1. Prostate cancer affects 1 in every 7 men in the US throughout life time (http://seer.cancer.gov/statfacts/html/prost.html). It's found in about 1 in every 3 men over 70 years old, and 1 in every 2 men over 80 years old. But don't be too worried, men! Most prostate cancer cases don't kill at all. Be aware, but don't get over-worried.
2. Association between tea drinking and prostate cancer has been studied a lot. I didn't realize this until recently when I saw a meta-analysis paper scrutinizing more than 2 dozen peer-reviewed articles in this topic. This means, even though we may not yet know everything about this topic, we do have some evidence-based opinions to look into.
Now as we go over some scientific studies on tea drinking and prostate cancer, I would like to recommend several rules in exploring health benefit of tea (or of anything).
Rule #1, Don't trust a striking headline on news media - many of them are (intentionally) wrong.
Headlines are made to grab public attention, not to give a correct message. A simple assertion on a health problem is almost always wrong. Sometimes reading the entire news article is better than reading just the headline. But sometimes, the news article isn't much better either.
An example is:
Seven cups of tea a day 'raises risk of prostate cancer by 50%' (Daily Mail 18 June 2012)
Another example is:
Two teas a day reduces prostate cancer risk 'by a third', but coffee offers no benefit (Daily Mail 26 March 2013)
Another example is:
Drinking more than five cups of tea 'cuts risk of prostate cancer by a third' (Daily Mail 26 July 2013)
I used all examples from Daily Mail because Brits do seem to care tea a lot more than Americans. American media have a lot of bad examples on reporting other health issues.
I don't know how many people, after reading these news report, would decided that, for the best of their prostate health, they should drink about 5 cups of two types of teas every day but never go for as much as 7 cups. Well, I'm sure you tea drinkers are smarter than this!
News media contribute to the public awareness of the newest scientific research. Once we are aware of the research, to find more accurate information about it, the best source is the original source.
Rule #2, To get the most accurate information about a research, go to its original publication (or abstract of it).
Just 10-15 years ago, it was not that easy to find the original publication that you are interested in, unless you are close to a large academic library. But today, it's very easy and information is usually just a few clicks away. A very useful search engine is Google Scholar (scholar.google.com). Once you learn of a research through news media, you could use keywords found in the news (such as names of the authors, name of the research project and institution, title of the study, etc.) to search on Google Scholar for the abstract of the original publication. For medical and public health studies, another very useful tool is PubMed (pubmed.gov), where you could also do a keywords-based search.
Today, results of vast majority of new studies are immediately available online - not all in full text, but at least their abstract with main results. It may take some specialized expertise to thoroughly understand a scientific article, but for the general public, just by reading an abstract, they could get a good idea about what was done in a study, and this first-hand information is always more accurate then what one could find from news stories.
Rule #3, Randomized control trial (RCT) is an important form of clinical study that makes fair and well-controlled comparisons. Other clinical studies could provide insightful information too, depending on how well the "noise" is adjusted in the comparison.
The key feature of a randomized control trial is that the two (or more) treatment groups are randomly assigned and are intended to be balanced in all their features, so that they are comparable, like "comparing apples to apples", and unlike "comparing apples to oranges."
An example of RCT is:
Randomized clinical trial of brewed green and black tea in men with prostate cancer prior to prostatectomy.(http://www.ncbi.nlm.nih.gov/pubmed/?term=25545744)
We can imagine that long-term RCT with tea drinking vs. no tea drinking is very hard to carry out. For such a trial, basically, you would need to randomize people in tea drinking group and no-tea group, and make sure they stick to the assigned habit for years. It's both extremely hard and somewhat unethical to keep participants adhere to their assigned life style. The above-mentioned example is one of the few RCT in tea drinking studies. It was made possible because it was a short-term study. Therefore, this study gave us some very inspiring information about impact of green tea on prostate tissues, but it cannot provide sufficient information on how long-term tea drinking influence prostate health.
Most studies on influence of tea drinking on prostate health must be non-RCT studies. In these studies, the tea drinking group and non-drinking groups were not randomly assigned, and inevitably there were imbalance in many other aspects of their life styles and health conditions. Usually the researchers would do their best to identify the potential imbalance factors and adjust them in the analyses, so to make the comparison as fair as possible. However, it's impossible to identify and adjust all imbalance factors. Therefore, each of such studies will add to the scientific understanding of the topic, while each bears its own bias. Health problems are often complicated. Therefore, to understand a problem, we will need to look into results of many relevant studies.
Rule #4, Do not draw a final conclusion based on just one or few studies, especially when they are not RCT.
Most of us don't spend all our days reading and studying prostate cancers. Where could we get results of many relevant studies? Systematic reviews and meta-analysis articles are good sources for us to get summarized results of many studies at one time. To find these articles, you could include keywords "systematic review" and/or "meta-analysis" in your search on Google Scholar or PubMed.
Here is an example of a meta-analysis:
The association of tea consumption and the risk and progression of prostate cancer: a meta-analysis.(http://www.ncbi.nlm.nih.gov/pubmed/?term=25550896)
On the top-right corner of this webpage for abstract, you can also find a link to the full-text of the article.
A great feature of a meta-analysis is that an exhaustive search on available studies is required, which means it must include all available studies that pass its inclusion criteria. Thus it can give us a broad enough view about what has been done in this field and what is known up to the time when the article was written.
Happy reading! And lastly,
Rule #5, Critical thinking and an open mind are the key!