What Is Fiber? Everything You Need To Know

What you need to know about the benefits of fiber.

Back in the good ol’ days, eating more fiber meant forcing down little bits of cardboard masquerading as cereal. But we’re not here to tell you that you have to eat cardboard to have good health. In fact, getting enough fiber can be both delicious and fun. For example, high fiber foods include beans, broccoli, berries, avocados, dried fruit, and even popcorn — and these foods comprise American favorites such as chili, go great on the side with steak, or work as easy-to-reach-for snacks. 

But to fully reap the benefits of a high fiber diet, you first need to understand what fiber is and why not all fiber is created equal. We’ll help you navigate what the research says about fiber, how it works, and what clinical advances say about using fiber to treat gut disorders.

How is dietary fiber beneficial to health?

Fiber is a key component of a healthy diet and has been shown to lower the risk of death from cardiac disease, stroke, type 2 diabetes, and colon cancer. It also lowers cholesterol and inflammation and helps people maintain a healthy body weight. 

Yet, 95% of Americans barely eat half of the daily amount of fiber recommended by the USDA. Hippocrates once said, “Let food be thy medicine.” It’s time to start eating the next superfood: fiber.

What is fiber?

Fiber is a type of carbohydrate that your body cannot digest. It comes from plants (i.e., fruits, vegetables, grains, and legumes), so the more plants you eat, the more fiber you eat. And although you can’t digest it, the bacteria living in your gut can. Some byproducts of bacterial fermentation (digestion) of fiber include different gases, which is why beans have earned the nickname “the musical fruit.”

Are all types of fibers created equal? 

If you’re confused about conflicting information around fiber — what type of fiber you should eat, how much, why, and for what — you’re not alone. Fiber research is complicated because there are so many factors that can affect how fiber works, including whether the fiber was synthetically produced or came from a food. 

Dietary fibers are those that are found in whole foods: Types of dietary fibers include whole grains, legumes, fruits, and vegetables. Isolated and extracted fibers can come from foods, but are typically consumed as dietary supplements.

The three main characteristics of all fiber are:

  • Solubility: does the fiber dissolve in water?
  • Viscosity: does the fiber thicken or form a gel when mixed with water?
  • Fermentability: is the fiber digested by bacteria in your GI tract?

These characteristics can vary widely from fiber to fiber, and they determine how each type of fiber works inside the human body. For example:

  • Soluble fibers, which include gums, pectins, and fructans, are digested in the small intestine and aid in nutrient absorption.
  • Insoluble fibers, such as lignin and cellulose, regulate digestion and help you avoid constipation.
  • Viscous fibers, which also are typically soluble fibers, help control blood sugar spikes after eating. They also help you feel full.
  • Nearly all fibers are fermentable, but some are more readily fermented than others. The byproducts of fermentation include beneficial molecules such as short-chain fatty acids (SCFAs), which support a healthy digestive system, glucose control, and immunity.

What are the health benefits of fiber?

There are numerous health benefits to including more fiber in your diet:

1. More fiber keeps your gut microbes happy.

Microbes in your gut, collectively known as your microbiome, influence a host of bodily functions, helping to regulate everything from your immunity and mood to your appetite and weight. Research has shown that ensuring that your gut bacteria get enough fiber has big consequences for your health:

  • Fiber has been shown to nourish microorganisms so that they continue to grow and diversify. And the more varied your gut bacteria are, the more they contribute to heightened immunity and reduced inflammation — both of which are associated with more positive health outcomes, including reduced risk of diabetes.
  • Prebiotic fibers, in particular, selectively stimulate the growth and metabolic activity of gut microbes. They are digested quickly by microbes that produce beneficial molecules for humans, including the important and beneficial SCFAs (butyrate, acetate, and propionate) — which are believed to play a role in regulating appetite, immune system function, and blood sugar. 
  • Examples of prebiotics include oligofructose, inulin, and galacto-oligosaccharides. While these prebiotics do exist as over-the-counter supplements, you can get them from fresh, whole foods such as chicory root, raw dandelion greens, Jerusalem artichoke, leeks, asparagus, onions, wheat, garlic, chicory, oats, soybeans, beans, certain root vegetables and certain dairy products. (Note that some of these foods are also considered FODMAPs, which can worsen IBS symptoms in some individuals.)

