Parabiosis experiments suggest that endocrine signals in the blood regulate hunger and appetite.

“This parabiosis experiment revealed something really important. When they lesioned the ventromedial hypothalamus in one of the rats that was connected to the other rat, that rat got very, very fat, just really obese, huge rat, super rat, jumbo rat. The other one however, got very thin, it actually lost weight, despite consuming the same amount of food that it had prior to the other one getting the lesion. So what does this tell us? This tells us that there's something in the blood that's being exchanged between the two animals, because it was their blood supply that was linked. And that tells us that there's hormone or endocrine signals that are involved in the desire to eat, and hunger and appetite.”