In our book and blog – we heavily promote exercise as one of the best ways (along with a healthy diet and quitting smoking) to prevent many different diseases, including heart disease, diabetes, strokes, and certain cancers in addition to improving quality of life and increasing life expectancy.
However, we mention that often doing just one type of exercise, or exercising too much (e.g. marathon running) can at times lead to injury and in the case of doing too much can actually damage the heart and weaken the immune system.
A recent study published in the Journal of Men’s Health shows a correlation between too much cycling and a risk of developing prostate cancer in men over 50. Data from over 5,000 men was analyzed; those who cycled more than 8.5 hours a week were over 6 times likely to report having prostate cancer. Those who cycled over 3.75 hours but less than 8.5 hours a week had an almost 3 fold increase.
The design of this study is such that it doesn’t per se prove that excessive cycling causes prostate cancer. For example, it’s possible that men who cycle more end up having more urological complaints stemming from sitting on a hard narrow seat for too long which leads to medical investigations that uncover cancer. However, the study did show that cyclists were no more likely to contact their primary care physicians than those who did not cycle. Furthermore, excessive cycling has been previously linked to testicular cancer as well as prostatitis (inflammation of the prostate), so that it’s at least plausible that the repeated trauma to the region of the prostate that happens during long bouts of cycling can lead to the development of prostate cancer.
The bottom line is that for now, it isn’t known for sure if too much cycling does cause prostate cancer. However, since it’s possible, and given that the magnitude of the potential harm (over a 6 fold increase in those that cycled the most) is huge, it probably is a good idea to cycle less than 3.5 hours a week and otherwise vary your workouts by incorporating other types of exercise.