect—a greater than 90% reduction with 50 grams (about 10 teaspoons) of milk in one cup of tea. Soymilk and several nondairy creamers were also tested, and they too pretty much killed the effect. By contrast, adding a jolt of lemon juice did not diminish the effect. So—milk bad, lemon good. (Hmm, perhaps this helps explain the decline of the British Empire . . . .)

Metabolic Syndrome—Slippery Slope to Diabetes

Enhanced insulin activity means better control over blood glucose (blood sugar) in people whose insulin function may be impaired and who are therefore at risk for type 2, or adult-onset, diabetes. If you’re even dimly aware of how devastating a disease diabetes can be, you can appreciate the implications of this fact. Anything that can enhance insulin activity is potentially of value in combating not only diabetes but also—and perhaps even more importantly—the prediabetic condition that puts so many people on the slippery slope to that dread, yet largely preventable, disease.

Ordinary tea, prepared just as 
humans normally consume it, was 
shown to increase the insulin 
activity in these fat cells by 
more than 15-fold—a dramatic 
effect by any standard.

The prediabetic condition has a name: it’s called the metabolic syndrome. (For a quick rundown on this vital subject, see the sidebar, “The Metabolic Syndrome Is a Killer.”) Paradoxically, most people have never even heard of it, despite its prevalence in our society—it is estimated that one-third of all middle-aged Americans have it to some degree—and the huge danger to our health that it represents.

The Metabolic Syndrome Is a Killer

The metabolic syndrome is a group of individual but interacting disorders that arise from a general derangement of the body’s metabolic functions. It includes obesity, high blood pressure, high blood levels of cholesterol and triglycerides (fats), and high blood glucose levels. Sound familiar?

There can be a genetic predisposition to the metabolic syndrome, but most people bring it upon themselves by committing the well-known cardinal sins of healthy living: poor diet and inadequate exercise (not to mention smoking, which is a form of slow suicide). These sins often lead to obesity, which is the number one risk factor for insulin resistance, a hallmark of the metabolic syndrome.

Insulin resistance, an age-related problem, develops when our cells become progressively more resistant to the action of insulin in trying to carry out its principal task, which is to facilitate the transport of glucose into the cells so that the glucose can be metabolized to produce the chemical energy we need to live. When insulin resistance becomes serious, blood glucose levels rise to abnormal and dangerous levels, and the result is type 2, or adult-onset, diabetes, a disease with horrendous consequences if not tightly controlled.

Having the metabolic syndrome thus puts you on the fast track to diabetes—and to cardiovascular disease, which often accompanies diabetes. You thus become a prime candidate for heart attack and stroke, and potentially for kidney failure and cancer as well. The form of obesity most strongly linked to these degenerative diseases, by the way, is abdominal obesity—a pronounced bulge in the midriff.

New evidence of how damaging the metabolic syndrome is has just come from Finland, where researchers examined a group of 1209 men aged 42 to 60 years who did not initially have diabetes, cardiovascular disease, or cancer.1 The researchers followed the men for 11.4 years to see what the consequences would be in those who developed the metabolic syndrome vs. those who did not.

Depending on how the metabolic syndrome was defined (there are different standards put forth by different medical organizations), anywhere from 8.8 to 14.3% of the men did develop the syndrome. Compared with the men who did not, and after correcting for relevant factors such as smoking or a family history of heart problems, it turned out that the men with metabolic syndrome were three to four times more likely to die of cardiovascular disease and twice as likely to die from any cause. The researchers said, “Early identification, treatment, and prevention of the metabolic syndrome present a major challenge for health care professionals facing an epidemic of overweight and sedentary lifestyle.”*

Are you getting the picture? The metabolic syndrome is a ticket to early death. If you have it, fight back! If you don’t have it, prevent it! You know how to do that. Don’t forget that certain nutritional supplements can help.

