Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

During easy exercise, like a gentle walk, you burn mostly fat from the supplies circulating in your bloodstream.9 As you speed up, you begin to add more carbohydrate to the mix, and by the time you’re panting heavily, the proportions have flipped and you’re burning mostly carbohydrate. The precise blend depends on a variety of factors: the fitter you are, for example, the greater the proportion of fat you burn at any given speed. (That’s simply because maintaining a given speed gets easier as you get fitter. As John Hawley, an exercise metabolism researcher at Australian Catholic University, points out, no matter how fit you are you’ll burn the same fat-carb mix at any given relative intensity.) Eating a diet high in either fat or carbohydrate also tilts your preferred fuel mix in that direction. But even taking these factors into account, carbohydrates dominate for any intense exercise: one study found that over the marathon distance, running at 2:45 pace relied on 97 percent carbohydrate fuel, while slowing down to 3:45 pace reduced the carbohydrate mix to 68 percent.