Your post got me curious so I read a little about endurance foods and eating habits. I am not trying to prove anyone right or wrong or saying what I read is right or wrong. Energy in an Enduro is pretty important and I will be trying to research this more in the future.
I havent done alot of research on how much your body uses during digestion, but I have seen them say 10% a few times. (I am no nutrition expert, or anything of that sort either. I am going from my brief readings and who knows what to believe on certain subjects)
I did come across an interesting article about endurance eating though one was about the eating habits of the Kenyan runners and one about endurance eating...
here are links for anyone interested:
Endurance Eating
http://www.johnberardi.com/articles/.../endeating.htm
Eating Habits of Kenyan Runners
http://www.active.com/running/Articl..._the_world.htm