I posted a little bit about this topic here
, but here is another tidbit of info from my Neuroscience book.
Apparently, animals have a "desired" weight that they will maintain at. Suppose you have a rat that is at a normal weight. If you force feed the rat, it will gain, but when the force feeding stops, it will voluntarily eat less until returning to its original weight. If you then starve the rat, it will lose weight, but when food is returned it will eat until it gains back to the original weight.
Maybe for some people the "desired" weight is screwed up? (That is entirely my own speculation, not from the book!)
Another interesting thing from the book that has nothing to do with this but was so bizarre that I have to tell you about it. Some rats don't naturally produce leptin (which I know has been discussed here). These rats are so huge they look like spheres. Apparently they can do a surgery where they take one of these spherical rats and fuse it to a normal rat, so they are sharing the same blood flow (BIZARRO I know). When they do that, the spherical rat returns to normal size! (But it has another rat attached to it like conjoined twins, which is really weird.) Anyway, I just thought it was pretty bizarre that they can even do a surgery like that.