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

by Alex Hutchinson

endurance athletes on a three-week high-fat diet became fat-burning machines to an extent few had imagined possible. By the end of a 25-kilometer time trial at their expected 50-kilometer race pace, the athletes were burning through 1.57 grams of fat per minute, which is two and a half times greater than the “normal” values seen in athletes eating a standard carbohydrate diet.37 That was the good news. The problem was that the fat-adapted athletes became less efficient, requiring more oxygen to sustain their race pace. This, it turns out, is a consequence of the cascade of metabolic reactions required to transform either fat or carbohydrate into ATP, the final form of fuel used for muscle contractions: the fat reactions require more oxygen molecules. If you’re out for a leisurely stroll, that’s no big deal, but if you’re running (or walking) a race at a pace that leaves you out of breath, anything that forces you to consume more oxygen is a liability. As a result, it was no surprise that the LCHF athletes ended up performing worse than the high-carbohydrate athletes in the Supernova study’s final and most important real-world test: a 10-kilometer racewalk.