Plenty of fiber: That’s long been the recommendation for a healthy diet. But why?

The main rationale has been that fiber is made up of undigestible bulk that prevents people from eating unhealthy food — and helps keep the digestive tract regular.

But new research suggests that dietary fibers actually play a critical role in feeding the trillions of microbes that reside in our bodies, known collectively as the microbiome. And that specifically for people with type 2 diabetes, a high-fiber diet along with a favorable gut microbiome can keep patients’ blood sugar and body weight under control.

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Researchers in China were able to pinpoint the specific “good” bacteria that ferment fiber into acids, and ultimately improves insulin regulation. These bugs, according to lead investigator Liping Zhao, chair of applied microbiology at Rutgers University, create an acidic microenvironment in the gut that helps beneficial, blood-sugar-lowering bacteria proliferate — and might even keep pathogens at bay.

The research was published Thursday in Science.

“The study really gets at the mechanistic reasons of why these fiber-rich, plant-based diets may be helpful, especially in patients with type 2 diabetes,” said Dr. Clare Lee, an endocrinologist at Johns Hopkins University who also studies the link between diabetes and the microbiome. She was not involved in the study.

“It’s an exciting step towards understanding potential mechanisms that can help us prevent and treat diabetes,” she said.

Fiber, of course, has long been shown to improve blood sugar, and diabetics are encouraged to eat plenty of it. But the benefits of fiber may be much more complex than scientists previously understood.

“Leafy greens, whole grains, fruits with fibers: There’s lots of evidence that microbes will digest foods our bodies can’t,” said Dr. Lynn Bry, director of the Massachusetts Host-Microbiome Center at Harvard University, who was also not involved in the research. “When you eat, you’re not only feeding yourself, you’re feeding your microbiota.”

In a randomized, controlled trial, 27 participants with diabetes were given large quantities of dietary fibers — including whole grains, traditional Chinese medicinal foods, and prebiotics. The 16 participants in the control group had about the same caloric and nutrient intake, but not the supplemental fiber. Both sets took the diabetes drug acarbose.

Study participants on the fiber-rich diet saw a substantial decrease in their HbA1c levels (the metric that gauges a person’s blood sugar levels over several months) and lost more weight. Because the only difference between the two groups was their level of fiber intake, the benefits can be solely attributed to diet, Zhao said.

His team then delved deeper — examining the microbial ecosystem of the participants’ guts before and after the dietary intervention. They found that as bacteria fermented the dietary fiber, the composition of the gut microbiome changed.

When bacteria from a person on a fiber-rich diet was transplanted into a mouse’s gut, its ability to control its blood sugar actually improved. But when that same person’s pre-treatment gut bacteria was transplanted to a mouse’s gut, its blood sugar worsened.

The gut microenvironment changed, too: Dozens of gut bacteria are able to digest dietary fibers, and produce short-chain fatty acetic acids and butyric acids as a byproduct. But only 15 strains can actually survive in the newly acidic environment that they create. Zhao dubs these “foundation species” that might have some therapeutic use.

The study also affirms a long-standing hypothesis that short-chain fatty acids like acetate and butyrate do indeed play a role in regulating blood sugar in humans. It’s been shown to some degree in animals, particularly cows — but the study marked the first time this mechanism has been shown in humans.

The study, while small, was still sufficiently robust, Bry and Lee said — and suggests that changes to the microbiome could substantially impact diseases like diabetes. Zhao, for his part, thinks the next step for this research is to see if changes to the microbiome could even reverse the course of diabetes.

“There are many ways to alter microbiota,” Bry said. “But the easiest way is your diet.”

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