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Around moderate exercise intensity, we tend to burn fat at the highest rate. When we exercise near or above VO2 max, we burn almost no fat.

Why can't we achieve high levels of fat oxidation near but below VO2 max pace?

Based on my understanding, fat can't be used anaerobically. What makes high intensity exercise anaerobic when it hasn't exceeded VO2 max yet?

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    Just so we're on the same page, where did you hear that you burn less fat near max pace? – Alec Oct 30 '19 at 14:13
  • @Alec Look up using keywords such as fatmax using Google images and you'll see that fat oxidation really drops once past the crossover point of around 65% of maximum heart rate. The crossover point is where fat oxidation peaks. – Brian Oct 30 '19 at 17:13
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The reason why it's hard to burn fat during high intensity exercise is that it's a slow and inefficient process of getting energy to the muscles. Your body will switch to using glycogen aerobically and/or anaerobically.

The infamous "fat-burning zone" concept is highly misleading. It's true that we don't burn a lot of fat during high intensity exercise, but we do burn a lot of calories (in the form of glycogen) that will be replenished later by increased metabolic activity, effectively burning the equivalent amount of fat. Since high intensity exercise burns more calories, at the end of the day, it burns more fat too. But there's a downside to that: you'll feel much more exhausted. The idea behind doing easy to moderate physical activities for weight loss is that if you're getting most of your energy from fat during the exercise, you won't feel as tired during the rest of the day, because your glycogen stores are intact and there's no need for muscle recovery.

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