Fibers found in food are a motley bunch: some are long and straight, others branched and broad. They can have opposite charges and varied solubilities, and our foods contain them in assorted combinations and dosages. Despite this diversity, researchers often lump them together into one group—“dietary fiber”—which can muddy each fiber’s individual effects on the human body. 

Researchers at Stanford University School of Medicine have started to untangle this figurative fiber jumble: they investigated the medical impact of two purified fibers in a randomized, longitudinal, crossover study in 18 volunteers published online April 27 in Cell Host & Microbe. The team found that various fibers have different, sometimes opposing effects on human health and uncovered some of the mechanisms that explain why. 

Dietary fibers are carbohydrates from plants that humans cannot digest. Gut bacteria, however, can ferment them into short-chain fatty acids. The same bacteria also produce vitamins such as vitamin B and K and influence the immune system. Prospective cohort studies have found that higher dietary fiber intake in general is associated with a lower risk of heart disease, and it’s well-established that certain fibers, such as beta-glucans in oats, lower cholesterol. One of the fibers that the Stanford researchers looked at, arabinoxylan (AX), is common in whole grains (it helps form the cell walls of plants) and is also one of the components of Metamucil. Another fiber from the experiment, called long-chain inulin (LCI), is found in onions, chicory root, and Jerusalem artichokes, and is added as a supplement to many nutrition bars and other foods.

In the study, the researchers found that consuming 30 grams per day of AX significantly lowered cholesterol, while LCI didn’t; in fact, at higher doses (30 grams per day), LCI increased inflammation and elevated liver enzymes associated with poor health. 

“Inulin is one of the common fibers that’s added to foods as a supplemental fiber source,” says University of Minnesota nutrition scientist and registered dietician Abigail Johnson, who didn’t work on the study. “A lot of people will report having intestinal discomfort and increased flatulence and side effects from just consuming 6 or 7 grams of fiber from inulin, so it doesn’t surprise me that high doses of inulin could trigger symptoms.”

See “Another Way Fiber Is Filling

To determine how these two single fibers individually help or hinder the body, the team gathered 18 adults with an average age of about 57 years and body weights ranging from normal to obese. They gave each person escalating doses of AX or LCI (10 grams per day for one week, 20 g/day for the second week, and 30 g/day for the third week), followed by a six-to-eight-week washout period to allow the microbiome to return to normal. They then gave the participants the other type of fiber in the same series of escalating doses and then, following a second washout period, a fiber mix that included escalating doses of AX and LCI plus glucomannan (a fiber supplement made from the root of the konjac plant), resistant starch (a type of carbohydrate digested by gut bacteria found in seeds and grains, as well as raw bananas and potatoes), and acacia fiber (a powder made from the gum of the acacia tree, used as a fiber supplement). Participants regularly gave plasma, serum, and stool samples, and had their heart rates and blood pressure taken, among other clinical measurements. The researchers used these samples and information to conduct a wide variety of experiments that included the proteomics, metabolomics, lipidomics, and metagenomics analyses to see the effects of purified fiber on participants’ bodies and microbiomes—all of which “is super costly, with repeated measures and all of this omics work,” Johnson notes. “Given the level of detail that they got from these participants, it’s a high-quality study.”

The team found that AX supplementation was associated with microbiome changes, and at the highest dose, many (but not all) participants saw a significant decrease in their low-density lipoprotein (LDL) cholesterol—the kind of cholesterol linked to an increased risk of heart disease—compared to baseline. When they stopped taking AX, LDL levels went back to baseline. Bile acids, which help metabolize and remove cholesterol from the body, also increased while taking AX. The authors write that the changes to the microbiome likely contribute to the increased bile flow, since levels of many key bile acids correlate with levels of certain microbes. “Bile acids are actually synthesized from cholesterol, and so we think an increased synthesis of bile acids. . . drains the cholesterol pool,” Stanford geneticist and study coauthor Samuel Lancaster tells The Scientist.

See “Gut Microbes Need Fiber, Too

Inulin supplementation also altered the microbiome composition but didn't change LDL levels. In fact, the highest inulin dose, 30 grams per day, was associated with a systemic inflammatory response and an increase in the liver enzyme alanine aminotransferase, a marker of an unhealthy liver, in most participants. “It highlights the importance of studying these different fibers and their different properties,” says Lancaster, since some could be harmful at certain dosages. Every participant’s response to each fiber was unique, and ingesting a mixture of five different fibers did not have as potent an effect on cholesterol as did AX by itself.

“That repeated sampling over time, that gives them an opportunity to examine both individual responses as well as group responses,” says microbiologist Tom Schmidt at the University of Michigan, who was not involved with the study. “For instance, everyone had a unique cholesterol response when consuming arabinoxylan, but they also found common responses . . . to me that presents the intriguing possibility of making both generalized recommendations for fiber consumption, dietary fiber supplements, as well as personalized recommendations.”

What to eat, then? “Most people in the US may be getting around 10 to 15 grams of fiber a day, nowhere near the recommended amount of 14 grams per 1,000 calories,” says Johnson. For women, that works out to about 28 grams per day, and for men, roughly 32 grams.

“I would encourage people to aim for whole food sources of fiber first, potentially because of the other phenolic compounds that come along,” says Johnson, referring to the many health-promoting antioxidants and other phytochemicals found in plants. “Focusing on looking for food sources of arabinoxylan, like whole grains, might be a good first step. And then if you realize that that’s not doing enough for you, potentially considering adding a supplement would make sense.”