Tea, to China’s 18th century Emperor Chien Lung was more than a whistle-wetting pick-me-up: It was “that precious drink which drives away the five causes of sorrow.”

Western businesses are banking on our buying into Chien Lung’s sentiments. In addition to selling a cornucopia of loose green teas, they have distilled the brew’s essence and added it to health bars, supplements, diet aids, gum, soft drinks and skin creams – even, in Asia, to Kit Kat candy bars.

Green tea is good for us: That mantra has been chanted in the West since the early 1990s, when studies reported that the infusion, sipped for centuries in China and Japan, appeared to help fight off cancers when drunk by lab mice or rubbed on their skin. Enthusiasm intensified after other studies revealed that green tea contained certain chemicals with cancer-fighting clout. Scientists rolled up their sleeves to figure out how it works.

Today, green tea imports are soaring.

“Ten years ago, 3 percent of imported tea was green tea. Now it’s 12 percent,” says Joe Simrany, president of the Tea Association of the U.S.A.

So confident was one doctor-turned-green-tea businessman that in 2004 he decided to petition the Food and Drug Administration to permit green teas to sport cancer-fighting health claims on their packages.

The FDA’s response: tepid. At best.

In June, the agency ruled that there was “no credible evidence” green tea fights cancers of the stomach, lung, colon, esophagus, pancreas or ovary. The agency acknowledged that the evidence for tea fighting breast or prostate cancer was somewhat better, although it also said the link was “highly unlikely” because the evidence on humans wasn’t conclusive enough.

Scientists say that despite the unanswered questions green tea still shows promise, not only as a potential cancer-protector but also against other health threats, such as cardiovascular disease and possibly Alzheimer’s. But they also are mindful that many a cell in a dish has been vanquished, and many a mouse cured of cancer, from therapies that don’t ultimately pan out in humans.

“You can build your case in cell studies and animal studies but ultimately you have to do it in humans or you can’t make a case that it works,” says Balz Frei, a professor of biochemistry at Oregon State University.

How it’s made

Green tea is made from the dried leaves of Camellia sinensis, an evergreen shrub native to Asia.

The plant is harvested and treated in different ways to produce green tea or black tea.

Green tea is made by steaming the crushed leaves shortly after harvest, destroying enzymes so that chemicals aren’t oxidized very much.

Leaves used for black tea ferment for days before they’re heated, causing the leaves to blacken and many chemical changes to within them.

Those processing differences may be medicinally important. Both types of tea are abundant in certain antioxidant chemicals called flavonoids, which obstruct the action of cell-damaging free radicals. Green tea, because it doesn’t ferment, has much higher levels of a group of flavonoids called catechins. A potent catechin, epigallocatechin gallate, or EGCG, is three to four times more abundant in green tea than black.

Cancer fighter?

Scientists cite three lines of green tea anti-cancer evidence.

First, there are test-tube studies. Green tea’s flavonoids interfere with cancer-related biochemical reactions: They may cause cancer cells to grow sluggishly, cease dividing or even self-destruct. Flavonoids also impede formation of carcinogens known as heterocyclic amines.

Then there are studies in rodents. In one fairly typical study, mice were injected with a tobacco carcinogen that caused them to develop lung tumors. Some of the mice drank green tea, and others did not. The tea-drinking mice got fewer tumors.

Similar studies have linked green tea to protecting against a range of cancers – such as those of the lung, skin, esophagus, colon, bladder and possibly the mammary glands.

EGCG isn’t the only thing having an effect. Caffeine is probably providing the lion’s share of protection in the case of the skin cancer experiments, and plays a big part in the lung ones, says green tea researcher Chung S. Yang, who chairs the chemical biology department at Rutgers University in New Jersey.

The third line of evidence is the one people care most about: What happens to humans when they drink tea? Such studies, because they’re usually not done in controlled groups, are tricky to interpret, partly because it’s hard to measure how much tea people drink, and partly because tea drinkers do a lot of other things. For example, it’s common in China that men who drink a lot of tea also smoke, Yang says.

Good prospects

The few human studies that have been done have produced mixed results. But despite the complexities, some studies do look good, scientists say.

For example, in an article published in the International Journal of Cancer in 2003, scientists looked at the eating and drinking habits of more than 1,000 Chinese, Japanese- and Filipino-American women in Los Angeles. They reported that women who drank green tea had a 43 percent lower risk of getting breast cancer compared with women who drank no tea. The more green tea the women drank, the lower their risk of breast cancer, according to the study.

Studies like these were enough to persuade Dr. Sin Hang Lee, a doctor in Connecticut, to start Dr. Lee’s TeaForHealth, a company that sells organic green tea, and to petition the government in January 2004. Lee asked that the FDA allow green-tea producers to label their products with a claim – known as a “qualified health claim” – stating that drinking 40 ounces a day of green tea containing specific amounts of EGCG may reduce the risk of certain forms of cancer. Lee’s FDA petition included stacks of green-tea studies.

When the FDA rejected Lee’s petition, it denied outright any claim for some cancers and awarded limp qualified health claims for breast and prostate cancer.

Here, for breast cancer, is the most that the government would grant: “Two studies do not show that drinking green tea reduces the risk of breast cancer in women, but one weaker, more limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of breast cancer.”

The FDA isn’t alone in its skepticism. The American Cancer Society also concluded that more research is needed to show that green tea helps prevent cancer, and many other scientists concur.