Experts recommend that adults eat 28 grams of fiber a day. On average, we get 17 grams. What’s more, much of the fiber in ultra-processed foods may not deliver the benefits you expect. Here are 5 fiber surprises.

1. Fiber targets aim at heart health. 

Why was the fiber target (or Daily Value) on Nutrition Facts labels set at 28 grams? 

“Most people would expect that the number was designed to promote regularity,” says Joanne Slavin, professor of food science and nutrition at the University of Minnesota. 

(Slavin has recently received research funding from Taiyo, which sells guar gum, and she serves on Olipop’s advisory board.) 

Instead, the target was aimed at preventing heart disease. 

Why? That benefit was backed by the strongest evidence. 

“People who ate more fiber had protection against heart disease in large prospective cohort studies,” explains Slavin, referring to research that tracks thousands of people for years. 

In other studies, people who ate more fiber had a lower risk of constipation, diverticular disease, type 2 diabetes, obesity, and even colorectal cancer. 

“But in the 1980s, the National Cancer Institute funded intervention studies to test whether high-fiber foods could prevent polyps that turn into colorectal cancer,” notes Slavin. “And those trials came up empty.” 

Researchers aren’t sure why. A smaller trial is now testing whether eating 1½ cups of beans a day can curb inflammation and cell proliferation that could lead to colorectal cancer. 

While further studies are underway, Slavin advises people to eat the kinds of foods that were linked to a lower risk of heart disease. 

“We need a variety of fiber from whole grains, beans, and other real foods, not just one fiber added to soda pop or snack foods.” 

Needless to say, the food industry has a different take. 

2. Added fibers may not do what you expect. 

Looking for fiber to make you feel fuller, stay regular, or lower your blood sugar or bad cholesterol? 

Don’t assume that just any processed food that’s been pumped up with fiber can help. 

“A 2001 report introduced the idea that there’s dietary fiber and there’s added fiber,” says Slavin, a member of the panel that wrote the report. 

The intact “dietary” fiber that occurs naturally in grains, beans, vegetables, and other plant foods is a mix of fibers. But “added” fiber can be a single type of fiber that’s isolated from foods or made in a lab. 

“The FDA made sure that the added fibers that they were approving had some physiological benefit in a human study,” says Slavin. 

But that may not be the benefit you’re looking for. 

Take inulin (aka chicory root fiber or extract). It’s used in most Fiber One bars, brownies, and donuts, as well as in thousands of other foods. 

“Inulin got accepted because it raises calcium absorption,” says Slavin. (See “Why was that added fiber approved?”) 

(Too bad the FDA didn’t require Nutrition Facts labels to list “Added fibers” on its own line and to disclose the benefit that got each fiber approved.) 

“Only cellulose got through on laxation,” says Slavin. “It takes an insoluble fiber like that to increase stool weight or do much for constipation.” 

Exception: Psyllium, a soluble fiber that’s sold as Metamucil and is added to Kellogg’s All-Bran Buds cereal, is good for regularity. 

But don’t expect Metamucil’s Fiber Gummies to help. They’re made with inulin and soluble corn fiber, not psyllium. Ditto for Benefiber gummies. 

Nature Made and MiraFIBER gummies (“from the makers of MiraLAX”) are also made with inulin. Olly uses inulin’s shorter-chain cousin, fructooligosaccharides. 

3. Added fiber gives foods a health halo. 

“The U.S. has one of the world’s biggest fiber gaps,” said McKenna Mills, a senior technical specialist at Cargill, in a 2023 brochure from the agribusiness giant. 

“Less than 3 percent of the population meets the FDA’s fiber intake guidelines,” noted the brochure. 

Its title: “Baked-in Goodness: Using soluble fiber to deliver indulgence and health in the bakery aisle.” 

Why bother with beans and veggies, when you’ve got the bakery aisle? 

“Muffins, cakes, cookies, bake-stable fillings, even icings—there’s really no limit to where this can go in the bakery space,” said Mills. 

Cargill’s soluble corn fiber can replace 30 percent of the sugar in baked goods, says the company. 

And it doesn’t take much for companies to be able to slap a “good source of fiber” claim on a cookie label. 

“Most bakery products inherently have some fiber from flour or other grains in the formula,” says Cargill. 

“As a result, achieving a ‘good’ source of fiber claim (2.8 grams of fiber per serving) is often a relatively easy task.” 

Just what Americans need: a claim to make us eat more sweets. 

ADM, another agribusiness titan, pitches Fibersol, its soluble corn fiber, as a perfect ingredient for ice cream, candy, cookies, and bars. 

(On ingredient lists, soluble corn fiber may show up as “soluble vegetable fiber (corn),” “resistant maltodextrin,” or just “maltodextrin.”) 

Fibersol can “help relieve occasional constipation” and “improve stool consistency” in “selected studies,” says ADM’s website. 

