I went on a 28-hour overnighter a few years ago with my only food consisting of peanuts, oats, and homemade sport drink mix. It was a fast and light trip: more jog than hike and more bivouac than camp. The weather was cooler than expected so I didn't use much of the drink mix but I finished the rest of the food just as I walked to the car. My hope was that the oats would supply enough fiber to allow me to "lighten my load" in the morning but had no such luck. A more well-rounded dinner the night before might have led to more spring in my step the next day and a net wash in average total weight (including body) but all in all it was a decent experiment. Since then I've gone pretty heavy on the nuts, mostly smoked almonds now, and haven't gotten sick of eating quite a lot of them for trips up to 5 days.
The experience got me thinking about the feasibility of foraging for fiber. I haven't had much luck finding enough wild food to be worth my while: prickly pair "nopales" are about the only thing that I can reliably positively identify and find in quantity. I'm wondering if something with minimal caloric value but high fiber, such as grass maybe, could serve an important digestive role without much risk of catastrophic mis-identification.