Believe it or not, you likely would gain a bunch of weight in the short-term and then it would STOP.

Fung presents a few studies illustrating this - one involved prisoners who were made to eat 4000+ calories a day and were carefully tracked to see that they were in fact eating them (they were).

They all gained weight in the short term, but it got so hard to keep up that many quit. And they didn't gain as much as predicted, as the body ramps up all sorts of mechanisms to counteract the weight gain (homeostasis). And most importantly, the moment they let the prisoners go back to their normal routines, they lost ALL the weight they gained very quickly, and IT STAYED OFF.

His point is that it's actually very hard to gain weight over your metabolic set point, as your body will fight you to pull your weight back down. It works in both directions - losing weight downwards, AS WELL as gaining weight upwards.

So contrary to what people think, it's literally impossible for them to eat themselves to 400 lbs unless they have a genetic disorder that allows them to do so (you'd probably know you had it if you were one of these folks.)