Your metabolism typically takes two big dumps in your lifetime. Once in your late teens/early twenties and once in your late thirties/early forties. Alex's terrible diet is making the first one far more severe than it has to be.
Alternatively he's trying to gain weight so his boobs/butt get bigger and doesn't realize it's all going to his belly because he's both stupid and a man.
I like the saying "tits & ass don't count if you're fat".