Prostate cancer is the second-highest killer of men among all types of cancer. At least 30 studies have tried to link level of physical activity to the risk of contracting the disease. None yet have yielded a definite answer, including this study, done by doctors from various institutions in Alberta, Canada.

What the researchers wanted to know: Once and for all, is physical activity linked to prostate cancer risk?

What they did: First, they amassed two groups of men: nearly 1,000 diagnosed with moderately invasive or more severe prostate cancer who were identified in the Alberta Cancer Registry, and slightly more than 1,000 who had not been diagnosed with cancer, found through a random telephone survey. In addition to being asked about medical history, family history, and lifestyle, each man was interviewed extensively about his lifetime physical activity–recreationally, on the job, and at home.

What they found: Unlike other forms of cancers, such as breast, colon, and endometrial (a type of uterine cancer), there is no clear link between overall activity level and risk for prostate cancer. The doctors found a small decreased risk in men who did recreational activity, those whose activity was very intense, and those who had more physical activity early in their life. But the correlations between activity and prostate cancer risk were weak and do not give a definitive answer.

What it means to you: True, the study is inconclusive, but that doesn't mean it's a license to spend the rest of your life on the couch. Previous studies have shown a link between physical activity and prostate cancer, though it is less marked than in other cancers, and a healthy lifestyle has been shown to decrease risk for most types of cancer.

Caveats: The study asked men to recall physical activity over the entire span of their life–a long time in many cases. After all, it can be hard to provide reliable data about how many times a week you went running 25 years ago–and even harder to recall how often you worked around the house. Also, they took into account a host of factors such as education, family history, and ethnic group, and in trying to combine and control for all these things, the data may have become skewed.

Find out more: The National Cancer Institute has a nice website that discusses physical activity and risk of various cancers. The National Institutes of Health also has a good website with lots of information about prostate cancer.

Read the article: C.M. Friedenreich, S.E. McGregor, K.S. Courneya, S.J. Angyalfi, and F.G. Elliott. "Case-Control Study of Lifetime Total Physical Activity and Prostate Cancer Risk." American Journal of Epidemiology. April 15, 2004, Vol. 159, No. 8, pp. 740‑749.