Your anus will be leaking, but you will be thin.
Actually, I think obesity has more to do with lack of exercise than what you eat. When I was a kid, we drank soda (not the diet kind), ate junk food, and so on. But we used to go outside and play jumprope, handball, stickball, whatever type of physical activity. So whatever calories we got from the crap food were burned off pretty quickly. Not like today's kids whose extracurricular activity consists of sitting in front of a computer playing vidoe games.