Reminder: correlation is not causality
February 27, 2014A research study by Martin Hoffman and Eswar Krishnan concludes, “Compared with the general population, ultramarathon runners appear healthier and report fewer missed work or school days due to illness or injury.”
The March 2014 issue of my local running magazine summarizes this study as follows: “Keep logging those miles, ultrarunners! Your body will thank you for it in the long run.”
See the difference?
The study itself simply notes that ultramarathoners are, by most measures, healthier than normal. The running magazine leaps (or perhaps sprints) to the conclusion that these runners’ training is what keeps them so healthy. But we can’t rule out the opposite: maybe these people’s good health is what allows them to run so much; maybe their impressive mileage tallies are an effect, rather than a cause, of their good health. Or maybe the ultrarunners surveyed differ from the general population in other ways, unrelated to running, that account for their superior health.
Numerous studies have provided strong evidence that running promotes good health, but this study isn’t one of them.
My Track Record 
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