You don’t just need more fiber—you need different types, experts say
From oats to beans to chicory root, each type of fiber acts differently inside the body. New research is revealing how fiber diversity shapes the microbiome—and why more isn’t always better.
Despite decades of dietary guidance, fiber remains one of the most consistently under-consumed nutrients in the U.S. According to the U.S. Department of Agriculture, as many as 97 percent of Americans consume less than the recommended daily intake of roughly 25-34 grams of fiber for adults.
“It's a nutrient of public health concern,” says Hannah Holscher, professor of nutrition at the University of Illinois Urbana-Champaign. “The vast majority of people aren't eating it…[and] it’s likely contributing to chronic diseases.”
But that doesn’t mean that maxing out on fiber supplements is the solution. Experts say its health benefits vary depending on the type of fiber consumed, its form, and how it interacts with the digestive system. Here is what you need to know.
Not all fiber is created equal
Despite what nutrition labels may suggest, dietary fiber is not a monolithic nutrient.
“Fiber is really an umbrella term that encompasses a lot of different types of complex carbohydrates,” Holscher says. “Because there are so many different types of fibers, they act in a lot of different ways depending on their origin.”
Part of the confusion is that “fiber” on a nutrition label doesn’t always reflect how that fiber behaves in the body. Nutrition scientists generally classify fiber into two main categories—soluble and insoluble—based on how it behaves in water and in the gut. Soluble fiber, such as beta-glucans found in whole grains like oats, dissolves in water, forming a viscous gel. This substance binds to cholesterol, slowing its digestion, increasing satiety, and reducing blood sugar spikes.
Insoluble fiber, like the cellulose found in nuts and legumes, doesn’t dissolve. Instead, it has a laxative effect, helping waste pass through our digestive tract more efficiently. Though they function differently, both soluble and insoluble fiber contribute to a healthy diet.
Beyond digestion, fiber plays a vital role in preventing chronic disease by cultivating our gut microbiome. Certain fibers—primarily soluble—are fermentable, meaning they are broken down by the bacteria in our large intestine.
Not only do microbes help us break down otherwise indigestible fibers, such as inulin found in chicory root, but they also produce metabolites, primarily short-chain fatty acids (SCFAs), which improve nutrient absorption, lower inflammation, and regulate the immune response.
“A lot of so-called Western diseases are this misregulation of our immune system towards a pro-inflammatory state,” says Erica Sonnenburg, senior research scientist in the Department of Microbiology and Immunology at Stanford University.
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Dietary fiber deficiency increases the risk of developing chronic inflammatory diseases, like Crohn’s disease or ulcerative colitis, as well as autoimmune diseases and even allergies. In contrast, eating a high-fiber diet can support a healthier body weight, reducing rates of obesity, cardiovascular disease, and colorectal cancer.
Emerging research suggests that fiber may also help mitigate the adverse effects of microplastics in our bodies. According to Katrine Whiteson, a professor and microbiome scientist at UC Irvine, “fiber will protect you from all different kinds of environmental toxicities” by creating a sort of matrix that will absorb and excrete them.
The exact mechanisms by which different types of fiber interact with the body remain an active area of research. But Holscher advocates for “eating a diverse array of plant-based fibers”—fruits, vegetables, whole grains, nuts, and legumes—to maximize the benefits.
Some studies suggest that eating a wider range of dietary fiber sources may support a more diverse gut microbiome. One large-scale study found that eating more than 30 different types of plants each week was associated with higher gut microbial diversity.
Still, Whiteson says the causation isn’t clear, noting that people in some resource-limited regions consume relatively few fiber sources yet maintain highly diverse gut microbiomes. “Like, the ones I got to work with in remote Ecuador, they have amazingly diverse gut microbiomes.”
At the end of the day, Whiteson believes the goal should be to “get more fiber and not be a perfectionist about it.”
Can supplements help?
It’s no coincidence that fiber intake has declined in modern American diets.
“In the industrialized world, as we’ve been eating more and more processed foods, our fiber consumption has really plummeted,” Whiteson explains. Processing often involves stripping out the fiber-rich ingredients—such as removing bran from wheat to produce white flour—to improve texture, taste, and shelf life.
To help close our dietary fiber gap, a surge of fiber supplements, fortified foods, and functional beverages have recently been hitting shelves. Supplements—typically capsules or gummies made from fibers like psyllium husk, methylcellulose, or inulin—promise to support digestive health, lower cholesterol, and aid satiety. However, studies on the benefits of supplements have shown mixed results, highly dependent on the type of fiber and the individual participants.
“Part of why we don’t have a good answer is everybody’s microbiome is unique,” Sonnenburg says. With a lack of human clinical trials, she adds, “there’s not great information about the efficacy of these supplements.”
While convenient, concentrated fiber can irritate the digestive system, causing bloating or diarrhea. Because liquids pass through the GI system faster than foods, functional beverages containing fiber are more likely to cause sensitivities than eating whole foods, Holscher says.
Fiber in supplements or functional foods also lacks the beneficial nutritional complexity found in plant-based fiber. While simple fibers like inulin—commonly added to prebiotic drinks—can be beneficial, they don’t give your gut the same benefits of a more complex fiber, according to Whiteson.
Fiber supplements can be helpful, “if you’re not going to get fiber otherwise,” Whiteson says. But experts agree that it’s best, whenever possible, to make plant-based foods the primary source of fiber.
“When you think of a plant-based food, it’s going to have this nice package of components,” including phytonutrients, protein, amino acids, and carbohydrates, Holscher says. Unlike pure fiber supplements, she explains, “plant-based foods, fruits, vegetables, whole grains, nuts, and legumes, seeds—[aren’t] just going to be providing one important nutrient.”
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From an evolutionary perspective, Sonnenburg says it’s unlikely that an isolated fiber could mimic the complex nutrients that our gut bacteria have adapted to digest. But without robust clinical trials, it’s hard to say definitively whether supplements can achieve the same benefits as plant-based fiber sources.
So how should you increase fiber intake? Experts recommend a gradual approach, paired with adequate hydration to support digestion. While fiber benefits most people, those with certain gastrointestinal conditions may need a more cautious or individualized plan.
Whiteson advocates for plant-based fiber sources that are cheap, accessible, and tasty; some of her favorites include frozen berries, dried beans, avocados, and chia seeds. “Everybody needs to find something that they actually like, that is doable for them, that they can do every day that will fill their fiber gap,” she says.
