While I don't know if there has been a formal study, there is plenty of documented evidence of reduced mental ability and fine motor control if you are sucking on internal reserves. Sure, you might survive if you just sit still and have enough water, but if you break your neck going to get water because your brain is fuzzy due to lack of food, you're still dead. You can get used to it, but lets face it, most of us are used to three or four meals a day, and that is what our body expects and it takes a while to adapt.

I'd call the several weeks on body fat a semi-myth. That is for people sitting in a rubber life raft, waiting for the PBY to spot them and hoping it really is a PBY and not the Japanese, while sucking on a pint of water a day under the sun. (To give you an idea of WHEN the figure comes from.)

The only saving grace I can give to that statement is that most people are found in under two days.