Comment by bitexploder

4 hours ago

"If you do aerobic exercise, almost all the energy comes from burning fat. " This is directionally incorrect. Your body will burn both concurrently. For low intensity aerobic exercise, fat is used as the dominant energy source. However even at moderate intensity levels like jogging and "zone 2" aerobic you are 50/50. At higher intensity you have crossed the inflection point and are using more glycogen than not. All strictly aerobic exercise. And it all works on a balance anyway. You use glycogen, it gets replaced until everything is topped off. Doing that means it isn't getting converted to fat.

Both forms of exercise are shown to have an "anti-hunger" effect.

And unless you are walking, your body is also shunting blood away from your gut which also has a secondary hunger dampening effect as it doesn't resume blood flow too it immediately.

So for anything we would call aerobic exercise, that is zone 2 "cardio" or greater, I would have to disagree with your main claims about it.

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bitexploder

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ralferoo  4 hours ago

> This is directionally incorrect. Your body will burn both concurrently.

For aerobic exercise, your body gets around 95% of the energy from burning fat. If you are doing exercise where you are 50/50, then it is by definition no longer aerobic exercise but anaerobic.

Anaerobic exercise starts at the point that your body is forced to use glucose from glycogen to provide energy because you have reached the limit of the energy your body can produce from burning fat, because your body can't provide oxygen at the rate required to do so.

  • kazinator  4 hours ago

    There are two exercise intensity thresholds related to respiration: VT1 and VT2 (ventilatory threshold).

    Everything from minimal activity far below VT1 to VT2 (a.k.a. "lactate threshold", LT, a.k.a. "anaerobic threshold", AT) is "aerobic".

    Near the VT2 limit, very little fat is used compared to glucose. Fat burning proportions as high as 95% are only reached under very light activity. (And/or in glycogen depleted exercisers whose body has switched to fat out of necessity). That doesn't represent the entire aerobic range.

    There is aerobic use of glucose (below the lactate threshold, "clean burning") and anaerobic (above AT, generating lactic acid).

    A useful parameter is the absolute fat burn rate. Maximal fat burning does not occur at exercise intensities that derive a large proportion of energy from fat. Supposedly, this "FatMax" exercise intensity fairly closely coincides with the VT1 threshold. Here, around 60% of the energy comes from fat.

    I'm "fat checking" all this as I type; I used to know more about this stuff, but forgot a lot.

    • kudokatz  1 hour ago

      As an example of this ...

      At LT1 (via lactate measurements) at peak 100k fitness with elite economy (n=1) ratios were roughly 23% fat, 77% carb. FATMAX was near 28% at slower speed. This is via training using the now-standard (at elite levels), high-carb approach for fueling ultra marathons.

      So many factors--including gut training and fueling--play into this. Most aren't even aware of the details, and the "we don't need no carbs for performance" folks still generally bury their heads in the sand. For performance, we're seeing huge skews to carb-based energy for endurance that were considered "wild" just 5 years ago.