> But it sounds like you're describing the possibility of calculating his expected weight gain based on how much he ate and how much he exercised, and I don't t

> But it sounds like you're describing the possibility of calculating his expected weight gain based on how much he ate and how much he exercised, and I don't think that's possible. Why? To a large extent this is possible, and with a bit of experience in how a specific person reacts, it's possible to do this very accurately. Bodybuilders will often have very specific meal plans over months, and know more-or-less exactly how much weight they will gain or lose, sometimes down to the week.