When I was in grad school and needed money I participated in a study comparing two different liquid diets that were formulated to be given via NG tube to unconscious trauma patients. Since they needed informed consent, they needed healthy volunteers. For two weeks (one week each for each regimen) I was confined to a hospital bed and fed only NG compatible liquid nutrition (which was nasty) - no real food at all, just this liquid.
After about 3 days I started having dreams about food and hunting (which I didn't do), and by the time it was over I would have eaten fresh roadkill raw.