The myth of the “fat-burning” zone


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I generally try not to get too excited about mouse studies, because there’s so much uncertainty about how the results will translate to humans. Still, I was very interested in a new study on fat-burning from Australian researchers at the Garvan Institute of Medical Research. I’ve always been skeptical about the various ways people suggest trying to convince your body to burn fat rather than carbohydrates, whether it’s by keeping your heart rate in a certain zone (usually by going slower than you would otherwise go, which seems silly to me) or by taking some sort of pill.

Anyway, there’s a very good press release describing the new study and providing context, but allow me to quote the key section:

Sydney scientists have demonstrated that mice genetically altered to burn fats in preference to carbohydrates, will convert the unburned carbohydrates into stored fat anyway, and their ultimate weight and body composition will be the same as normal mice.

The research related to an enzyme called ACC2 (acetyl-CoA carboxylase) that controls whether cells burn fats or carbohydrates. There’s no doubt that this is still a very complicated area of research, so I’m certainly not claiming this is the “last word” on this topic. But it reinforces my impression that our primary focus should be on burning (or avoiding) calories, not burning fat.

3 Replies to “The myth of the “fat-burning” zone”

  1. What about marathon training? I understand an important part of most training plans, the long, steady run, as trying to engage the fat burning system as preparation for the late stages of a marathon? Or is this a different thing (because it kicks in after the carb-burning system is exhausted)?

  2. Yeah, I was wondering about the implications of this for all the training routines that aim to optimize fat-burning. I don’t think it’s really relevant, because it’s operating on a different time-scale. They showed that, if you genetically engineer mice to burn more fat, they EVENTUALLY convert stored carbohydrates to refill the fat stores. But I’m not sure that would happen in real-time during a race or training session. Competitive runners are most concerned with their fuel stores on a timescale of minutes rather than days or weeks.

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