*It is a tragic indictment of our fat, lazy lifestyle that “adult-onset” diabetes, which used to be a disease of aging people, has become fairly common in young adults and even children, who are thus dooming themselves to a shortened life beset by debilitating degenerative disorders, such as memory loss, heart failure, kidney failure, and blindness.

  1. Lakka H-M, Laaksonen DE, Lakka TA, Niskanen LK, Kumpusalo E, Tuomilehto J, Salonen JT. The metabolic syndrome and total and cardiovascular disease mortality in middle-aged men. JAMA 2002;288(21):2709-16.

Perhaps it’s poorly known because the name sounds obscure and unthreatening. There is an analogous term, though (not a synonym, but close): insulin-resistance syndrome. That doesn’t help much, does it? If it had a catchier name, such as, say, your-vital-systems-are-breaking-down! syndrome, perhaps more people would pay attention.

True Teas Are Antidiabetic

In the USDA study, the researchers tried over 40 true teas, i.e., teas made from leaves of the tea tree, Camellia sinensis, which is native to China (sinensis means Chinese). All of them—whether green tea, oolong tea,* or black tea (the only three kinds of true tea there are)—showed significant insulin-enhancing activity. Instant teas, however, did not, except for one brand, which the authors did not identify. Herbal teas and commercially prepared iced teas showed only minimal activity. The presence or absence of caffeine in the teas made no significant difference, and caffeine alone had no insulin-enhancing activity.

*Never heard of the color oolong? Actually, oolong comes from the Mandarin Chinese term for “dark dragon.” So that’s a color, sort of.

The reason for the differences between true teas and herbal teas is that only true teas (and apparently only those that have not been processed for use as instant or iced tea) contain significant amounts of the chemical compounds primarily responsible for the insulin-enhancing effect. These compounds, called catechins (pronounced cat’-eh-kins), belong to a larger group called polyphenols, which are members of a still larger group called flavonoids. All these compounds are known for their potent antioxidant properties and a host of documented benefits against cancer, heart disease, vision disorders, allergies, viral infections, and more.

Antioxidants neutralize free radicals, highly reactive molecules that are responsible for much cellular damage. Free radical damage is, in fact, believed to be one of the principal causes of aging, which is why antioxidants are so important for our health and longevity. (From childhood we’ve always heard how important it is to have plenty of vitamins and minerals in our diet. We should have been hearing how important it is to have plenty of vitamins and minerals and antioxidants in our diet.)

Antioxidant Power Is the Key to Tea’s Benefits

In writing about the myriad health benefits of tea—summarized as “antibacterial, antiviral, anticarcinogenic, and antimutagenic”—the USDA scientists wrote (citing abundant literature references):

Tea protects against chemically induced tumor initiation and promotion and progression of benign tumors to malignancy. The majority of the benefits associated with tea and atherosclerosis, hypertension, infectious diseases, immune response, and longevity are generally attributed to the antioxidant activities of tea.

They go on to say that folk remedies have also included the antidiabetic properties of tea for decades and that a certain variety of tea has been used as a traditional Chinese treatment for diabetes. This, of course, is what prompted them to investigate—and confirm—the insulin-enhancing effect of tea, using modern scientific methods. (Tea attacks diabetes in yet another way—see the sidebar, “Green Tea Also Suppresses Blood Sugar.”)

Green Tea Also Suppresses Blood Sugar

There are two main avenues of attack in trying to prevent or control diabetes: you can enhance your insulin function, which helps control blood sugar levels, or you can suppress your blood sugar levels in some other way (careful control of your diet being the most obvious). An intriguing aspect of tea is that it may have both effects, not just the insulin-enhancing effect described in the accompanying article.

The authors of the USDA tea study cited other research in which the antidiabetic effects of tea on human beings were investigated. When humans in clinical trials ingested tea catechins (200–500 mg) before ingesting 50 g of starch (which is converted to glucose by digestive enzymes, mainly alpha-amylase), glucose production was suppressed, apparently because the catechins inhibited the enzyme action.

