Gain weight without eating?
The Law of Conservation of Mass says that’s impossible.
OSMOSIS!
You get the calories from watching cooking shows, restaurant, soft drink & beer commercials.
Inactivity slows the body’s metabolism.....duh! Assuming normal activity, a person could probably consume 1500 calories, but a person who sits on the couch probably doesn’t burn much more than their Basal Metabolic Rate, which is more in the 1000-1100 range. And of course not eating at all, will put the body in “starvation mode”, which will tend to conserve the fat stores, at the expense of muscle.