High-Fiber Generates Gut Bacteria Good for Type 2 Diabetes

A select "guild" of gut bacteria responsible for the benefits of high-fiber diets in type 2 diabetes has been identified in a study in which those patients on the high-fiber diet showed improved control of HbA1c.

Effectively, eating the right dietary fibers may rebalance the gut microbiome and lead to reduced blood sugar and body weight, and may pave the way for a new nutritional approach to preventing and managing type 2 diabetes, say the researchers.

The specific bacteria thought to be effective produce short chain fatty acids (SCFAs).

"Targeted promotion of the active SCFA producers...via personalized nutrition may present a novel ecological approach for manipulating the gut microbiota to manage type 2 diabetes and potentially other dysbiosis-related diseases," write the authors led by Liping Zhao, PhD, from the School of Environmental and Biological Sciences, Rutgers University-New Brunswick, New Jersey.

The research was conducted in China, and was published in the March 9 issue of Science.

However, separately, in an article published online November 1, 2017, in Gut, a whole-grain diet failed to alter insulin sensitivity and the gut microbiome in healthy individuals at risk for development of metabolic syndrome. But the high-fiber diet did lead to lower body weight and less systemic low-grade inflammation.

Certain Fibers Could Become Part of the Treatment for Type 2 Diabetes

In their paper, Zhao and colleagues explain that gut microbes play a range of roles in response to food intake, and they suggest that chronic diseases, such as type 2 diabetes, may in part result from a deficiency in SCFA production from carbohydrate fermentation in the gut.

Prior clinical trials have shown that increased intake of nondigestible but fermentable carbohydrates (dietary fibers) helps alleviate type 2 diabetes, but treatment responses vary considerably. The authors note that improved understanding of how gut bacteria respond, both as individual species and via interactions with each other, may improve clinical outcomes of dietary interventions.

In this study, patients with type 2 diabetes were randomly assigned to either a control group, in which they received usual care comprising patient education and dietary recommendations (n = 16), or the high-fiber diet treatment group, in which they were prescribed a diet composed of whole grains, traditional Chinese medicinal foods, and prebiotics (n = 27).

Next, the researchers characterized the dynamics of the gut microbiota and its effect on patients' glucose levels, including cataloging bacterial genes to see how the increased dietary fibers altered the overall composition of the gut microbiota. Specifically, they examined the genes involved in the production of glucose metabolites.

HbA1c levels, the primary outcome measure, fell significantly from baseline in a time-dependent manner in both groups. However, from day 28 onward, there was a greater reduction in the high-fiber treatment group.

The proportion of participants who achieved adequate glycemic control (HbA1c < 7%) at the end of the intervention (12 weeks) was significantly higher in the treatment group (89% vs 50% in the control group; P = .005).

Of 141 strains of SCFA-producing gut bacteria identified, only 15 were promoted by consumption of the high-fiber diet, and these are the ones most likely to be involved in driving the health benefits seen. These 15 formed the so-called guild that boosts deficient SCFA production (mainly butyrate and acetate) from the gut ecosystem, say the researchers.

"[P]romoting this active group of SCFA producers not only enhanced a beneficial function but also maintained a gut environment that keeps detrimental bacteria at bay," Zhao and colleagues observe.

They add that considered as a functional group, the abundance and evenness of this guild of SCFA producers correlates with host clinical outcomes.

"Our clinical data indicate that increased availability of nondigestible but fermentable carbohydrates is sufficient to induce clinically relevant metabolic improvements in patients with type 2 diabetes."

In a news release from Rutgers University, Zhao points out that their study opens the possibility that, for patients with type 2 diabetes, "Fibers targeting this group of gut bacteria could eventually become a major part of your diet and your treatment."

Gut Paper: Benefits of Whole Grain Appear Independent of Microbiome Changes

Meanwhile, in the research published in Gut, 60 Danish adults at risk of developing metabolic syndrome were randomly assigned, in a crossover trial, to two 8-week dietary intervention periods comprising a whole-grain diet (mean, 179 g whole grain/day) and a refined grain diet (13 g/day), separated by a washout period of more than 6 weeks.

Intake of a diet rich in whole grains was associated with lower energy intake and body weight and a significant reduction in circulating markers of inflammation, such as interleukin 6 and C-reactive protein, report Henrik Munch Roager, PhD, from the National Food Institute, Technical University of Denmark, Kongens lyngby, and colleagues.

However, "in contrast to our hypothesis," compared with the refined grain diet, the whole-grain-rich diet did not significantly modify fecal microbiome composition, nor did it affect glucose homeostasis. So the health benefits of this specific diet rich in whole grains "appeared to be independent of changes in the gut microbiome composition within an 8-week diet study," they observe.

Nevertheless, "Whole grain consumption has beneficial effects on blood markers of subclinical inflammation in adults at risk of developing metabolic syndrome, and higher intake of whole grains should be encouraged in those at risk of inflammation-related diseases," they conclude.

One of the authors of the Gut paper was partly supported by an unrestricted grant from cereal Partners Worldwide, a joint venture between Nestlé Sa and General Mills Ltd. The other authors have disclosed no relevant financial relationships.

Gut. Published online November 1, 2017. Abstract

Science. 2018;359:1151-1156. Abstract

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