In the end, it really is just metabolic math. As a former wrestler who did some insane weight-losing in my youth, the muscle mass loss is due to extreme calorie restriction. Besides, just sitting around starving is not a good way to become healthier in the long run, and that's where the muscle loss will be worst. To be a true journey towards better health, dieting should be accompied by at least a lot of walking and some light weight training, in the minimum.
And don't forget your supplements, kids (addressing general readership), because our foods are less nutrient dense in 2025, so find out if some supplements help energy levels on the downslope. Me and my teenagers have taken a sip / mini-gulp of a product called 'Orgain' daily for a month or so, and I have found my energy better, while having less cravings. An entire little 11oz single-size drink is way too much for me (too many vitamins, makes me sleepy), having 'just a little bit' is not at all scientific, but we seem to be thriving on that little bit of supplementation.
And, as always, avoid those processed sugars, kids! Whole sugar cane is very, very good for us, but only in moderate doses. Processed sugar is the crack cocaine of sugar cane, as opposed to just chewing a leaf of the coca plant.