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March 9, 2018 -- Scientists say they’ve found a direct connection between blood sugar and gut bacteria. By exploiting that connection with a very high-fiber diet, they’ve successfully treated a small group of people with type 2 diabetes.

The finding could be important not only for the 100 million American adults with diabetes or prediabetes, but also for anyone who’s trying to manage their weight.

Doctors and nutritionists have long known that fiber is important for good health. Studies have shown that people who eat diets high in fiber and low in saturated fat have lower risks for many chronic diseases, including heart disease, diabetes, and some intestinal problems like constipation and diverticulitis.

What hasn’t been well-understood is exactly how fiber gives you these benefits.

Evidence has been mounting that fiber plays a key role in the types of bacteria that thrive in our guts and how they work.

Last year, new research from the Finnish Diabetes Prevention Study showed that people who ate more fiber had more of an anti-inflammatory chemical in their blood called indolepropionic acid, which is made by gut bacteria. They were also less likely to go on to get type 2 diabetes.

The new study builds on that by showing how fiber helps grow bacteria in the gut that produce chemical signals that help to regulate appetite and blood sugar.

“Overall, this study adds to what we know about how important the gut microbiota is when it comes to the development of some chronic diseases, like type 2 diabetes,” says Vanessa de Mello Laaksonen, PhD, an assistant professor in nutrigenomics at the University of Eastern Finland, who was not involved in the research.

Could High-Fiber Diets Stave Off Diabetes?

About 1 in 11 Americans has diabetes. Type 2 diabetes is the most common form and is on the rise in the U.S. in both children and adults.

For the study, a team of scientists in China put 27 people with type 2 diabetes on a very high-fiber diet and followed them for 12 weeks, measuring changes to their blood sugar, and also to their gut bacteria.

They compared this group with 16 others who also had type 2 diabetes who got standard advice on exercise and eating right for diabetes along with a diet for the study that racked up roughly same number of calories, but had a lot less fiber.

The high-fiber group was getting a whopping 37 grams of fiber a day, while the group getting the average, healthy diet was eating about 16 grams of fiber a day -- which, by chance, is about what American adults average.

Both groups were also taking a medication called acarbose to help manage their blood sugar. Acarbose acts a bit like fiber in that it prevents some carbohydrates from being digested in the gut for energy. Before the study, all participants were weaned off any other medications to manage their blood sugar. If they used insulin, it was adjusted as needed throughout the study.

Both groups improved, but by the end of the experiment, the group on the very high-fiber diet was healthier than the group getting standard care. They had better control of their blood sugar, and they’d lost a bit more weight. About 90% of the high-fiber group achieved good control of their blood sugar -- keeping their hemoglobin A1c under 7%, the target recommended by the American Diabetes Association -- compared with just 50% of the group on a standard diabetes diet.

Here’s why that could be true.

All that fiber, which came from a wide variety of food sources, fertilized the growth of 15 strains of bacteria that produce certain short-chain fatty acids. Those acids act as both a fuel source for cells and as messengers.

In the new study, these short-chain fatty acids signaled the gut to make more glucagon-like peptide-1 (GLP-1) and peptide YY (PYY).

GLP-1 is a hormone that tells the body to make more insulin, while PYY dials down appetite. The two are critical in keeping blood sugar and weight under control.

Some diabetes drugs also work by increasing GLP-1. The increase in short-chain fatty acids also made the gut walls unfriendly to other kinds of bacteria that block GLP-1, boosting the effect.

“This is one benefit, one reason why high-fiber diets work,” says study researcher Liping Zhao, PhD, a professor of applied microbiology at both Rutgers University in New Brunswick, NJ, and Shanghai Jiao Tong University in Shanghai, China.

To make sure it was the change in gut bacteria that was driving the results -- and not something else -- the scientists then transferred the gut bacteria from their study participants to mice that were bred to be germ-free. Because they were bred not to have any of their own gut bacteria, scientists could later introduce bacteria and watch what happened as they started growing.

The mice that got the bacteria from the humans on the high-fiber diets had better fasting blood sugar levels than mice that got bacteria from people on the usual diabetes diet, even though they were eating the same chow.

The study was published in the journal Science.

Getting More Daily Fiber

The study authors say it would be difficult for the average person to copy their high-fiber diet. People in the high-fiber part of the study  ate a specially prepared gruel made from oats, white beans, yellow corn, red beans, yams, peanuts, and lotus seeds, along with two whole grains that are unfamiliar to Westerners -- green buckwheat and adlay, or Job’s tears. They also got more fiber in the form of powders that included bitter melon, kudzu starch, inulin, and resistant dextrin.

What’s important, researchers say, is the principle of all this: The good bacteria in our guts, the ones that keep us healthy, need a lot more fiber than we’re now giving them to thrive.

The U.S. Department of Agriculture recommends that healthy adults eat somewhere between 22 and 34 grams of fiber every day, according to the government’s dietary guidelines.

Most of us only get about half that much.

Kids and adults are so short on fiber in the U.S. that the expert panel that put together the most recent version of the dietary guidelines singled out fiber as one of four “nutrients of public health concern.”

Lacking fiber, the study suggests that the bacteria that help us regulate our appetite and control our blood sugar get out-competed by other bacteria that actually suppress those all-important signals.

The researchers who are leading the Finnish Diabetes Prevention Study think high-fiber diets are doable with a little planning.

“We think that some 30-40 grams per day may not be unrealistic if you use whole-grain products, fruit and vegetables several times a day, instead of low-fiber bread, sugar-containing juices, and meat,” says Laaksonen.

Sources of fiber in their healthy Nordic Diet focuses on whole grains like rye, barley and oats, berries, and beans and peas.

The study also shows something else that’s important to understand about gut bacteria: They’re already there, and they change in response to our diets.

Most studies have shown that probiotic supplements, which promise to deliver millions or billions of helpful bacteria to our guts, have few effects that usually don't last long. That’s because the bacteria that live inside us are there because of what we eat.

The best way to change them in a lasting way is to change their food source -- our diets.

“When you introduce a large amount of diverse fibers into the gut, it’s just like you threw some new nutrition into a pond. You disturb the system. Some members will take advantage of that to grow and increase their population levels. Some others may decline,” Zhao says.

“That’s exactly what happens in the gut ecosystem,” he says.

Gut bacteria, he says, live off two food sources -- the food we can’t digest, which is  fiber, and the products of digestion that are made locally in our intestines.

Lacking fiber, he says, this enormous ecosystem in our intestines, made up of an estimated 100 trillion cells, can become underfed in ways that encourage disease instead of health.

If you decide to boost the fiber in your diet, Zhao cautions that it takes some time for the bacteria to adjust -- about a month. That period of adjustment can come with some uncomfortable side effects like bloating and gas.

Show Sources

Liping Zhao, PhD, professor of applied microbiology, Rutgers University, New Brunswick, NJ; professor, Shanghai Jiao Tong University, Shanghai, China.

Vanessa de Mello Laaksonen, PhD, assistant professor in nutrigenomics, the University of Eastern Finland, Kuopio, Finland.

Science, March 8, 2018.

Scientific Reports: "Indolepropionic acid and novel lipid metabolites are associated with a lower risk of type 2 diabetes in the Finnish Diabetes Prevention Study."

Nutrients: "Ten-Year Trends in Fiber and Whole Grain Intakes and Food Sources for the United States Population: National Health and Nutrition Examination Survey 2001-2010."

American Diabetes Association, Statistics About Diabetes, accessed March 8, 2018.

U.S. Department of Agriculture, Dietary Guidelines for Americans, 2015-2020, 8th Edition.

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