From grandmas to general physicians, plenty of people extoll the virtues of fiber. Research shows that a fiber-rich diet—with plenty of fruits, veggies, and whole grains—could stave off conditions like heart disease, stroke, and diabetes, but nobody knows exactly why dietary fiber (the indigestible portion of plant foods) is so good for us. Now, The New York Times reports that scientists think it could benefit the gut microbiome, which in turn affects our immune systems.

Our bodies don't produce enzymes that break down roughage, but some types of bacteria in our guts do. This prompted scientists to hypothesize that the gut microbes are intrinsically linked to fiber's wellness benefits. Two separate studies, both of which were recently published in the journal Cell Host & Microbe, put this theory to the test by examining mice on low-fiber diets.

The first study, led by Georgia State University scientist Andrew Gewirtz, looked at mice on a low-fiber, high-fat diet. In the second study, which was led by University of Gothenburg biologist Fredrik Bäckhed, the mice were transitioned from a high-fiber to a low-fiber diet.

In both studies, the mice developed unhealthy imbalances of different gut bacterial strains, shrunken intestines, and thinner protective mucus layers in their colons. This newly permeable membrane allowed bacteria to invade the organ, which in turn caused an immune reaction. And in addition to colon issues, the mice in Gewirtz's group ended up gaining weight, developing high blood sugar and insulin resistance, and experiencing severely reduced gut bacterial levels.

In contrast, when both groups gave mice doses of a type of fiber called inulin, the rodents ended up developing healthier guts, and were also healthier overall. This wasn't a coincidence, experts say.

"Diets that lack fiber alter the bacterial composition and bacterial metabolism, which in turn causes defects to the inner mucus layer and allows bacteria to encroach, something that triggers inflammation and ultimately metabolic disease," says Gunnar C. Hansson, a senior researcher who worked alongside Bäckhed, according to Science Daily. Yet another reason why it's important to kick leftover holiday cookies to the curb and start eating more leafy greens.

[h/t The New York Times]