I think most of the ultra nutrionists also recommend that ultra runners take on board fat while they are running, hence the cheese/meat/avocado/peanut butter wraps and sarnies that appear at aid stations. From what I recall, the reason for this is that the brain requires fat to keep functioning and this runs out after 35-odd miles.
Ultrarunnerpodcast.com has some very good podcasts on this subject with the wonderfully named nutritionist Sunny Blende. They can be downloaded from iTunes.
Edited: 23/04/2012 at 23:07