September 13,
1997
A growing number of studies in the past few years have turned up signs that consumption of green tea helps slow the development of heart disease and certain cancers. As chemists probe how this brew works its medicinal magic, they have increasingly homed in on a family of antioxidants known as catechins (pronounced cat-i-kins). Members of the flavonoid family, these chemicals occur in a number plant-derived foods.
Teas, however, possess some of the most active antioxidant catechins. At the American Chemical Society meeting in Las Vegas this week, Lester A. Mitscher of the University of Kansas presented new data demonstrating that at least three of green tea’s catechins outperform a number of common natural antioxidants.
In one test, for instance, the catechins proved 100 times more powerful than vitamin C in halting oxidative damage to DNA and 25 times more powerful than vitamin E. Their oxidation-quenching activity also surpassed that of the resveratrol in grapes and wine and the BHA used as a commercial food preservative.
"What’s new here," Mitscher told Science News Online, "is that we’ve gotten them all in the same study -- side by side -- so that we can relate [these antioxidants] one to another."
Oxidation is a reactive chemical process that forcibly strips an electron from a molecule. In most cases, the attacked molecule looks to immediately replace the lost electron -- so that it can become chemically stable again -- by stealing one from its neighbor. However, it’s not as simple as merely trading electrons around. The reactions that remove and donate electrons tend to be very energetic and often result in severe damage to neighboring tissue.
In some cases, oxidation proves beneficial. It’s one means by which the body destroys cells that have lived beyond their usefulness or kills noxious invaders, such as germs and parasites. However, oxidation also has a dark side. When not held in check, it can foster many diseases of aging -- especially atherosclerosis, cancer, cataracts, chronic inflammatory diseases, emphysema, heart attacks, and stroke.
Though the body generates antioxidants to turn off oxidation locally when it’s no longer needed, the production of these reaction quenchers falls with age, rendering the body increasingly vulnerable over time to the ravages of runaway or inappropriate electron theft. That’s one reason why medicinal chemists often refer to oxidation as a primary mechanism of aging.
Luckily, many foods contain antioxidants that can help make up the shortfall as the body’s production wanes. Mitscher’s new data suggest that green tea can be a particularly rich source.
In one series of tests, Mitscher’s University of Kansas team compared the catechins’ ability to protect cells from the development of the mutations that might trigger cancer development. They used the Ames test, which employs bacteria that have been genetically damaged so that they cannot live without supplementation of an essential amino acid (known as histidine). When they subject these cells to certain mutagenic agents, some share undergo a transformation that restores their ability to make the missing amino acid. While this new mutation is helpful to the bacteria, the ability of a chemical to initiate this mutation serves as one gauge of its ability to also initiate other random mutations -- some of which might prove harmful.
Mitscher’s team quantified the ability of their mutagens to transform cells in the Ames tests with and without the addition of an antioxidant. In most cases, the antioxidant substantially cut the mutation rate observed in the cells, suggesting the antioxidant could protect cells from damage.
In a second set of tests, the University of Kansas scientists isolated strands of genetic material from cells and subjected this naked DNA to mutagenic chemicals. Again, the antioxidants tended to cut the number of strand breaks that the mutagens provoke. In all these tests, Mitscher says, the catechins proved most protective.
The three top performing catechins have unwieldy monikers that are unlikely to ever become household names: epigallocatechin gallate, epigallocatechin, and epicatechin gallate. Even chemists ignore these long names in favor of their initials -- EGC, EGCG, and ECG.
In each of the tests, Mitscher says, "these three always came out on top, though sometimes one leaped ahead of the other" within that top-three ordering.
Food chemists Edwin N. Frankel and Shu-Wen Huang are finding much the same thing in their studies at the University of California, Davis. In the just published August Journal of Agricultural and Food Chemistry, they applied any of at least five catechins to various oily substances and then heated the materials to promote oxidation. Which catechins retarded oxidation most effectively depended on the specific environment, with no one antioxidant always leading or trailing the pack.
