Scientists Uncover New Cancer-Fighting Benefits in Tea

American Institute for Cancer Research
Tuesday, 7 December 1999

Recent research sponsored by the American Institute for Cancer Research (AICR) has revealed several new ways that both black and green teas can help to inhibit cancer. Such discoveries place tea on a short list of substances with the demonstrated ability to fight cancer at every stage of its development: initiation, promotion, progression and metastasis.

Tea has long been recognized as a potent antioxidant capable of neutralizing "free radicals." In laboratory experiments, cells exposed to tea were able to keep unstable oxygen molecules from causing the kind of genetic damage that begins the cancer process.

But Dr. Roderick Dashwood of Oregon State University has uncovered new evidence that antioxidation alone is not sufficient to explain tea's remarkable cancer-fighting potential.

"We studied tea's effectiveness against cancers initiated by heterocyclic amines (HCAs), which are carcinogens produced when meat is charred," Dr. Dashwood said. "Rats given doses of HCAs along with regular-strength black tea showed resistance to the kinds of cellular changes that signal the beginnings of cancer."

In Dr. Dashwood's study, some rat subjects drank tea prior to being exposed to HCAs, while others drank tea during HCA exposure. Both groups showed increased resistance to cancer.

"This indicates that more than one anti-cancer mechanism is at work," Dr. Dashwood said. Specifically, these results suggest that compounds in tea afford protection at both the initiation and promotion phases of cancer progression.

During initiation, the first stage of the cancer process, exposure to carcinogens gives rise to genetic damage. At this stage, a potent antioxidant like tea may prevent or inhibit cancer formation.

Once a cell has been initiated, certain changes in the cellular environment can encourage the formation of tumors - this stage is called promotion. Dr. Dashwood's study was among the first to demonstrate tea's protective activity against this stage of cancer formation.

"Other recent studies have identified ways that tea components can inhibit progression and metastasis, the final cancer stages," said Dr. Dashwood. During these phases, cancer cells form an invasive mass and spread to other sites in the body.

Certain naturally occurring substances in tea are thought to be behind the drink's cancer-fighting potential. Both black and green tea contains a class of substances called flavonoids, which have been isolated and tested for their anti-cancer potential.

Dr. Zigang Dong of the Hormel Institute has found evidence that tea flavonoids inhibit substances called cell-growth factors, which transform normal cells into cancerous ones. "In other experiments," Dr. Dong said, "flavonoids kept foreign substances from attaching to genetic material, and suppressed the action of enzymes integral to the cancer process."

Human Trials Define the Promise of Tea

In light of these findings, the real-world potential for tea as a cancer-fighting tool has become a subject of inquiry worldwide. Experts are investigating whether the anti-cancer activity exhibited by tea in animal studies is applicable to humans.

"There is marked difference between the amount of tea that produces anti-cancer activity in cell cultures and the amount needed for the same activity to show up in test animals," said Dr. Chung S. Yang of the Laboratory for Cancer Research at Rutgers University. Further tests need to be done, he said, to determine if tea can play a preventive role at levels typically found in the human diet.

Experts agree that human trials will be necessary to sort through some of the questions and complications that arise in laboratory research. Dr. Dashwood noted that future experiments must take many factors into account. "Both green and black teas show anti-cancer activity," he said, "but they have slightly different components, and are processed and prepared using different methods. In order for us to make practical recommendations about drinking tea for cancer prevention, we will have to examine all possible variables very carefully. This will take some time."

Dr. Dashwood said that green teas, which are less processed, contain more of one type of flavonoid. Black teas, however, contain two additional varieties of flavonoids.

"In rats, its seems that green tea is more effective in fighting cancer than black tea, if both teas are prepared in the same way. For humans, the issue is more complicated. We have a very different physiology, and different groups of us use very different methods to make our tea."

Typically, Dr. Dashwood said, black tea is brewed for three to five minutes in boiling water. Green tea, on the other hand, is generally brewed for only two minutes in water that is heated but not boiled.

"There are other differences," Dr. Dashwood continued. "Russians drink their tea thick, with sugar. Americans drink theirs thin, with ice cubes. The British drink lots of tea with milk and sugar, and Indians often brew their tea with milk rather than water."

Results to date have left experts with some specific questions. "Does using boiling water interfere with tea's cancer-fighting potential?" asked Dr. Dashwood. "If black tea were to be brewed longer, would more anti-cancer compounds be released? How does adding things like sugar or honey affect the anti-cancer effects observed in a laboratory setting?"

Dr. Dashwood again stressed the need for human trials. "There is some new research suggesting that adding milk, for instance, interferes with tea's cancer-inhibiting actions," he said, "but we don't have enough information to say for certain.

"Of course," he continued, "a larger question needs answering: How much tea do we have to drink to see these protective benefits? How many cups a day? I'm confident the next round of human studies should shed more light on this issue."

This question continues to elicit debate in the scientific community. Some experts argue that when it comes to tea consumption, the world should follow Japan's lead. There, the per capita consumption of green tea averages 3-4 cups each day, and Japan traditionally shows lower rates for many cancers. Other evidence indicates that as little as one cup a week of black or green tea will provide some measure of protection.

"For now," said Dr. Dashwood, "I think most researchers would agree that making tea a daily part of a healthy diet - one that includes a variety of fruits, vegetables, grains and beans - is a sensible idea."

A 1997 AICR report entitled Food, Nutrition and the Prevention of Cancer: A Global Perspective examined over 4,500 international studies on the link between diet and cancer. The report's expert panel recommended a predominantly plant-based diet, regular exercise, maintenance of a healthy body weight and the avoidance of tobacco for everyday cancer prevention. They estimated that following these guidelines could lower worldwide cancer rates by as much as 70 percent.

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org