February 19, 2014 — Lorelei Mucci, associate professor of epidemiology at Harvard School of Public Health (HSPH), studies the factors that influence prostate cancer risk and progression. She answers three questions about recent research findings in the field that have generated confusion for men trying to make the right choice for their health.
Q: Two recent studies from the same research group found that high consumption of fatty acids from fish increased men’s risk of developing a high grade prostate cancer. But previous studies have shown that diets rich in fish lower men’s risk of developing advanced or lethal prostate cancer, as well as protecting against cardiovascular disease. Should men cut back on fish?
A: Prostate cancer is challenging to study because so many men get diagnosed—one in six men in the U.S. will be diagnosed in their lifetime. But many of those men have a very slow-growing cancer that was only diagnosed because they were screened with the PSA [prostate-specific antigen] test. The proportion of those cancers that will go on to be fatal is very small.
The results of epidemiologic studies on risk factors for prostate cancer can be influenced by whether the investigators considered how much PSA screening was happening in their study population. Men who participate in healthy behaviors such as eating fish are also the ones who are more likely to get screened. And if you get screened, we know that you are more likely to be diagnosed with these slow-growing cancers. So if the investigator has not carefully accounted for rates of screening, that can lead to a biased result. Moreover, of the men diagnosed with prostate cancer in these two studies, less than one percent had advanced stage cancer, and there are limitations using high-grade cancer as a surrogate of lethal prostate cancer.
Our group undertook a meta-analysis of studies that looked at whether fatty acids from fish might lower the risk of advanced or lethal prostate cancer. We found that when screening rates were considered, the results showed that men who ate the most fish had a lower rate of developing prostate cancer that would ultimately be fatal.
There are so many known health benefits of fish, particularly when it’s eaten in lieu of red meat. I feel confident in saying that eating fish will not increase the risk of lethal prostate cancer and there are many important health benefits. I don’t see these two studies as being anything to be concerned about.
Q: Another recent study found that taking statins [cholesterol-lowering medications] lowered men’s risk of developing lethal prostate cancer. Should all men start taking statins?
A: The study found that men who had prostate cancer and were taking a statin had a much lower risk of dying from their prostate cancer. In particular, it looked like the men who were taking the statins before they were diagnosed with prostate cancer had the most benefit. Several studies have shown similar results.
These findings might point to the mechanisms behind why some men with prostate cancer have a fatal disease and others do not. While the results are quite interesting, I don’t think that all men should start taking a statin to prevent lethal prostate cancer if they don’t need it to lower their cholesterol.
Q: Recommendations regarding whether men should get the PSA test, at what age they should have it, and what they should do in the event of a diagnosis are inconsistent. What is your advice?
A: Much of the confusion comes from the findings released last year from two big randomized trials looking at how effective the PSA is in reducing prostate cancer mortality — one in the U.S. and one in Europe. The U.S. study found no difference between the men who did and did not get screened, whereas the European study found that PSA screening had a significant benefit in reducing mortality.
I think the U.S. study probably got the wrong answer. Many of the men in the US study who got randomized to the group not getting screening actually received PSA screening on their own. This would dilute any possible benefit since both groups were essentially getting screened. This is the key problem. Also, screening is thought to catch cancer ten years earlier than symptoms would. In the most recent publication, only half of the men had been followed for 13 years. So if you want to see any benefit for mortality, you have to follow people for really much longer.
The take home message for me is that PSA screening likely reduces mortality from prostate cancer. What then becomes complicated is that it seems to diagnose many men whose cancer will never go on to develop metastasis in their lifetime. A lot of those men get treated and those treatments have side effects. It’s really important for men to become informed and to talk to their doctor about the risks and benefits of PSA and how that relates to them and their own health.