The bottom line, Frankel says, is that gauging the biological value of these food-derived antioxidants "is more complicated than most people appreciate." In their recent study, Frankel and Huang tested the tea catechins in three different systems -- resembling a bottle of salad oil sitting on the pantry shelf, an emulsion of salad dressing, and the oily membranes on the surface of cells.
To retard the rancidity that oxidation can trigger in salad oil, manufacturers currently add antioxidants. The Davis pair found that in bulk corn oil, EGC outperformed EGCG, ECG, and a couple other catechins. But when it came to the oil-in-water emulsions, all catechins could promote oxidation equally . In tests with liposomes -- the materials used to model cell membranes -- EGCG proved most effective, followed by several other catechins, with ECG and EGC near the bottom of the list.
Sometimes, the catechins effectively lost their antioxidant property entirely and became, instead, a promoter of oxidation. In other words, Frankel says, in some situations "if you have too much of one [catechin], it can actually make the situation worse." Luckily, he adds, the situations where this is most likely to occur tends to be somewhat unusual.
The catechins’ transformation from oxidation quencher to oxidation promoter -- a change akin to Dr. Jeckyl’s metamorphosis into Mr. Hyde -- occurred in the presence of free copper. Many metals are good at donating electrons, thus fostering oxidation. Ordinarily, to keep such potentially dangerous electron transfers under control, antioxidants and other materials bind to free metals so that their electrons stay put.
However, Frankel observes, under stress, such as when the body is fighting an infection, coping with inflammatory disease, or burdened with a metal overload, some of the metals may become freed. And under these circumstances, he says, the antioxidants may actually boost the free metals’ ability to donate electrons -- and foster oxidation.
Most of the catechin studies have been performed in test tubes or isolated cells. That’s a long way from modeling a human being, whose internal chemistry will be complex, Frankel points out. However, he acknowledges, hosts of human dietary studies so suggest that "green tea is apparently a very beneficial drink." Moreover, he says, "there are data to show that [this tea’s catechins] are better absorbed than the simpler catechins present in fruits, wines, and grapes."
Mitscher was so compelled by his own new data that he now consumes a nutritional supplement of green-tea catechins. It’s premature to make any health claims about their benefits, he says, "but used in conjunction with a healthful diet and exercise program, it’s like an insurance policy. It increases your odds of avoiding or postponing diseases associated with [oxidation]."
But what if you, like 80 percent of the world’s tea drinkers, consume black tea instead? No problem, Mitscher says, because it contains the same catechins, just in smaller quantities.
In fact, green tea, black tea, and oolong tea all come from the same plant. The different brews depend on how long the leaves were allowed to oxidize after picking, Mitscher explains.
Green tea’s leaves are steamed immediately after harvest to inactivate the enzyme that promotes oxidation. Oolong leaves are allowed to oxidize a bit before steaming. Black tea leaves darken to their distinctive hue during a longer fermentation period that fosters heavy oxidation, and they end up with only about 40 percent as much of the high-performing catechins characteristic of green tea. "Of course, there are other antioxidant compounds produced during the making of black tea, and they too contribute its potential antioxidant effects," Mitscher says.
References
Huang, S-W., and E.N. Frankel. 1997. Antioxidant activity of tea catechins in different lipid systems. Journal of Agricultural and Food Chemistry 45 (August):3033.
Pennisi, E. 1991. Tea-totaling mice gain cancer protection. Science News 140(Aug. 31):133.
Raloff, J. 1997. Versatile cancer weapon in grapes. Science News Online (Feb. 8).
_____. 1996. Of tea and heart disease. Science News 150(Sept. 7):150.
_____. 1996. A couple of heart-friendly dark brews. Science News 149(May 4):286.
_____. 1995. New support for tea’s heart-y benefits 148(Dec. 9):399.
_____. 1993. Add tea to that old ‘apple a day’ adage. Science News 144(Oct. 30):278.
_____. 1992. Another reason to drink green tea. Science News 141(Apr. 18):253.
Sources
Edwin N. Frankel
Department of Food Science and Technology
University of California
Davis, CA 95616
Lester A. Mitscher
Department of Medicinal Chemistry
University of Kansas
4010 Malott Hall
Lawrence, KS 66045
This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.
Food for Thought Archives! 