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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.

- Alex Hutchinson (@sweatscience)

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These days, everyone’s interested in boosting their fat-burning abilities. After all, we have a “limitless” supply of fat, so why not rely on it during extended exercise? That’s the goal of current ideas like “train low, compete high,” where you basically practice training in a low-carb state to enhance your fat-burning abilities.

With that in mind, I quite like a nugget of info that John Hawley dug up from the archives (this 1993 paper, to be precise) about our dependence on carbohydrate during extended exercise. Researchers from George Brooks’s group at Berkeley tested runners doing a treadmill marathon in either 2:45 or 3:45, and measured what fuels they were oxidizing. The slower group used 68% carbohydrate, and the faster group used 97% (which means, as Hawley pointed out, that Geb is probably using PURE carbohydrate).

Doesn’t mean that fat-burning abilities aren’t important — but it’s worth keeping in perspective.