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It's well known that the most optimal exercise for fat utilization/oxidation or simply "to burn fat" should be low intense aerobic exercise where HRavg should not exceed 65% HRmax. In addition doing exercise in fasted state and after high intense anaerobic session should even increase the amount of "burned" fat.

Regular anaerobic activity such as running or cycling does contribute to overall endurance and therefore higher effort is needed to bring HR over anaerobic threshold >65 HRmax (where body switch to carb (glycogen) as main source of the energy).

Based on these two facts, my questions are:

  1. The higher the athlete's VO2max, the more effort is needed for athlete to raise HR over the 65% HR max threshold. Does this mean that time (during the exercise) spent inside aerobic zone (<65% HR max) will be longer, therefore the body will run for more time using FAT as main source of energy?
  2. Does this imply that the higher the athlete's VO2max, the more fat they will "burn" during the exercise, for exercise types that are not intended as anaerobic/HIIT training sessions?
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The short answer is that the "fat burning" heart rate zone is a myth. Exercise at low intensities does not result result in greater rates of fat loss compared to exercise at higher intensities, because both burning fat as an energy substrate and burning glucose or glycogen will have the same long term effect on body fat stores.

For an explanation, see: https://www.ironmagazine.com/2011/myths-under-the-microscope-the-fat-burning-zone-on-trial/

To answer your questions to the extent that they're still relevant:

  1. If the athlete is working out at >65% of their max heart rate, their heart rate will rise to >65% quite rapidly, and the time spent at <65% will probably be insignificant compared to the rest of the workout.
  2. The higher the athlete's VO2max, the more energy they will be capable of burning during any exercise session.
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  • PubMed is full of articles which are telling/proving the theory that which source of energy will body use is somehow managed by intensity of exercise. And here we come to the question of HR’s role in this process.
    – bumlabumla
    Aug 2, 2021 at 16:36
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    I think it's more accurate to say that the myth is you lose fat faster by staying in the "fat burning zone" of cardio. In reality, fat oxidation is the same throughout the entire day when calories are equated. You do utilize more fat in the "fat burning zone" cardio but you utilize more fat post-exercise if you exercise in the anaerobic zone. It ends up being the same at the end of the day.
    – DeeV
    Aug 2, 2021 at 18:05
  • @DeeV this is exactly correct. The myth is you need to be in the "fat burning" zone to burn fat, not that energy substrates used during exercise can vary. Over 24 hours it makes no difference: europepmc.org/article/med/8883001 Aug 3, 2021 at 2:14
  • @bumlabumla your question implied that low intensity exercise is optimal for fat loss, but fat oxidation is not the same thing as loss of body fat. Can you please clarify then whether you're trying to achieve maximal fat oxidation even if it does not result in loss of body fat, versus just maximal loss of body fat? Aug 3, 2021 at 2:26
  • Agreed - it's worth bearing in mind that the most fat you can actually burn whilst exercising is around .5g/min, or maybe 1g/min in extreme circumstances. So focusing on fat oxidation throughout the day would be more beneficial for most people.
    – John M
    Aug 4, 2021 at 14:22

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