I really wonder that a lot. Why some people just fall apart if they skip a single meal. I really suspect a lot of it's down to fat people just being in such bad shape that they can't even properly utilize their fat reserves when needed. And with all fats and a big chunk of normal sized people also hit hard by a poor diet that their body is struggling with.  I mean I've known tons of skinny hippies who can happily go through extended fasts while staying pretty active. Meanwhile most people in our culture act like skipping breakfast is going to send someone into the hospital.