Carbohydrates vs. fat in marathons


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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.

2 Replies to “Carbohydrates vs. fat in marathons”

  1. Did you read the paper? The subjects ran a complete marathon on a treadmill, which is quite an accomplishment by itself, with a catheter in the arm, a mask, all of this after an overnight fast!!!! Crazy! I hope they got paid for this torture!
    As for the science itself, it is very interesting that 4 different studies came up with very different curves of CHO vs Fat over time. Whereas this study observed a rise, followed by a peak of quite high CHO consumption at ~60 min, and then a progressive decrease for subjects running at 65-73% VO2max; Cox et al. (2010) observed a continuous, almost linear decrease in the CHO/Fat ratio for subjects cycling 100 min at 70% VO2max; Yeo et al., (2008) observed a decrease and then a stabilization of the CHO/Fat ratio for subjects cycling 50 min at 70% VO2max; Stepto et al. (2001) observed a near logarythmic decrease in the CHO/Fat ratio over 8X5min cycling intervals at 86% VO2max. What’s going on here? Are these huge variations linked to different analytic protocols or indicates a highly individual parameter? Interesting subject!

  2. Hi Felix — Yes, those volunteers must have been mentally tough bastards! 🙂

    The topic of CHO/fat ratio is definitely a convoluted one. For example, in Asker Jeukendrup’s recent sports nutrition book, he says “A trained person may have his maximal fat oxidation at 70% VO2max or 45% VO2max and the only way to really find out is to perform one of these Fatmax tests in the laboratory.” Whether it’s truly just a case of massive individual variation, or whether there’s some “hidden variable” in the protocol or subject characteristics that dictates it that we simply haven’t figured out yet, I don’t know.

    But even bearing all this variation in mind, I think the point Hawley was making in the talk I quote above is that carb usage is still quite high even in exercise lasting several hours. Whether it’s 68% or 97%, or even just 40%, it’s still a significant chunk of your overall fuel — so if you compromise your carbohydrate storage or burning capacity in pursuit of better fat burning, you’re likely shooting yourself in the foot.

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