In my experience, those who can cover 15, 20 or even 25 miles over the span of 10-12 hours at a comfortable cruising speed have very little trouble eating a regular diet during the day, or even a slightly higher fat diet, with generous amounts of things like whole milk, cheese and nuts. I'm comfortable up to ~ 45% of calories from fat, but that's probably more than most people would want to eat.
Trail-running is a very different story. Even many of those who can withstand a pretty normal diet up to moderately high exercise intensity on a bicycle can't do it with the churning of a run.
Things certainly vary from person to person, but among the ten or so people with whom I've packed extensively, all of them can eat regular food while doing 15 mile trail days at moderate altitudes. (Yes, there may be a little self-selection at work.) Change the conditions and you change the story, at different rates for different people.
Change enough of them, and you'll come back to a more invariant outcome. The combination of high intensity exercise, churning/bouncing, altitude, and food different from what one's used to is a potent recipe for indigestion. :)