When one twin runs more than the other


As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

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Interesting new study from Paul Williams’ National Runners’ Health Study — no, wait, this is a new one, not the one I wrote about a few days ago! This one looks at identical twins to find out the extent to which your body shape is dictated by your genes. Yes, the old nature vs. nurture debate…

Williams has an enormous data set of over 100,000 runners, including 926 identical twins. He explored the relationship between two quantities: the difference in how much a pair of twins ran, and the difference between how much they weighed. Various studies have found that genetic factors account for 40% to 70% of the variation in BMI. But Williams found that the more the active twin exercised, the less genetics seemed to matter. Here’s a graph:

The numbers on the right-hand side represent how much more the active twin runs than the less-active twin. Of course, for some people the results will seem absurdly obvious: the greater the difference in activity levels between two people, the greater the difference in their BMI. Still, it’s a good reminder that genetics isn’t destiny. Here’s what Williams concludes:

Extrapolating the coefficients of Table 3 shows that BMI inheritance might be eliminated completely by running 7.05 km/day (23 mi/wk) in women and 13.51 km/d (60 mi/wk) in men, a projection that is consistent with our previous finding of uncorrelated BMI values in 35 pairs of MZ twins whose running differed by an average of 8 km/d [31], but a projection that should nevertheless be considered with caution.


3 Replies to “When one twin runs more than the other”

  1. Or somebody with a low BMI would run more. Genetics might not be destiny but diet might be.

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