Yawn doesn't realise how badly these c9s fuck with her metabolism, especially since she does them so often. I can really mess up your hormones;, food is fuel. I'd imagine that a long period of essentially starving oneself would invoke a binge eating episode at the conclusion of each c9 too. Resulting in weight gain.
I currently wear clear braces and only eat 2-3 meals per day to ensure they're on my teeth for the required 22 hours. I'm used to snacking, so now I'm often so hungry, that when I do eat, I wolf down much larger portions of mains and if available, chocolate, than I ever used to. But I'm still eating a normal amount of daily calories - if I were to starve myself on some bullshit shake diet for 9 days then I'd be wanting to eat everything I laid eyes on, for probably another 9 days until I'd resolve to do another c9.
I'm always a bit disappointed with mums of girls who are constantly dieting - I remember a friend's mum dieting all the time; the mum wasn't hugely overweight, but she definitely affected my friend's self-image- my friend actually became very overweight over the years and which I imagine was due to a messed up attitude to dieting in that households.