Many people have begun touting green tea as a way to reduce the risk of cancer. This isn't as strange as it might sound - many foods can affect your risk of cancer, either positively or negatively. It's not particularly well-understood at the moment, but there have been clear groupings of people found who eat particular foods or drinks frequently and have reduced risks of various diseases. For example, red wine in moderation has been tentatively found to lower the risk of certain cancers - this is because red wine contains antioxidants, which are substances that protect cells against damage from certain kinds of molecules that are called "free radicals." Cell damage is one of the potential causes of cancer - it can cause the DNA to go haywire and go out of control. So what about green tea?
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Recent research suggests that green tea does indeed have a positive effect on the risk of cancer. The exact mechanism is not perfectly understood, but there have been a number of studies with generally positive results. There is a good summary of the results of several of them here. Essentially, the link was first suggested by survey research - efforts to identify the rates of cancer among people who traditionally drink green tea in China or among people who drank it for the purpose of the study. The results were fairly positive - a survey of 18,000 people in China found that it reduced stomach cancer by half. There are some contradictory data there, however - a study of consumption of green tea in people who already had prostate cancer didn't show any measurable improvement. And another study using black tea did not find any link. In animals, the results were clearly positive - both green and black tea were found to reduce the size of tumors in mice. That's a little hard to generalize what that means for people, however. It's easy to control the diet of a lab rat - people eat and drink a variety of things and even if green tea has a positive effect it might be hard to achieve it with people because something else about our lifestyles might interfere with it. But the general summary of the testing results is that there doesn't appear to be any negative side effect from drinking it, and there is good reason to think it has a positive effect. It clearly helps at least among mice and people living certain lifestyles in China.
So how does green tea reduce the risk of cancer? There are two suggested ways. One of them is the same as in wine - the antioxidant theory. Green tea contains antioxidants, and it can help prevent cancer in the same way as wine without the negatives of the alcohol. The other has been discovered more recently - green tea has other chemicals that inhibit a molecule called aryl hydrocarbon or AH. The molecule AH can disrupt genetic material, particularly in people who smoke. So green tea might be reducing cancer in both ways.
The Caveats:
There are a few, as always, for you to think about:
1) No food research is certain. People's diets vary so much that it is very hard to be absolutely sure about the benefits of green tea. Additionally, there have been frequently reversals in the science on the benefits or detriments of various foods in the past.
2) Different types of green tea may be different. There are vastly different means of production and growth of different brands. No one can tell you that one particular brand is the "cancer solution," and if someone does you shouldn't believe it. There just haven't been any trials, and it may be that some particular type of green tea is most effective. It also may be a particular method of preparation.
3) There are some negatives to overdoing caffeine, so be sensible about it. There aren't any known negatives other than this, and use of green tea is widespread enough that if there were any people would probably know about it. But if you overuse caffeine, you can cause nervousness, insomnia, or problems with your heart rate.
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