We’ve known for years the largely indigestible stuff was good for us. Now we understand what it does.
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We all have heard it many times—a diet with ample amounts of fiber leads to a slew of other benefits. Besides keeping you regular, it helps reduce the risk of type 2 diabetes, helps reduce pain associated with arthritis, and even helps boost immunity.

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But it hasn’t always been clear as to why a steady source of fiber is essential at all, given that the stuff is, by definition, the indigestible portion of fruits and veggies.

A new study finally puts the role that fiber plays into perspective, and it doesn’t have to do with calorie intake—but with our gut health instead.

It turns out the answer is that it’s food for our gut microbiome. That’s the name for the multitudes of bacteria in our intestinal system: which keep us healthy and aids in digestion.

It turns out those helpful little guys rely on dietary fiber for their food—and we benefit. While the gut might not have all the enzymes needed to completely break down each and every plant we eat, some of the bacteria within the gut actually have the power to break down these otherwise indigestible foods.

The New York Times reports that the discovery was made by Dr. Fredrik Bäckhed, who tested his theory on mice.

The mice who ate a diet poor in fiber had drastically fewer good—or bad—bacteria in their guts. What’s more, the bacteria there ended up closer to the animal’s intestine wall, causing ailments such as chronic inflammation. Within a few weeks, the mice developed higher blood sugar levels and began to put on weight.

The mice with high-fiber diets suffered from none of these problems.

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But there are indications that the situation is not too difficult to reverse. After several weeks, Dr. Bäckhed administered a boost of fiber called inulin to a group of rodents who were eating the low fiber diet. Despite the fact that their diet was poor, the gut biomes in the mice improved.

While there’s a clear link between dietary fiber and aiding your healthy gut, Dr. Bäckhed noted that the mice with added inulin weren’t completely resorted to perfect health. It turns out not all fiber is equal—and the many different kinds of microbes need fiber from a wide variety of sources to stay healthy.

“It points to the boring thing that we all know but no one does,” Dr. Bäckhed told the Times. “If you eat more green veggies and less fries and sweets, you’ll probably be better off in the long term.”