It depends on the person and their fitness level as to how much to eat, if anything at all. I have dropped 31 pounds since June 1st. Part of my routine has been riding fasted. The most I have done is 70 miles, with 3,800 ft of climbing. I averaged 17.7, and would've averaged more, but I was riding with a newer rider and had to slow down or him at times. I can honestly say that I felt great the entire ride as far as my energy level was concerned.
This method didn't work on Labor Day weekend when I did a century@ 21.1 mph. It was mostly flat, but the heat and pace broke me and by the time I was feeling like crap, it was too late. I started eating on some snacks I had brought along, but it didn't do much good at that point. I finished the ride, but it wasn't pretty. I learned my lesson on that one.