Jean Liew, Boston University
The numbers are clear: Americans aren’t getting enough fiber.
Research has consistently shown that high-fiber diets are linked to significant health benefits, including a lower risk of cardiovascular disease and colorectal cancer, as well as improved arthritis symptoms.
Despite these benefits, there remains a significant fiber gap in the typical American diet. The recommended daily fiber intake is 25 grams a day for women and 38 grams a day for men. From 2013 to 2018, only 4% of U.S. adults met their recommended adequate intake for dietary fiber. The average fiber intake was about 16 grams a day.
Adding fiber to your diet through eating more fiber-rich foods or by taking fiber supplements has become a viral health trend. At the same time, the 2025 update to the American dietary guidelines appears to downplay fiber’s role in a healthy diet, placing high-fiber whole grains at the bottom as the smallest part of the inverted food pyramid. This can be confusing, because the written guidelines themselves continue to highlight the importance of eating whole grains.
What does the science say about the health benefits of a high-fiber diet? And what steps can you take to incorporate more fiber into your diet? These and similar questions my patients have asked me as a physician-scientist prompted me to look further into the latest research on dietary fiber.
Basics of dietary fiber
Dietary fiber can be found in many sources, and the choices can be overwhelming.
Dietary fiber refers to carbohydrates your body can’t digest. Many whole foods are high in fiber. High-fiber foods include raspberries, avocados, oats and other whole grains, and lentils and other legumes.
Fiber also comes in supplements such as psyllium husk powder, known as the brand Metamucil, which comes from a plant called Plantago ovata. Isolated fibers such as inulin are now increasingly added to foods, including those marketed as high-fiber snack bars, keto-friendly carbs and even soda.
Moreover, not all types of fiber are created equal.
For example, psyllium is a viscous fiber, meaning it forms a gel when added to water. This gel can contribute to early satiety, meaning you feel fuller after eating. Viscous fiber can help lower cholesterol, improve blood sugar levels and blood pressure, lead to a small amount of weight loss, and make bowel movements more regular.
Other popular types of fiber are fermented, meaning they are eaten and processed by bacteria that live in your gut. These gut bacteria make proteins that can lower inflammation in the body. Inulin is one type of fermented fiber and can be extracted from chicory root and Jerusalem artichokes.
Nutrition experts recommend getting a variety of fiber-rich foods in your diet because of the added benefit of other nutrients in these foods, such as antioxidants.
Probiotics versus fiber
Besides fiber, adding probiotics to your diet may also benefit your gut health.
Probiotics are a category of fermented foods that still contain live microbes when you eat them. These include fermented foods such as sauerkraut, yogurt and kimchi, which are changed by the addition of live microbes, such as bacteria or yeast. Although the terms sound similar, these fermented foods that have “good” bacteria added to them are not the same as the fiber that gets fermented by bacteria already in your gut.
Think of it this way: Fiber provides the material for the gut bacteria to work with. Probiotics provide more “good” gut bacteria to join the community that already exists.
In a 2021 study, researchers examined the gut microbiomes of 36 healthy adults who either ate a diet high in fiber or a diet high in fermented foods. They found that those who ate a high-fiber diet had a gut microbiome that was more active in making the enzymes that break down fiber. That means these microbes got better at using the fiber they were fed to make byproducts helpful for the body.
In contrast, those who ate a diet high in fermented foods had a gut microbiome that was more diverse, meaning they had many different species of bacteria in their guts. They also had lower levels of inflammation. Many chronic conditions can raise levels of inflammation in the body, and exposure to inflammation can put stress on the body, contributing to issues such as cardiovascular disease.
The results of this small study point to the possibility that combining high-fiber and fermented foods in your diet could synergize their health benefits in nourishing the gut microbiome.
How can you get more fiber in your diet?
With so many options available, what’s the best way to increase your fiber intake?
Some barriers to eating enough dietary fiber include cost, accessibility and tolerability. Some high-fiber whole foods, such as berries, can be expensive. Highly fermentable fibers, such as inulin, often cause bloating and discomfort, especially if you aren’t used to eating them. This can be a turnoff for high-fiber drinks and snacks, since they contain so much fiber in a single serving.
There are many ways to increase the amount of fiber you eat on a daily basis. The general recommendation is to do it slowly to allow your body to get used to the higher amount of fiber and to stay hydrated so your body can use the water along with the fiber for optimal bowel health.
Most importantly, try a variety of fiber sources to figure out what’s right for you. Some people will be able to reach the recommended fiber levels by eating whole foods alone; others will add supplements or eat foods with added fiber. There is no one-size-fits-all healthy diet.
Unlocking the full benefits of fiber
More focused research is needed to understand more about the types of fiber that are beneficial to health, in what doses, and in what combinations. But these nutrition studies can be tricky.
My research focuses on finding links between fiber and different diseases in observational data and figuring out how to test them in a clinical trial. For example, one large observational study found an association between higher self-reported intake of whole grains and lower risk of arthritis. But researchers still need to test this connection in clinical trials, which requires deciding the right type and amount of whole grains to recommend, as well as the right length of time to test this diet.
While there have been many small clinical trials on fiber, they have not been big enough, specific enough or broad enough to provide answers that can be put into guidelines for everyone. Adequate funding has been one major hurdle to starting such large, well-designed trials.
At the same time, scientists already know many confirmed benefits of dietary fiber, and the gap lies in educating the public and making changes so that foods high in fiber are affordable and accessible to more people.
Jean Liew, Assistant Professor of Medicine, Boston University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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