Years ago, the only place you were likely to find green tea was in an Asian restaurant. Now nearly every supermarket sells it, both regular and decaffeinated. In the last decade green tea has acquired a reputation as a preventive of many scourges, including heart disease and cancer. But is it? How good is the evidence? And is there a downside to drinking lots of it?
The evidence is conflicting, but to date it does not indicate green tea is a panacea or the elixir many believe it to be. For example, Japanese researchers reported March 1 in The New England Journal of Medicine that among 26,311 residents of northern Japan followed for eight years, the consumption of green tea showed no effect on the risk of developing stomach cancer, a leading killer among cancers in Japan.
Still, there are probably some benefits -- and perhaps a few risks -- associated with the frequent consumption of green tea.
What's the Magic?
Green tea is not the only form of the plant Camellia sinenis that has acquired a health-promoting aura. Black tea (the kind consumed by most Westerners) and red tea (oolong, a popular Chinese tea) also have enjoyed much positive press of late. All originate with the same plant.Continue reading the main story
The difference between them lies in the degree of fermentation after the leaves are picked. For green tea, the leaves are steamed, rolled and dried, and that process inactivates the enzymes that cause oxidation. For black and red teas, the leaves are first partially dried, then crushed and stored in a temperature- and moisture-controlled environment where they oxidize, or ferment, longer for black tea than for red. The extra fermentation results in the darker color of black tea.
All tea contains potent chemicals called polyphenols that theoretically protect people against a host of common health problems and possibly some effects of aging.
Polyphenols act as antioxidants, protecting cells and body chemicals against damage caused by free radicals, reactive atoms that contribute to tissue damage in the body, from the general environment and from normal metabolism. For example, when low-density lipoprotein (LDL) cholesterol becomes oxidized, it can become glued to arteries and cause coronary heart disease.
Polyphenols can also block the action of enzymes that cancers need for growth, and they can deactivate substances that promote the growth of cancers. The polyphenol most strongly associated with cancer prevention is epigallocatechin-3-gallate, or EGCG. Both green and black teas and isolated polyphenols from tea have been shown in laboratory studies to act as scavengers of oxygen and nitrogen free radicals, protecting the fatty membranes of cells, proteins and DNA.
For many years, green tea was thought to be a much richer source of polyphenols than either black or red tea, but recent evidence indicates that all teas (except decaffeinated tea, which has less) have about the same amount of these active chemicals, though in different proportions. So, if ''green'' is not your cup of tea, you're likely to get the same benefit (or no benefit) from black tea.
A Role in Health?
Most evidence of the health benefits of green or any other kind of tea comes from laboratory and animal studies, but the results of human studies have been inconsistent and have yet to prove anything one way or another. For example, when animals were fed diets high in fats and cholesterol, polyphenols from green and black tea prevented the expected increase in blood cholesterol and clogging of coronary arteries. But in dogs given green tea extract equivalent to 10 cups of tea a day for four weeks, no significant effects were noted in cardiovascular risk factors.
Only 4 of 13 studies in people have found a cholesterol-lowering effect from drinking green or black tea. In a long-term Dutch study, those who drank the most tea had the lowest risk of death from coronary heart disease and stroke. A follow-up study in Rotterdam linked tea drinking to less severe clogging of the aorta, the body's main artery. Boston researchers noted that people who drank one or more cups of black tea a day had about half the risk of heart attacks as those who did not drink tea. But in all these studies, other unmeasured factors in the diets or habits of tea drinkers may account for the cardiovascular benefits. Drinking tea may be only an indicator of a healthier lifestyle.
In mice, the polyphenol EGCG inhibited the growth of transplanted human breast and prostate cancer cells. And various other animal studies have suggested that drinking tea may help prevent cancers of the skin, lung, esophagus, stomach, liver, small intestine, pancreas, colon, bladder, prostate and breast. In most of these studies, the animals were given tea as their only beverage.
But animals are not people, and as already mentioned, the strong belief that green tea can prevent stomach cancer was not borne out by the Japanese study. And in a large Dutch study, drinking black tea had no effect on people's risk of developing stomach, colorectal, lung or breast cancers.
But some human studies indicate that tea may protect against cancer. In Japan, for example, women who drank more than 10 cups of tea a day (mostly green tea) had a lower risk of developing all cancers. And in patients with breast cancer, tea drinking was associated with a lower rate of metastasis and recurrence.
Studies in northern Italy have suggested a protective effect of tea against cancers of the mouth, throat and larynx. And a study of cancer patients in Shanghai indicated that frequent consumption of green tea reduced the risk of esophageal cancer. In a study of 35,000 older women in Iowa, having at least two cups of tea (mostly black) a day was linked to a lower risk of cancers of the digestive and urinary tracts.
Most reports of a cancer-protective effect of tea come from Asia, where green tea is the main beverage. In European studies, black tea showed a benefit only infrequently, suggesting that the cancer-preventive activity of green tea may be greater than that of black tea.
As for other benefits, a study of women 65 to 76 linked tea to greater bone density, consistent with an earlier finding of a lower risk of hip fractures among tea drinkers. In mice given a disease that mimics human rheumatoid arthritis, green tea polyphenols reduced its incidence and severity. And a human study has suggested that tea consumption may decrease the risk of cataracts.
But whatever protection tea may afford, it is not a free lunch. The American Journal of Clinical Nutrition just reported a study showing that green tea extract significantly impaired the ability of young women to absorb iron from vegetables. That impairment could increase their risk of anemia. And for those sensitive to caffeine, tea, whether black or green, has about half the amount of this stimulant as brewed coffee. But canned and bottled teas have less of the protective polyphenols, if any.
The bottom line? Tea, green or otherwise, is a fine beverage. But don't count on it to protect your health, at least not yet.Continue reading the main story