I got a notification in my email today about a newly published study showing that people who drink green tea had a lower risk of developing coronary artery disease (CAD). This was contrasted to black tea, which did not reduce the risk of CAD. The study was a meta-analysis, which is a type of super-analysis where the results of several trials were pooled. This methodology generates larger numbers of patients, giving it more power to detect effects, but since each trial uses different criteria to enroll patients, the method can make the results a meaningless jumble of mixed messages.
It is important to remember that nearly all studies of food consumption and risk of diseases have many significant limitations. They are almost never randomized trials, but observational. They are limited by what people remember. Do you remember the last time you drank green tea? Unless you drink it every day, you might have a hard time remembering. Let's say you do drink green tea every day, do you drink the same amount every day? Sometimes you have one cup, sometimes four. Now, tell me the average number that you drink every day over the last five years. After all, you will almost certainly see no significant benefit from a single cup of tea. Observational studies are also limited by the fact that there could be differences between tea drinkers and non-drinkers that you cannot control for. Think about tea drinkers you know, are they healthier than other people in other ways? Do they tend to be vegetarian? Are they nonsmokers? Do they exercise more than other people? You can imagine how this could get confusing.
Despite the limitations of observational studies and meta-analyses, green tea does seem to have some beneficial effects. After checking out the first study, I clicked on the top four related results, all of which were analyses of green tea for other diseases, like breast cancer, (green tea beneficial) lung cancer, (green tea beneficial, black not) stomach cancer, (mixed results) and stroke (green and black beneficial). Furthermore, there is some plausible science behind the idea that green tea could be healthy. Tea does contain antioxidants, catechins, and myriad organic compounds of unknown effect on humans. The difference between this and quackery, though, is that someone has taken the time to review the literature and find all the pertinent studies on green tea for several diseases and analyze them to find the effect. It is not enough to say A) green tea has antioxidants and B) it is therefore good for you. Some effort needs to be made in actually measuring effects to see if they are repeatedly observed.
Arguably, the organization that is the best at doing these sorts of analyses is the Cochrane Collaboration, and the PubMed search I mentioned before also included the Cochrane review of the effect of green tea on cancer. This review looked at 51 studies with 1.6 million patients and found that there is "insufficient and conflicting" evidence on the subject (yes, even with 1.6 million patients, we do not know for sure). Further, the vast majority of studies came from one area of the world (Asia), which does not mean that the studies are biased, but if green tea really works, it should work regardless of where it is studied. They do also conclude, however, that green tea does not appear to be harmful and therefore if you desire, moderate habitual use, drinking 3 to 5 cups a day would be reasonable.