Are Marathons and Extreme Running Linked to Colon Cancer? A small, preliminary study found that marathoners were much more likely to have precancerous growths. Experts aren’t sure why.
From the New York Times.
My initial take is that marathoners and extreme runners aren't dying of cardiovascular diseases or pulmonary disease (including lung cancer), but eventually have to die of something, and that something is often colon cancer.
Another non-causal explanation could be that marathoners and extreme runners are more health conscious, and hence more likely to get their colonoscopies done, causing their colon cancers to be noted statistically, while less health conscious people skip their colonoscopies (an entirely understandable sentiment) and so if they have precancerous growths at the same age, they go undetected and aren't noted statistically.
In the same vein, almost no one discovers that they have precancerous pancreatic cancer growth, or even pancreatic cancer itself, because it is usually asymptomatic until it is very advanced.
Or, it could just be a statistical fluke, which is something that is common in small studies and is further influenced by a bias towards only publishing results showing a potential positive or negative disease correlation (which creates a "look elsewhere effect" that reduces the statistical significance of the studies that are published since only locally statistically significant outlier results get published). The look elsewhere effect is widely recognized but can be hard to quantify.
It could be more than that. It is certainly plausible that a more stressed body might be more prone to cancer, and long distance running does indeed stress your gastrointestinal system. This is especially plausible if the true effect size is modest. Indeed, the confounds noted above could turn a small effect size into a seemingly much larger one.
But it also might be just be entirely due to some of the non-intuitive aspects of how epidemiology and health statistics work.
2 comments:
I'll be less skeptical with a larger and more thoroughly randomized cohort with its own control (vs. genpop statistics). The target for this study is a tiny sample of the overall population, yet still one that is pretty well-studied, especially in the last 20 years. If this is a real effect, I'd also be shocked if a direct cause didn't present itself - thus my skepticism that an effect like this has been hidden for so long.
I'll admit that the results in the story are strong enough to at least merit further study.
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