It has also been found that the uptake of glucose by the intestine for transfer to the bloodstream is markedly suppressed by green tea polyphenols (which may help to explain green tea’s weight-loss effect). This suppression is strongest with the same catechin compounds that are the most active in enhancing insulin activity (recall that catechins are a subcategory of polyphenols).

Another health benefit attributed to green tea is its thermogenic effect, i.e., its fat-burning ability, which makes it helpful in weight-loss formulations (see Green Tea Helps Burn Fat in Life Enhancement, July 2001).

EGCG Is Tea’s Star Component

Epigallocatechin gallate (EGCG), the principal source of green tea’s many health benefits.
In the USDA tea study, one compound stood out among all the catechins as being by far the most potent of the lot: epigallocatechin gallate, or EGCG. Those with long memories and a familiarity with the health-giving properties of green tea will recall that this is the very same compound that is primarily responsible for green tea’s remarkable anticancer effects.

Thanks largely to EGCG, which is considered by some scientists to be among the most promising natural anticancer compounds ever discovered, green tea is renowned among nutritional supplements for its role in helping to prevent certain cancers in humans. Well-controlled clinical trials have shown that green tea can prevent cancers of the pancreas, colon, small intestine, stomach, breast, and lung.2 No other supplement can make such a claim. There is also good evidence that green tea can provide protection, from the inside out, against skin cancer caused by harmful solar ultraviolet radiation (see Green Tea: What’s New Under the Sun? in Life Enhancement, August 2001).

Is it a coincidence that EGCG, among all the catechins, is so extraordinarily effective in enhancing insulin activity* and inhibiting the growth of cancer cells, or is there some deep biochemical connection between the two effects? We don’t know, but it is certainly an intriguing question. It’s worth noting, too, that, according to researchers at the University of Kansas, the antioxidant power of EGCG is about 100 times greater than that of vitamin C and 25 times greater than that of vitamin E in protecting DNA from the kind of free radical damage that is thought to increase the risk of cancer.3

*Bear in mind that the insulin-enhancing effect was seen only in rat cells in the laboratory; it has not been demonstrated in human beings, which is much more difficult to do.

Green Tea Is Best in Some Regards

Curiously, roughly the same insulin-enhancing activity of tea was found in all three kinds of true tea: green, oolong, and black (all other names for true tea refer to varieties that fall into one of those three categories). It’s curious because it is known that EGCG and other catechins are largely destroyed by the oxidative fermentation process that turns green tea (which is unfermented) into oolong tea (a partial fermentation) or black tea (a full fermentation). This is why, in fact, the anticancer properties of tea are found largely in green tea; oolong tea has some activity, and black tea has almost none. (The antioxidant power of green tea, incidentally, is six times greater than that of black tea.2)

In the tea study, however, chromatographic analysis showed that the chemical compositions of the green and oolong teas were very similar, and the authors state that in both cases, EGCG was the main active ingredient in terms of the insulin effect. The teas also contain certain compounds called tannins and theaflavins, however, and the authors suggested that these compounds, as well as a catechin called epicatechin gallate, also display insulin-enhancing activity and may account for the effect observed with black tea. They also mentioned, by the way, that diluting the teas reduced their effectiveness accordingly—which one would expect.

Milk Revisited—and Feynman’s Joke

It’s time for full disclosure about the milk thing, which will let our British friends off the hook. What we said before was true, but there’s more to it than that. It turns out that, even though milk destroys most of the insulin-enhancing activity of tea in the laboratory, it almost certainly does not do that in human beings. In a cup of tea, the tea catechins bind to milk proteins and are thus, in effect, precipitated from solution. This renders the “milked” tea impotent in the test-tube experiments on cells.

