• Posted August 4, 2022

Which Source of Dietary Fiber Works Best?

If your diet is low in fiber, you can do your gut some good by adding more -- regardless of the fiber source, new research suggests.

Many people know fiber as the nutrient that keeps you regular. But it's also a key player in the makeup of the gut microbiome -- the vast collection of bacteria and other microbes that reside in the digestive tract.

When bacteria in the gut break down the fiber, they produce certain short-chain fatty acids that are the main source of nutrition for cells in the colon. Research also suggests the fatty acids play a role in regulating functions as vital as metabolism and immune defenses.

But it hasn't been clear whether any one type of fiber supplement is better for people's gut bacteria than others.

In the new study, researchers tested three common fiber-powder supplements: inulin (an extract of chicory root), wheat dextrin (in this case, the brand Benefiber), and galactooligosaccharides (Bimuno).

They recruited 28 healthy adults and gave them each of the supplements to use for one week, with one week off in between each product.

Overall, the study found, no one supplement outperformed the others in changing consumers' gut microbiome. Each supplement boosted the production of butyrate -- an important fatty acid that helps control inflammation.

If a study participant churned out more butyrate after using one fiber supplement, they responded just as well to the other two, said Jeffrey Letourneau, a doctoral student at Duke University in Durham, N.C., who was part of the research team.

But while the fiber supplement didn't matter, the person did: Supplements revved up butyrate production only in participants who normally ate few fiber-rich foods, the study found.

That does make sense, according to Letourneau: It's the "low fiber consumers" who would be making a substantial change by adding a daily fiber supplement.

But that term also describes most Americans, he pointed out.

Experts generally recommend that women strive for 25 grams of fiber per day, while men should aim for 38 grams. The average U.S. adult, however, consumes only in the neighborhood of 30% of those amounts.

And in the grand scheme of human history, Letourneau said, even the recommended fiber amounts probably fall far short of what our ancestors downed. He pointed to research showing that members of the Hadza tribe, in Tanzania, still consume a whopping 100 to 150 grams of fiber a day -- owing to diets high in foods like berries, honey and tubers.

So the new research -- published July 29 in the journal Microbiome -- emphasizes the importance of getting more fiber, whatever the source.

The study focused on supplements, in part, because they are easy to study, Letourneau said. Researchers gave each participant pre-measured individual doses of the fiber supplements, so they simply had to dump the powder into a drink once a day.

Those doses amounted to 9 grams of either inulin or wheat dextrin, or 3.6 grams of galactooligosaccharides, per day.

Fiber from food, however, would be preferable, according to a registered dietitian who was not involved in the study.

Plant foods provide not only various forms of fiber, but also a range of vitamins, minerals and beneficial "phytochemicals," said Nancy Farrell Allen, a spokeswoman for the Academy of Nutrition and Dietetics and an instructor at Rosalind Franklin University of Medicine and Science in North Chicago, Ill.

"I believe that food is the best way to meet fiber needs," she said.

Farrell Allen pointed to a long list of fiber-rich foods, including an array of vegetables and fruit; bran cereals and whole grains like farro; "pulses" such as lentils and chickpeas, and legumes like soybeans and peanuts.

She also had a caution on fiber supplements: They can cause unpleasant gas, bloating and prolonged indigestion.

Letourneau agreed that whole foods have "real benefits" that cannot be captured in a supplement. But given the importance of fiber -- and the dearth of it in Americans' diets -- he supports getting more of it, however you can.

"My attitude is: Whatever you can fit into your life, in a sustainable way, is good," Letourneau said.

In some more good news, it doesn't take long for any added fiber to make a difference to your gut bacteria. In a separate study, the Duke researchers found that fiber supplements began to alter people's gut bacteria within a day -- changing the microbiome makeup and activity.

"Things do seem to change really quickly," Letourneau said.

The research was funded by the U.S. National Institutes of Health and other government and foundation grants.

More information

Harvard University has more on fiber and health.

SOURCES: Jeffrey Letourneau, BS, doctoral student, molecular genetics and microbiology, Duke University, Durham, N.C.; Nancy Farrell Allen, MS, RDN, spokeswoman, Academy of Nutrition and Dietetics, Chicago, and nutrition instructor, Rosalind Franklin University of Medicine and Science, North Chicago, Ill.; Microbiome, July 29, 2022, online; ISME Journal, July 23, 2022