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Even though the United States seems caught in a coffee craze, tea drinkers are having their day. What’s more, they’re getting some potent health benefits in addition to the simple pleasure of sipping away at a steaming hot cup of tea or savoring a frosted glass of iced tea on a summer’s day.
Tea consumption in the United States has doubled in the past decade, keeping pace with scientific literature that suggests cardioprotective and anticarcinogenic benefits of tea drinking, including evidence from an unpublished Boston University study that suggests tea drinking has a powerful effect on blood vessels.
In results presented at the American Heart Association annual meeting in New Orleans last November, Boston University researchers reported that drinking two cups of black tea dilates the arteries and improves blood flow in heart patients within two hours.
Lead researcher Joseph Vita, MD, professor of medicine at Boston University, gave four cups of ordinary black tea, similar to what one would buy in the supermarket, per day for four weeks to 50 subjects with known heart disease. The study was controlled for the effects of caffeine and there was a washout period of water drinking for an equal period of time.
Using ultrasound to measure blood flow in the forearm, investigators found that the tea helped blood vessels function more effectively. Blood vessels in people without heart disease normally dilate 11%, but patients with heart disease only dilate 6%, Vita says. Tea drinking restored blood vessel response to near-normal levels after the tea was consumed.
"The effects were visible as soon as two hours after drinking the first cup, and those effects persisted after the four weeks," Vita says.
The effects are not due to the caffeine in tea, Vita says. Some of the participants were given caffeine tablets equal to two cups of tea and then repeated the artery response test with no effect on blood vessel function.
Clearly, the various antioxidant effects in tea are responsible for the response in blood vessels, says Vita, but it will take further research to determine which of the multitude of flavonoids, polyphenols, and catechins are to be credited. In both green and black tea, potent antioxidant polyphenols account for 36% of the dry weight.
Antioxidants neutralize the effect of oxygen-free radicals, which are known to be a major factor in aging in chronic diseases, including cancer, cardiovascular disease, and diabetes. Free radicals can cause DNA injuries, resulting in DNA mutations and possible cell proclivity toward malignancy. Free radicals are a by-product of normal metabolism and exposure to cigarette smoke, sun, and environmental pollutants.
The antioxidants in just one cup of tea, according to a Chinese study, can provide the same potential for improving antioxidant status as 150 mg of pure vitamin C.1 Research has shown that antioxidants prevent the oxidation of LDL cholesterol, thereby preventing the formation of atherosclerotic plaque in blood vessels.2
Vita’s research takes this theory a step further. "We know that abnormalities in the endothelium or cell lining are associated with heart disease, and it has been shown these endothelial abnormalities can be reversed by the use of antioxidants," he says. By aiding the blood vessels to dilate, increased blood flow also improves the ability of the blood vessels to release vasodilating nitric oxide. "That may explain why tea drinkers do better," Vita adds.
Vita cautions against a blanket recommendation that tea will protect against heart disease. "It’s way too early to say that yet, but there is no known harm from drinking tea in the amounts we used," he says.
While there is little literature on cardiovascular effects of tea drinking, there is substantial evidence of the cancer protection effects of tea drinking.
Animal and other preclinical laboratory studies suggest that both green and black tea decrease the risk of several cancers. Studies regarding tea’s effect on lung cancer in humans are inconclusive, but animal studies suggest tea may inhibit tobacco-induced cancers.3
The Iowa Women’s Health Study, which looked at 35,369 postmenopausal women, showed those who drank two or more cups of black tea per day had a decreased risk of digestive and urinary tract cancers. However, other cancers were not affected, including melanoma and cancers of the pancreas, lung, breast, uterus, and ovary.4
Another study indicated that men who drank one to six cups of tea daily had a decreased risk of developing all types of cancer. However, heavy tea drinkers (more than six cups per day) increased their overall risk of getting cancer.5
And a recent report from Japan found that green tea also might enhance the effectiveness of chemotherapy.6
The increase in tea sales in the United States is a sign of increasing awareness of the health benefits of tea drinking, says Susan Kundrat, MS, RD, outreach director of the University of Illinois Functional Foods for Health Program in Urbana. "We recommend drinking three to four cups of tea a day and that people look at ways they can fit functional foods like tea into their daily lives on a consistent basis," Kundrat says.
Many of the antioxidants found in tea also are found in fruits and vegetables, and Vita and Kundrat both note that tea comes from leaves, so it could nominally be considered a vegetable.
The cogent point they make is that the antioxidants in tea make it an important nutrient and "it may be easier to ingest that some other foods," says Kundrat.
1. Benzie I, Szeto Y. Total antioxidant capacity of teas by the ferric reducing/antioxidant power assay. J Agric Food Chem 1999; 47:633-636.
2. Yoshida H, Ishikawa T, Hosoai H. Inhibitory effect of tea flavonoids on the ability of cells to oxidize low density lipoprotein. Biochem Pharmacol 1999; 58:1,695-1,703.
3. Chung F, Wang M, Rivenson A, et al. Inhibition of lung carcinogenesis by black tea in Fischer rats treated with tobacco-specific carcinogen: Caffeine as an important constituent. Cancer Res 1998; 58:4,096-5,101.
4. Anonymous. Tea drinking and cancer in women. Nutrition Research Newsletter 1996; 15:104.
5. Anonymous. Tea and cancer. Nutrition Research Newsletter 1997; 16:19-20.
6. Sadzuka Y, Sugiyama T, Hirota S. Modulation of cancer chemotherapy by green tea. Clin Cancer Res 1998; 4:153-156.