Oolong tea, being partially fermented, has more catechins than black tea, but less than green tea, despite it being considered a type of green tea.

There are over 200 biologically active constituents in green tea, including 6 types of catechins. Those catechins are Epigallocatechin gallate (EGCG), EGC, EC, GC, and C. EGCG is the most powerful, although its' effects are sharpened by the other molecules in green tea. And scientists have found that these catechins can inhibit the growth of cancer cells, without damaging normal cells.

There have not been a lot of studies on green tea and cancer done with people, so the role of green tea in preventing and/or treating cancer is definitely in its early stages, though it does look promising. This is particularly true for some types of cancer. An epidemiological study conducted in China, and published in the Journal of the National Cancer Institute (1994), found that those who drank green tea had their risk of developing esophageal cancer cut by as much as 60%.

Another study conducted in China, at the Beijing Dental Hospital, looked at people who had a type of pre-cancerous plaque called leukoplakia. It found that those who drank about 2 cups of green tea a day, in conjunction with the use of a topical tea extract, were able to reduce the size of leukoplakia, as well as its' spread.

It does not appear to benefit prostate cancer patients, however. According to an unpublished trial conducted by NCI researchers, only 1 person out of 42 experienced any benefits from drinking 4 cups a day for nearly 4 months, and that was only a temporary improvement.

However, drinking at least one cup of green tea a day seems to reduce the risk of stomach cancer, in some studies.

Green tea and cancer studies relating to colon, esophageal, stomach, and lung cancer have on the whole been inconclusive. This means that there have been conflicting results in the various studies. Some studies showed beneficial effects, others were neutral or showed negative effects.

These conflicts may well be explained by other factors in the studies, as some of them have been population based, which can bring in a host of other factors. And sometimes it is difficult to compare studies. In some, black tea is used in addition to, or instead of, green tea. And the two have different health benefits and chemical makeup. So, scientists are further investigating these possible links before drawing any official conclusions.

Green tea and cancer studies relating to breast cancer are also inconclusive. However, it's worth mentioning a couple of things. One study found that, overall, drinking tea of any type did not reduce the risk of breast cancer. This was a very large study. Yet when the researchers looked at a subset, that of women under the age of 50, they found that those women who drank 3 or more cups of tea a day had a 37% reduced risk of developing breast cancer, compared with women who didn't drink tea.

The study of green tea and cancer may be in its' infancy. And it may be that for some types of cancer that it isn't beneficial. But it certainly shows promise with others. And even though there is no hard data to back that up yet, in most cases it won't hurt to add green tea to your daily regimen.

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References:

1. http://www.web-us.com/l-theanine_anxiety_reducer.htm

2. http://www.cancer.gov/cancertopics/factsheet/prevention/tea#1

3. http://www.sciencedaily.com/releases/2003/08/030805072109.htm

4. http://www.bccancer.bc.ca/PPI/UnconventionalTherapies/GreenTea.htm

5. http://www.ncbi.nlm.nih.gov/pubmed/15946418?dopt=Abstract

6. greentealibrary.com/Green%20Tea-Cholesterol%20Studies.htm

7. http://www.umm.edu/altmed/articles/green-tea-000255.htm

8. http://www.sciencedaily.com/releases/2008/03/080330200640.htm

9. http://www.sciencedaily.com/releases/2007/01/070116132946.htm

10. http://www.sciencedaily.com/releases/2007/01/070116132946.htm

11. http://www.sciencedaily.com/releases/1999/06/990623062746.htm