Dietary fibers can have different effects on human health

A new study published in Cell Host & Microbe highlights the association of fibers with the microbiota.

What is already known on this topic

High-fiber diets reduce the risk of heart disease by lowering cholesterol levels. Studies have shown that fibers, which are indigestible by humans, can promote a healthy gut microbiota once bacteria ferment them into short-chain fatty acids. However, little is understood about how individual types of fiber modulate the microbiota and in turn human health.

What this research adds

Researchers analyzed the effects of supplementing the diet of 18 adults with two common fibers: arabinoxylan, which is found in whole grains, and long-chain inulin, which is found in onions, chicory root, and Jerusalem artichokes. People who consumed high doses of arabinoxylan per day had substantially lower cholesterol levels compared to baseline, while those who consumed inulin didn’t. People who consumed high doses of inulin daily showed an increase in inflammation and in the levels of a liver enzyme associated with poor health. At lower doses, inulin was associated with an increase in the abundance of Bifidobacterium, a gut microbe that produces healthy short-chain fatty acids. However, responses were variable across study participants, and some participants saw little or no changes in their cholesterol levels.

Conclusions

The findings highlight the association of fibers with the microbiota and show that the benefits of fiber consumption vary across individuals and may depend on the type of fiber and the dose consumed. The study also suggests that high inulin consumption can have deleterious health effects.

Dietary fiber, or the indigestible part of plant foods, is often touted as beneficial for human health. But the health benefits of dietary fiber vary across individuals and may depend on the specific type of fiber and the dose consumed, according to a new study.

The findings, published in Cell Host & Microbe, highlight the association of fibers with the microbiota. They also suggest that high consumption of certain types of fibers can have deleterious health effects. “Our results demonstrate that the physiological, microbial, and molecular effects of individual fibers differ substantially,” says senior study author Michael Snyder at Stanford School of Medicine. “Further, our results demonstrate the tantalizing prospect of using targeted fibers, mediated by the microbiome, to drive health and systems biology in a predictable, personalized direction.”

Fiber is mostly found in vegetables, fruits, whole grains and legumes. Indigestible by humans, fibers can promote a healthy gut microbiota once bacteria ferment them into short-chain fatty acids. Studies have shown that diets rich in fiber reduce the risk of heart disease by lowering cholesterol levels, but little is understood about how individual types of fiber modulate the microbiota and in turn human health.

To address this question, Snyder and his colleagues assessed the effects of different dietary fibers on a group of adults.

Fiber benefits

The researchers recruited 18 people with an average age of about 57 years and body weights ranging from normal to obese. Then, they analyzed the effects of supplementing the participants’ diet with two common fibers: arabinoxylan, which is found in whole grains, and long-chain inulin, which is found in onions, chicory root, and Jerusalem artichokes. Inulin is often added to foods as a supplemental fiber source.

Study participants consumed 10 grams of fiber per day during the first week, 20 grams per day during the second week, and 30 grams per day during the third week. The researchers collected blood and stool samples, as well as clinical measurements, from the participants and conducted a wide variety of tests on them to assess the effects of each type of fiber on their health and gut microbes.

Compared to baseline, consumption of arabinoxylan altered the gut microbiota composition and, at high doses, it was linked to lower levels of low-density lipoprotein (LDL), the so-called ‘bad cholesterol’ that is linked to increased risk of heart disease. Consumption of arabinoxylan also led to an increase in bile acids, which likely contribute to the cholesterol reduction. Bile acids are known to correlate with numerous microbes: for example, ursodeoxycholic acid is associated with Bifidobacterium, which produces short-chain fatty acids and is recognized as a ‘healthy’ gut microbe.

Variable responses

Consumption of inulin was associated with changes in gut microbiota composition but didn’t influence LDL levels, the researchers found. People who consumed high doses of inulin daily showed an increase in inflammation and in the levels of a liver enzyme associated with poor health, compared to baseline. At lower doses, inulin was associated with an increase in the abundance of Bifidobacterium

“Several high-fiber foods have cholesterol-reducing effects, and our study suggests that these reductions may be driven by individual constituents of the mix of fibers in unrefined plant foods,” Snyder says.

However, participants’ responses to each fiber varied, and some participants saw little or no changes in their cholesterol levels, the researchers found. “Overall, our findings show that the benefits of fiber are dependent on fiber type, dose, and participant — a landscape of factors resulting from interactions between fiber, the gut microbiome, and host,” Snyder says. “These results have important implications in personalized response and interventions.”