> 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.

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kazinator 2 hours ago | parent [-]

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 8 minutes ago \| parent \| next [-]
		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.
	▲	aix1 42 minutes ago \| parent \| prev [-]
		This article has a graph from a lab test: https://knowledgeiswatt.substack.com/p/20-120-vs-90-gh-of-ca... I thought it would help illustrate what you're saying but, gosh, those Y axes aren't making things easy to interpret. For those willing to do the mental arithmetic, 1g of FAT is 9 kcal and 1g of CHO is 4 kcal. :) P.S. It also only starts at 150W.