Selected, indeed. When the FDA evaluated soluble corn fiber in 2016, it improved laxation in only 1 out of 5 studies. Instead, the agency approved soluble corn fiber for boosting calcium absorption. Apparently, that’s not the strongest sales pitch. 

4. Excess inulin may boost gas and inflammation. 

“Diets high in fiber can help keep your digestive system on track,” says Fiber One’s website.

That’s true for some fibers, but not for inulin, the main fiber in most Fiber One sweets. 

“Inulin has no effect on laxation,” says Slavin. “And some people are very intolerant to it.” 

Ditto for fructooligosaccharides and galactooligosaccharides, she adds. 

“They’re all very fermentable. And when they get fermented, short-chain fatty acids get produced.” 

So does gas. 

In a recent study, researchers gave 18 people a hefty 10, 20, or 30 grams of inulin for one week each. Bloating and flatulence increased at all three daily doses.

At 30 grams, the researchers also saw a rise in inflammatory markers. And in three people, a liver enzyme (alanine aminotransferase, or ALT) spiked outside the normal range, leading the scientists to quickly stop the inulin. 

You won’t get 30 grams of inulin in one serving of any food. But many fiber gummies supply 5 to 8 grams of inulin or fructooligosaccharides per serving. And it’s easy to overdo gummies. 

“Inulin is added to a lot of foods, and you can’t always tell how much,” says Andrew Gewirtz, professor at the Institute for Biomedical Sciences at Georgia State University. 

And the popularity of obesity meds (which can cause constipation) is spurring companies to add fiber to even more foods. 

Gewirtz’s advice: “For now, people would be better off getting their fiber from a variety of diverse plant foods.” 

5. How fiber affects the microbiome is still unclear. 

Keeping the microbes in your gut healthy matters. 

“People with more diverse gut microbiomes are often healthier,” says Justin Sonnenburg, professor of microbiology and immunology at Stanford University. “They have lower markers of inflammation and better metabolic markers.” 

How might microbes help? 

“The microbes produce short-chain fatty acids, which dampen inflammation,” says Sonnenburg. 

And—unlike protein, fat, and sugars, which are absorbed in the small intestine—the intact fiber in plant foods reaches the large intestine, where it can feed those microbes. 

“If we’re not feeding the microbes, they may use the mucus lining of our gut as a backup food source,” notes Sonnenburg. “If the mucus erodes and the microbes get too close to the intestinal cells, that can incite inflammation.” 

So Sonnenburg teamed up with Christopher Gardner, professor of medicine at the Stanford University School of Medicine, to test two strategies that might lower inflammation and boost the diversity of microbes in the gut. 

“We randomly assigned 36 people to eat either as much fiber-rich food or as many servings of fermented foods like sauerkraut, kimchi, and yogurt as they could,” says Gardner. 

“After ramping up for four weeks, the fermented-food group was eating five or six servings a day, and the fiber folks went from about 22 to 45 grams of fiber a day.” And both groups kept it up for another six weeks. 

The results were a surprise. 

“In the fermented- food group, microbial diversity increased and 20 inflammatory markers decreased,” notes Gardner. “So that was good news. But it looked like nothing was going on in the fiber group.” 

Overall, neither microbial diversity nor inflammation changed in the fiber eaters. But a closer look revealed that inflammation dropped in some and rose slightly in others, says Gardner. 

“So when we put them all together, they canceled each other out.” 

What might explain those divergent responses? Some participants may have started the study without enough of the type of gut microbes that could break down the incoming flood of fiber. 

“Those with a rise in inflammation started the study with low microbial diversity,” says Gardner. “On the high-fiber diet, they were churning out lots of fiber in their stool, as if to say, ‘Whoa, I can’t handle this fire hose of fiber.’” 

In contrast, those with a drop in inflammation started the study with more microbial diversity. 

“Maybe they had less fiber in their stool because their microbes were chewing it up for fuel,” Gardner suggests. 

But those “exploratory” findings need to be tested in a new study. 

“Maybe someone who hasn’t been eating much fiber needs to ramp up more slowly, or stay on the high-fiber diet for a longer period of time before their gut microbes can handle the extra fiber,” says Gardner. 

That’s only one open question. “There are hundreds of types of microbes in your colon, and hundreds of inflammatory markers,” notes Gardner. “We need to know which ones matter most.” 

He and Sonnenburg have now launched a new trial. 

“We’ve enrolled 130 pregnant women at the end of their first trimester,” says Gardner. “For their last two trimesters, we’re putting them on the high-fiber diet, the fermented-food diet, both, or neither.” 

“And the women will have an option to continue for a month after the baby’s birth. That will give us a chance to see what happens if people eat the diets for longer.” 

The study’s key goal: “We want to see to what extent the maternal microbiome gets picked up by the child, so we’re going to look at the kids’ poop for five years,” says Gardner. 

His bottom line: “You can’t go wrong with whole foods. We don’t know exactly which fibers are best for your microbiome. But with beans, grains, fruits, vegetables, nuts, and seeds, you get different types of fibers for different microbes, so you’re hedging your bets.”