Or microbes will eat out your stomach lining. Or something like that.

Fiber has long been linked to better health, but new research shows how the gut microbiota might play a role in this pattern. One investigation discovered that adding more fiber to the diet can trigger a shift from a microbial profile linked to obesity to one correlated with a leaner physique. Another recent study shows that when microbes are starved of fiber, they can start to feed on the protective mucus lining of the gut, possibly triggering inflammation and disease.

...

As gut microbes are starved of fermentable fiber, some do die off. Others, however, are able to switch to another food source in the gut: the mucus lining that helps keep the gut wall intact and free from infection.

In a recent study presented at the Keystone meeting, Eric Martens of the University of Michigan Medical School, postdoctoral researcher Mahesh Desai and their colleagues found that this fuel switch had striking consequences in rodents. A group of mice fed a high-fiber diet had healthy gut lining, but for mice on a fiber-free diet, "the mucus layer becomes dramatically diminished," he explained at the meeting. This shift might sometimes have severe health consequences. Research by a Swedish team, published last year in the journal Gut, showed a link between bacteria penetrating the mucus layer and ulcerative colitis, a painful chronic bowel disease.

A third group of mice received high-fiber chow and fiber-free chow on alternating days--"like what we would do if we were being bad and eating McDonald's one day and eating our whole grains the next," Martens joked. Even the part-time high-fiber diet was not enough to keep guts healthy: these mice had a mucus layer about half the thickness of mice on the consistently high-fiber diet. If we can extend these results to humans, he said, it "tells us that even eating your whole fiber foods every other day is still not enough to protect you. You need to eat a high-fiber diet every day to keep a healthy gut." Along the same lines, Swanson's group found that the gut microbiomes of his adult subjects reverted back to initial profiles as soon as the high-fiber bars were discontinued.

I guess pandas never suffer from ulcerative colitis, what with all that bamboo they can't actually digest. *shakes fist* Pandas!!