But when humans of the British persuasion drink such ghastly cups of tea, their digestive systems break the milk proteins down into their constituent amino acids. The catechins are thus set free and can enter the bloodstream, which carries them throughout the body to do their good works; this is not conjecture, but fact.4 (The milk story illustrates how dangerous it can be to assume, without hard evidence, that what works in laboratory experiments, or even in animal tests, will work in human beings, or vice versa. Always demand evidence.)

EGCG is the very same compound 
that is primarily responsible for 
green tea’s remarkable
anticancer effects.

One other thing: it would be a big mistake to put both lemon and milk in your tea, because the lemon’s citric acid would cause the milk to curdle. Therein lies a story. When the great theoretical physicist Richard Feynman was a young graduate student at Princeton, he attended an afternoon tea. A genteel lady asked him whether he would like lemon or milk in his tea. Not knowing which one was “correct” (or perhaps just being the smart aleck that he always was), Feynman said, “Both,” at which the shocked lady gasped, “Surely you’re joking, Mr. Feynman!” That phrase later became the title of a wonderful book about the mercurial and amusing genius.

“Supplement Up” with Green Tea

Owing to its insulin-enhancing activity, green tea, along with its many other proven health benefits, may also be able to help forestall or alleviate the metabolic syndrome, and thus type 2 diabetes. If this is not a worthy goal in anyone’s personal health regimen, nothing is. So if you are at all concerned about this aspect of your health, drink up! Whether you take your tea the American or British or Japanese way, or some other way, doesn’t matter, as long as you drink plenty of it.

Some physicians recommend four to six cups a day; others recommend eight to ten cups (the average in Far Eastern societies where green tea is a staple is about five to ten cups a day). That’s a challenging amount for even tea lovers to imbibe on a daily basis. Fortunately, researchers have concluded that it’s just as effective to take green tea supplements—so “supplement up”! And may your life be long and healthy.

Caution: If you have diabetes, do not take any supplement that may affect your blood sugar levels without first consulting your physician. Diabetes is a serious disease requiring careful professional management.


  1. Anderson RA, Polansky MM. Tea enhances insulin activity. J Agric Food Chem 2002;50:7182-6.
  2. PDR for Herbal Medicines, 2nd ed. Medical Economics Co., Montvale, NJ, 2000, p 370.
  3. Kalyn W, ed. The Healing Power of Vitamins, Minerals, and Herbs. The Reader’s Digest Association, Pleasantville, NY, 1999, p 311.
  4. van het Hof KH, Kivits GA, Westgate JA, Tijburg LB. Bioavailability of catechins from tea: the effect of milk. Eur J Clin Nutr 1998;52:356-9.

Dr. Anderson Is on a Roll (a Jelly Roll?)

The tea discovery described in the accompanying article was made by Dr. Richard A. Anderson, a chemist at the Human Nutrition Research Center of the U.S. Department of Agriculture, and a colleague. Dr. Anderson is well known for his work with the trace mineral chromium, having demonstrated its role in the prevention of diabetes and cardiovascular disease. He is also the same man who startled the medical world in 2001 with his discovery that a component of cinnamon called methylhydroxychalcone polymer, or MHCP, is a potent insulin-mimetic compound, i.e., one that simulates the action of insulin and produces essentially the same effects. (See Revitalize Yourself: Cinnamon Extract for Healthy Blood Sugar in Life Enhancement, March 2002.)

In screening dozens of foods for potential antidiabetic activity, Dr. Anderson and his colleagues found initially that apple pie, of all things, seemed to help regulate blood sugar levels. (No one in his right mind would have predicted that.) Looking further, they traced the effect to the spices used in the pie, then to cinnamon in particular, and finally to the MHCP in the cinnamon—good scientific detective work!

And now we have the tea discovery. What next, Dr. Anderson? Would you mind having a look at chocolate? And what about bacon? Hope springs eternal . . . . (It has been said, by the way, that there is no food that could not be improved by the addition of either chocolate or bacon. Think about that.)

Will Block is the publisher and editorial director of Life Enhancement magazine.

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