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Because the body can only extract so much energy per minute from all of the fat in your body. If that's not enough, muscle is used, etc.

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Terr_ 4 days ago | parent [-]

> Because the body can only extract so much energy per minute from all of the fat in your body.

Was curious about this, went hunting for some rough data, this [0] suggests every kilogram of fat held can be drawn down at ~70 food-calories per day.

So someone with 25% body fat weighting 100kg (~220lb) could draw 1750 food calories per day, which strikes me as pretty ample unless they're also adding a bunch of physical activity.

[0] https://pubmed.ncbi.nlm.nih.gov/15615615/

	▲	cthalupa 4 days ago \| parent \| next [-]
		> which strikes me as pretty ample unless they're also adding a bunch of physical activity. It seems likely we've evolved to reduce energy expenditure in other ways when we regularly induce physical activity, too. Walk 20,000 steps or spend a couple of hours on the treadmill? Your body finds ways to reduce your energy expenditure elsewhere. https://pmc.ncbi.nlm.nih.gov/articles/PMC4803033/
	▲	nkmskdmfodf 4 days ago \| parent \| prev [-]
		It's not going to be linear though. 1750 cal per day ~= 73 cal per hour. If, for example, you're already in a calorie deficit for the day, and then do a nice hour long workout (or demanding mental work), you're going to burn some muscle.