In one episode, Mr. Ratburn was shown to be overweight as a kid, before he began playing ping pong and lost weight. It's safe to assume his love of cake helped him gain weight in the first place.
in "To eat or not to eat" the big boss bars were extremely addictive and toxic, even worse the main ingredient was basically a recreational drug which means it most likely caused brain damage when it was eaten and almost every kid in the school were addicted to them....