The list of foods and beverages that supposedly prevent cancer is a long one and is constantly being updated. Everything from apples and mushrooms to red wine and the seeds of mandarin oranges has been cited as preventing the disease.

Green tea was added to this list more than two decades ago. In the 1980s, scientists and physicians began to focus their research on green tea's high concentration of catechins, which are also known as tea tannins.

"The large amount of epigallocatechin gallete, a type of catechin in green tea, is said to have an antioxidant effect," says Hirohiko Matsumoto, a surgeon specializing in digestive illnesses and a pioneer of research on the relation between cancer prevention and green tea. "This means it can prevent DNA mutating or breaking down."

According to Isaac Berenblum, an Israeli doctor specializing in cancer research, the development of the disease involves a multistage process: initiation, promotion and progression. Initiation is the primary stage, when a carcinogenic substance damages the genes and causes mutation. Promotion is when those damaged genes begin to produce cancerous cells. In the final stage, progression, the cancerous cell or cells begin to divide and increase.

"In order to prevent cancer, it is crucial that we stop the spread at any of these stages," says Matsumoto. "And [green tea's] antioxidant effects, I believe, can prevent such illnesses as cancer."

Saitama Cancer Center is conducting an ongoing epidemiological survey begun in 1986 on 8,552 Saitama Prefecture residents. According to the most recent data, the occurrence of cancer among those people in the survey who consume more than 10 cups of green tea per day is nearly half that of people who drink fewer than three cups.

"We are making progress in demonstrating the effect of catechins on cancer, but we have no concrete proof yet," says Matsumoto. "This is because cancer is a disease that takes years to spread. This research has just started so it will be at least 10 years, most likely more, before we have an answer."

The apparent benefits of catechins aren't limited to cancer prevention. They have also been found to prevent other types of age-related illnesses, including arteriosclerosis (hardening of arteries), as well as cerebral and myocardial infarction, which is caused by blood clots. The antibacterial actions of catechins prevent all kinds of bacteria, from cavities to colon bacilli.

Although comprehensive research on green tea's medicinal effects is still in its early stages, the restorative powers of green tea have long been known in Japan.

In the 13th-century book "Kissa Yojoki," Zen priest Eisai recorded the physiological effects of green tea. He wrote that it could cure hangovers, shake off drowsiness and revive the spirit.

"These are all the characteristics of tea caffeine, the existence of which was discovered in 1827," says Kimio Sugiyama, a professor of agriculture at Shizuoka University. "Out of nearly 300,000 plants, there are very few that contain caffeine, which makes green tea, along with coffee and cocoa, a very important beverage."

But since Asian teas -- oolong, black and green tea -- are derived from the same plant, Camellia sinensis, why is green tea getting the most attention?

"The difference is not in the ingredients, but in the production process," says Sugiyama. Unlike black tea, for which leaves are fermented, or oolong, made from semi-fermented leaves, no fermentation is used in the making of green tea. Instead, the leaves are steamed to halt fermentation.

"Preventing oxidation is the key to the medicinal effects of green tea," says Sugiyama. "Fermentation leads to a drastic decrease in easily oxidized substances, such as catechin and vitamin C, which is why oolong and black tea lack these."

Along with catechins, caffeine and vitamin C, green tea contains two lesser-known substances: saponin and theanine.

The main characteristic of saponin, found in many plants, is its behavior as a surfactant. In the old days, people used plants that contained a lot of saponin as soap.

Saponin, along with catechin, is the source of green tea's bitter taste. Both also have antibacterial effects.

The amount of saponin in tea leaves is minuscule (an estimated 0.2 percent), but researchers are studying how it assists the function of catechins.

Theanine, on the other hand, greatly affects a tea's flavor, as amino acids in it contribute greatly to the taste. It is said that a tea's quality corresponds to its theanine content. Furthermore, theanine reportedly has a relaxing effect on the brain and the nervous system.

With all of these substances affecting our bodies, it is no wonder that bottled green tea has become a big hit, available at vending machines and convenience stores everywhere. There is catch, though.

As Matsumoto points out, one problem with these kinds of bottled teas is that, "In order to make them tasty, [tea companies] have removed the bitterness. And yes, bitterness is caused by catechin, the key source of green tea's medicinal effect."