2. High-fiber foods also help with blood sugar control.

The more fiber you pack into meals, the slower your digestive system will turn what you eat into glucose, helping to slow the rise in your blood sugar levels. The result: fewer spikes and dips in energy, resulting in reduced inflammation and lower demands on various bodily functions. 

3. A diet high in fiber can help you stay slim.

As mentioned previously, high fiber diets play a role in enhancing gut bacteria diversity and beneficial metabolic activity. Not only can these diverse gut bacteria directly help maintain weight, but their anti-inflammatory effect also plays a role in maintaining a healthy weight. Weight control is important because obesity is a risk factor for metabolic syndrome and type 2 diabetes. 

4. Fibers enhance nutrient absorption.

Fiber isn’t just good for your gut; some fibers may also help you obtain more nutrients from the foods you eat:

  • Fructans increase calcium absorption.
  • Some fibers can help you better absorb fat-soluble vitamins.

But take care when selecting fiber; some fiber types actually make it harder for you to get nutrients:

  • Cereal-derived fibers, while typically a great source of iron, zinc and calcium, may also contain factors (e.g., bran) that make it harder to absorb those nutrients.
  • Some fibers have been shown to increase the passage of vitamins in stool.

5. Fiber has a role in medical management.

Researchers are still trying to figure out which types of fiber are beneficial for particular gut disorders, but data and conclusions collected to date include the following:

  • Diets lacking fiber are associated with imbalanced (i.e., dysbiotic) gut bacteria communities. Dysbiotic communities have been associated with a number of gastrointestinal (GI) disorders, including inflammatory bowel syndrome (IBS) and inflammatory bowel disease (IBD). In inflammatory bowel syndrome (IBS), insoluble fibers appear to make symptoms worse, while soluble fibers can improve symptoms; however, the impact of fiber also seems to depend on whether the patient has constipation-predominant IBS or diarrhea-predominant IBS.
  • Dysbiosis may also aid in the progression of insulin resistance, putting you at increased risk for diabetes (because the body is no longer able to produce enough insulin or use the insulin it makes effectively). The end result: more sugar in your blood. While it’s unclear exactly what mechanisms are at play here, dysbiosis “may reshape intestinal barrier functions and host metabolic and signaling pathways, which are directly or indirectly related to the insulin resistance” in type 2 diabetes, according to a 2019 study.
  • In inflammatory bowel diseases (IBD), research points to prebiotic supplementation to favor beneficial bacteria and increase production of anti-inflammatory SCFAs; additionally, fiber was more beneficial in patients with ulcerative colitis (UC) than in patients with Crohn’s disease (CD).
  • There is little evidence that fiber benefits patients with diverticulitis.
  • Although fiber has been shown to reduce constipation and improve bowel movements, there is little evidence that fiber can benefit patients with functional constipation (i.e., chronic constipation in which patients have hard, infrequent, difficult to pass bowel movements).
  • Early studies suggest that administering different types of fiber together (co-administration) could provide significant benefit to patients with gut disorders.
  • Natural fibers (fibers obtained from eating whole foods) might elicit the same benefits as co-administration but with the added benefit of additional micronutrients; however, caloric intake could rise as well.

Key takeaways

Fibers’ mode of action is dictated by a combination of solubility, viscosity, and fermentability. For most people, consuming a diet high in fiber (i.e., whole grains, vegetables, fruits, and legumes) supports a healthy community of gut bacteria, helps maintain a well-functioning digestive system, aids in regulating blood sugar levels, helps keep weight in check, and enhances nutrient absorption. Fiber also plays an important role in the medical management of certain GI disorders. 

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