Fasting is the ultimate "calories in, calories out" diet. Can't get any lower than zero. It's weird that Josh suddenly wants to talk about regaining weight, adding complexity to the topic after years of boinking fat chicks over their head with "it's just thermodynamics" talk.
Now, if forgoing the "calories in, calories out" oversimplification, then we're mostly talking about behavior. How hard it is to stick with it, how miserable you feel, how full you feel. That stuff will vary with the individual, at the end of the day. Fasting, like God told us to do, might be the right choice for PPP, while stuffing his fat face with pizza could be the right diet for Josh.