[thefz]

> Even amongst traditional calorie deficits, rapid weight loss results in greater loss of muscle mass when compared to gradual weight loss,

This does not make any sense. Why would the body prefer anything over the most dense and available calorie store? Protein in muscle gives shit calories per gram, it is hard to build back and generally less available than fat: the number one energy store, doing exactly what it does.

[beejiu]

I don't think anyone knows for sure, but I think the prevailing theory is it being a survival mechanism.

When our ancestors faced famine, it makes sense for the body to shed as much muscle as possible, since this reduces the metabolic rate in the medium-long term.

Muscle is more metabolically active than fat. Although fat can be used up for energy more readily, but muscle takes more energy to maintain. Burning fat just to maintain (unnecessary) muscle doesn't make sense in terms of survival.

[Jensson]

Could just be its for winter where you don't need to move much for a few months, otherwise normally you need that muscle to gather food even when starving, someone has to gather it and it wont be someone who shed most of their muscle.

[cthalupa]

Whether or not it makes any sense to you, it's not a matter of any scientific debate - being in a deficit puts you in a catabolic state where the body will break down muscle mass for energy. It does it less if you have lots of protein and are providing frequent muscle stimulus.

[thefz]

Source?

[cthalupa]

For protein intake helping decrease this: https://faseb.onlinelibrary.wiley.com/doi/abs/10.1096/fj.13-...

https://faseb.onlinelibrary.wiley.com/doi/epdf/10.1096/fj.13...

https://faseb.onlinelibrary.wiley.com/doi/epdf/10.1096/fj.14...

https://www.sciencedirect.com/science/article/pii/S216183132...

https://www.mdpi.com/2072-6643/12/8/2457

For weight lifting helping decrease this: https://www.mdpi.com/2072-6643/11/11/2824

https://journals.physiology.org/doi/full/10.1152/japplphysio...

https://link.springer.com/article/10.1007/s00726-013-1506-0

https://journals.physiology.org/doi/full/10.1152/ajpendo.005...

https://onlinelibrary.wiley.com/doi/abs/10.1002/mus.21780

https://journals.physiology.org/doi/full/10.1152/ajpregu.004...

https://www.mdpi.com/2072-6643/10/4/423

These are just a tiny subset of the studies done - google scholar can find you many dozens more, if you desire. And, of course, the fact that these studies exist it all necessarily implies that you lose muscle mass when in energy deficit, as you will see in the control groups for them.

[gls2ro]

(Not a doctor) My understanding is that it is more rapid to extract energy from muscle than from fat.

[morgengold]

The body breaks down some muscle tissue beacause it can make glucose from by gluconeogenesis. You need about at least 80 g glucose or so per day (brain), even if you do not eat any carbohydrates. The body cannot make glucose from fat.

[nkmskdmfodf]

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.

[Terr_]

> 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]

> 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]

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.