Kiwi- I should have given a better answer. Most elites I know DO consume less than the field behind them. If we're talking about a 100 miler, they're going to exhaust their usable glycogen stores before the end of the race and aren't consuming enough to avoid having to rely on fat metabolism. So how do they do it?
Obviously training methods vary tremendously, but all of the elites or near-elites I know simply don't eat before or during many of their long, fast runs. If you hit that gylcogen-depletion wall frequently, your body begins to adapt and allow you to run faster longer. I used to do this a lot when I trained at higher mileage, and it works exceptionally well. I still do it when I can, but it's not enough to create a significant difference during races.The key is to run long enough to get a good crash, then keep running after the crash. It seriously sucks at first, but the body adapts quickly.
The physiology geeks like to talk about the impossibility and limits of various training adaptations, but they don't look closely at the research that determined the supposed "limits" of the human body. We like to talk about ultras as if we've mastered our knowledge about the human body. There's precious little research on the effects of endurance effects on the human body. The research that is available has serious methodological flaws... in most cases because of the inherent difficulty of measuring physiological responses in the middle of a typical ultra. The best data we have comes from Western States, and that research is still in its infancy (see
Marty Hoffman).
Using Bighorn as an example (starting the race in about 5 hours), I could either eat enough calories to mostly avoid crashing, which allows me to run faster, or run until I crash, which will force me to slow wayyyyy down. Had I regularly trained in that deficit, I could maintain a faster pace after crashing. In the absence of that specific training, I'll eat. Both strategies produce the same end result- faster finish time.
Low intensity training (Maff) gets a lot of press among barefoot runners, especially lately. It's a good method to maximize fat utilization and will produce good results. However, it's not the only means to dealing with glycogen depletion.