Of all the different major nutrient groups that we eat, fiber is the one component of our diet that directly feeds our gut health + microbiota. When we eat protein, for instance, we digest it and absorb it in our small intestine. The same thing happens with fats and most sugars.

Dietary fiber, also known as roughage or bulk, includes the parts of plant foods your body can’t digest or absorb. Unlike other food components, such as fats, proteins or carbohydrates—which your body breaks down and absorbs—fiber isn’t digested by your body. Instead, it passes relatively intact through your stomach, small intestine and colon and out of your body.

In the case of non-digestible fibers, we do not have the enzymes needed to break them down and digest them. Only gut bacteria can do that. They digest fibers and produce short chain fatty acids, whose beneficial effects on health are well documented. By eating fiber, then, we are ensuring those trillions of microbes are well-fed so they can help us stay in good health.

Soluble Fiber vs. Insoluble Fiber

Fiber is commonly classified as soluble, which dissolves in water, or insoluble, which doesn’t dissolve.

• Soluble fiber. This type of fiber dissolves in water to form a gel-like material. It can help lower blood cholesterol and glucose levels. Soluble fiber is found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium.
• Insoluble fiber. This type of fiber promotes the movement of material through your digestive system and increases stool bulk, so it can be of benefit to those who struggle with constipation or irregular stools. Whole-wheat flour, wheat bran, nuts, beans and vegetables, such as cauliflower, green beans and potatoes, are good sources of insoluble fiber.

The amount of soluble and insoluble fiber varies in different plant foods. To receive the greatest health benefit, eat a wide variety of high-fiber foods.

How Fiber Impacts Gut Health

Feeding well your gut microbiota is also key for your physiology. The gut’s defensive wall, the intestinal barrier, keeps our friendly bacteria where they should be. This intestinal barrier is covered by a mucus layer. When gut microbiota is not properly fed, they forage on the mucus layer and there is the risk of potentially harmful bacteria spreading all over the body, causing infection or disease.

A 2017 study shed some light on the role gut microbiota play in providing benefits from a high fiber diet.

“Diets that lack fiber alter the bacterial composition and bacterial metabolism, which in turn causes defects to the inner mucus layer and allows bacteria to come close [encroach], something that triggers inflammation and ultimately metabolic disease,” says Gunnar C Hansson, professor at the University of Gothenburg and a coauthor of the study. “It is not enough just to add fiber to your diet; it also depends on which bacteria you carry.”

This is because different dietary fibers feed different groups of microbes. Certain bacteria even produce specific enzymes that can break down only certain types of fiber. A diverse, balanced, and rich microbial community is part of a diverse, balanced, and rich diet of fiber.

What Fiber Should You Eat for Gut Health?

A team of researchers from Washington University School of Medicine, in St. Louis, MO, along with international collaborators, recently conducted a study with the goal of gaining a long term view of developing what they call microbiota directed foods, to improve our gut health. The researchers provided a base diet of high quantities of saturated fats and low quantities of fruits and vegetables to mice that had been given isolated human gut microbiota. The team used this as a model of a Western diet, which is typically high in fats and low in fiber. To each base diet, they added different fiber types.

The team tested 34 different sources of dietary fiber, including pea protein, citrus peel, citrus pectin, tomato peel, orange fiber, apple fiber, oat hull fiber, cocoa, chia seeds, and rice bran. In total, this resulted in 144 different diet combinations. They then analyzed how the 20 different bacterial strains reacted to the presence of the various fiber sources.

Specifically, B. thetaiotaomicron abundance increased in the presence of citrus pectin and pea fiber, while B. ovatus levels rose in the presence of barley beta-glucan and barley bran. Other fibers that resulted in an increase in members of the Bacteroides strains in the study were high molecular weight inulin, resistant maltodextrin, and psyllium.

Whole Grains

Oatmeal, whole grain bread (gluten-free or otherwise, if you’re not sensitive to gluten), brown rice… the list can go on (and actually includes popcorn too).

Avocados

Have you noticed that avocados can be kind of… stringy? That stringy-ness is what makes them a great prebiotic for gut health!

Leafy Greens

Probably the easiest to increase in your diet, spinach, kale, collard greens, cabbage, and more provide some of that roughage your body needs.

Berries, Apples, and Other Fruits

Add raspberries and blueberries, or an apple to your oatmeal every day for a fiber boost.

Interested in how your specific gut can benefit from dietary fiber? See our RDN, LD Bobbi Horner for support!

Dr. Cassie Wilder

Dr. Cassie Wilder is a registered Naturopathic Medical Doctor (NMD) and founder of MIMC. Her passion is empowering her patients through education, understanding, and support through their healing journey. After graduating from Iowa State University with a Bachelors of Science in Kinesiology and Health, Dr. Wilder earned her Doctorate of Naturopathic Medicine from Southwest College of Naturopathic Medicine & Health Sciences, a fully accredited and nationally recognized institution in Phoenix, AZ. During her clinical training, she received extensive hands-on training with many leading experts in the field of functional medicine and developed a passion for treating hormonal imbalances, thyroid disorders, cardiovascular concerns, and adrenal fatigue.