Rural populations still have lots of the gut bacteria that break down cellulose. //

Amazingly, humans also play host to bacteria that can break down cellulose—something that wasn't confirmed until 2003 (long after I'd wrapped up my education). Now, a new study indicates that we're host to a mix of cellulose-eating bacteria, some via our primate ancestry, and others through our domestication of herbivores such as cows. But urban living has caused the number of these bacteria to shrink dramatically. //

Present-day hunter/gatherers and those living in a rural environment, both of whom eat very high fiber diets, still had about 20 percent prevalence of these cellulose-digesting species. By contrast, those in industrialized countries had a prevalence under 5 percent.

In general, the more fiber in the diet of a culture, the more diverse their cellulose-digesting bacteria were. So, their diversity in humans has been going down as more of our population has shifted into urban living. //

You could try to rebuild your gut biome. If you haven't already, try eating live culture (not pasteurized after fermentation) fermented foods (think kosher refrigerated picklees rather than sterile jars) - yoghurt, kefir, kombucha, sauerkraut, pickles, kimchee, etc - this will help. As many different types of these as possible - each type of food has different species/strains of microflora, so the more types you eat, the wider a population will end in your gut. Kefir & kombucha especially, as they have dozens of strains of bacterias and yeasts.

Taste can sometimes be an issue. I personally don't like the raw yeast taste that sometimes can be found with kefir and kombucha. Other people really don't like the vinegar-y taste of kombucha, pickles, etc. I hide the yeast taste of kefir & kombucha in daily smoothies. Have no solution for those philistines that don't like pickles or kimchee. //

This is not virtue or anything like it. No doubt much of it is the blind luck of genetic chance. But evidence increasingly suggests that having absolutely wallowed in a diverse biological environment for my first couple of decades is the smartest thing I've ever unknowingly done.