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