With each passing day the saying ?not for all the tea in China? takes on a larger and health-negative meaning as researchers continue to discover tea?s wellness benefits worth, well, all the tea in China. Specifically, green tea seems to be the shining star of the Camellia sinensis family, with those unfermented leaves looking great on numerous fronts.
In a July 2001 article in The Journal of Nutritional Biochemistry, researchers at Agriculture and Agri-food Canada?s Food Research and Development Centre reviewed the latest research findings at that time on ?the health promotion properties of tea.? They found that tea is the best dietary source of the antioxidant catechin; animal studies indicate that tea inhibits many types of cancer, including those of the skin, lung and digestive tract; tea components appear to inhibit cardiovascular disease; and tea has potential beneficial effects on renal function, diabetes, skin and eye tissue, arthritis and dental caries (decay).
?Green tea is one of the most important antioxidant products available, and it?s been around, of course, for thousands of years. Now people in the West are beginning to recognize the benefits that the people in the East have been enjoying,? says Jonathan Selzer, Ph.D., technical director of the Wallingford, Conn.-based green tea products company HerbaSway Laboratories.
Following are some of those benefits of green tea—at least science?s understanding of them. Of interest is the difference between green tea in supplement form versus as a beverage. Anthony Almada, chief science officer at IMAGINutrition in Laguna Niguel, Calif., says it?s important to remember that much of the scientific work done thus far on green tea supplements has been in animals or in vitro. Also, studies on drinking green tea have shown more benefit than in extract or supplement form. Almada—who drinks nearly half a gallon of oolong, green and black teas every day—says, ?The initial impetus [for switching to green tea] was that all the studies among green tea drinkers—when they drink it for decades—almost without exception suggest or indicate a positive or protective effect.?
The International Journal of Cancer published a study in 2003 that found a significant reduction in breast cancer among Asian-American women consuming green tea in beverage form.
In April, a study presented at the annual meeting of the American Association for Cancer Research found that those taking a green tea extract for one year had a significantly lower risk for prostate cancer. One out of 32 men at high risk for prostate cancer developed the disease compared with nine out of 30 in the control group. The GTE was caffeine-free. However, a study published in the March-April issue of Urologic Oncology showed a minimal effect against prostate cancer that doesn?t respond to hormone therapy.
Clinical Cancer Research published a study in February that found that a GTE interrupted a cellular process that allows bladder cancer to spread. This in vitro study conducted at the University of California at Los Angeles ?provides further support for the use of GTE as a chemopreventive agent,? the researchers wrote.
In 2003, researchers discovered that a GTE inhibited a gene that tobacco uses to cause cancer. The study, published in Chemical Research in Toxicology, found that two substances in green tea, epigallocatechin gallate and its cousin, epigallocatechin, worked to stop the harmful effects. Recently, those same researchers discovered that GTE also blocks a protein that drug companies are studying for its links to cancer.
Researchers writing in February in the journal Diabetes found mixed results regarding the link between green tea and diabetes. The study, done on rats for one year, found that GTE improved several ?diabetes-related cellular dysfunctions,? but worsened others. Results were compared with rats treated with water or antioxidants.
The Journal of Agricultural and Food Chemistry published a study in April by researchers at the University of Scranton in Pennsylvania that showed that both green and black teas ?significantly inhibited diabetic cataracts? in rats. They also found that green tea significantly lowered triglycerides.
In 2004, researchers at Toyama Medical and Pharmaceutical University in Toyama, Japan, found that mice given a GTE had improved glucose metabolism. In the study published in BMC Clinical Pharmacology, an online journal, the researchers concluded that green tea has an antidiabetic effect.
Endurance and weight loss
A study published in November 2004 in the American Journal of Physiology?Regulatory, Integrative and Comparative Physiology found that a GTE was beneficial in exercise tests using mice. Researchers at Kao Corp.?s Biological Science Laboratories in Tochigi, Japan, found that swimming times to exhaustion for mice fed the extract were prolonged by 8 percent to 24 percent. They concluded, ?These results indicate that GTEs are beneficial for improving endurance capacity.? Researchers also found a benefit from the green tea component EGCG.
In 1999, The American Journal of Clinical Nutrition published a human study concluding that a combination of EGCG and caffeine ?has thermogenic properties and promotes fat oxidation? and ?green tea extract may play a role in the control of body composition.? The study, conducted in Europe, was done on 10 healthy men, and treatment with only caffeine found no effect on energy expenditure. Almada warns that the extract used in this study was later pulled from the market due to liver toxicity concerns.
In March 2004, the British Journal of Nutrition published a study that found no weight loss benefits for green tea. Researchers investigated whether a GTE could improve weight maintenance after weight loss, and 104 participants received either green tea supplements or placebo for 13 weeks. The researchers concluded, ?Green tea did not improve weight maintenance. ? The stronger weight maintenance with green tea in the low-caffeine consumers compared with the high-caffeine consumers indicates that the magnitude of habitual caffeine intake may affect the effectiveness of green tea administration.?
A study published in the September 2004 issue of The American Journal of Clinical Nutrition found that a GTE of EGCG reduced liver damage in mice. Liver injury was induced in the mice, and two different doses of EGCG were tested against placebo. A dose-dependent reduction in damage was observed. Researchers concluded, ?Green tea polyphenols can be a useful supplement in the treatment of liver disease and should be considered for liver conditions in which proinflammatory and oxidant stress responses are dominant.?
Previously, researchers at the University of North Carolina at Chapel Hill had found that GTE protects against early alcohol-induced liver injury in rats. The study, published in 2002 in the journal Biological Chemistry, revealed that rats given a GTE had a significantly lower increase in markers for liver disease and reduced necrosis of liver tissue. Researchers concluded that GTE most likely prevented the liver injury by reducing oxidative stress.
A study published in January in Clinical Biochemistry found a large reduction in oxidized LDL (?bad?) cholesterol, as well as reductions in P-selectin, a marker for atherosclerosis. However, the study was done strictly with adult male smokers. Also, the study used green tea as a beverage.
A meta-analysis of tea?s affects on cardiovascular disease was done in 2001 in the American Journal of Epidemiology. Researchers concluded that there was ?a decrease in the rate of CVD outcomes with increasing tea consumption.? However, the study looked at all forms of tea, and there was a strong association between geographic location and reduction in CVD risk. Continental Europe had the lowest risk.
HerbaSway?s Selzer sums up the future of green tea research. ?The science is phenomenal. I just went on the Web site Medline to do a quick search of how many articles have been written on the health benefits of green tea since January this year, and already it was 116 scientific research papers. Each one from a different laboratory, and each has a team of scientists. So really there are hundreds and hundreds of people working on green tea and its health benefits.?
Natural Foods Merchandiser volume XXVI/number 7/p. 34, 38