Fiber is much more than a way to keep you regular. It has several benefits for our health including weight loss, blood sugar control, and lowered risk for chronic disease. It is also the key to maintaining the health of our gut bacteria. However, with all these benefits, most of the world does not get enough fiber. The WHO (World Health Organization) recommends, on average, about 30 grams of fiber daily, however, Americans eat between 10-15 grams of fiber per day. This is not quite surprising when we consider that fiber only comes from plant foods, something Americans (and other Westernized countries) usually consider as an afterthought or a chore.

Let’s dive into the types of fiber and the role they play in our health, specifically our microbiome. We’ll also discuss how to keep our gut bacteria well fed and the importance of eating a variety of high-fiber foods.


Types of Fiber

At its core, fiber is a type of carbohydrate that cannot be digested by our body. In fact, we don’t have the enzymes necessary to break them down, so they pass through our digestive system unchanged. Fiber comes by many names, each describing their unique biochemical structure. Names such as cellulose, pectins, oligosaccharides, dextrins, beta-glucan, polydextrose, galactan, inulin, and more.

Despite these various names, fiber comes in two main types: soluble and insoluble. It is best to eat a mix of both types unless instructed otherwise by a functional medicine physician. Those who are having a flare-up of IBD (Inflammatory Bowel Disease), or who are in treatment for SIBO (Small Intestine Bacterial Overgrowth), have diverticulitis, or have chronic diarrhea, are candidates for limiting certain types of fiber. 

Soluble fiber dissolves in water and is fermentable. It forms a gel-like substance in our gut and slows down bowel transit time (the time it takes for food to pass through our system). This allows soft, well-formed stools. 

Insoluble fiber does not dissolve in water, remaining intact throughout our digestive system. It is the main bulking agent, speeding up our bowel transit time and helping with constipation. Most insoluble fiber is not fermentable. 

Most plant foods contain both soluble and insoluble fiber.


Food For Gut Bacteria

Feeding our gut bacteria perpetuates the growth and variety of bacterial species while playing crucial roles in our health. Beneficial gut bacteria help with weight control, blood sugar balance, cognitive function, immune function, inflammation, digestive disorders, and more. There are three sources of food that our gut bacteria uses to function properly.


Fermentable Fiber

You may have noticed the mention of fermentation when describing the types of fiber, above. When a food is fermentable (like soluble fiber and some types of insoluble fiber), it means our gut bacteria literally ferment it (or digest it) once it reaches our large intestine. Fermenting these fibers is how our microbiome gets its fuel for energy. Fiber serves as their food source. 


Resistant Starch

Another food source for our gut flora is resistant starch. Starches are the main type of carbohydrates in our food and some of them resist being digested. Much like fiber, these resistant starches pass through our digestive tract unchanged. Although not a type of fiber, resistant starches are fermentable just like soluble fiber. This makes it another food source for our gut bacteria. 



Similar to fermentable fiber and resistant starches, prebiotics are also carbohydrates that are non-digestible by our bodies, making them a great food source for our gut bacteria. Most prebiotics are found in plant foods and are considered a type of soluble fiber. However, other prebiotics come from non-fiber sources such as lactulose (made from lactose) and polyunsaturated fatty acids (such as that found in fatty fish).

The main difference between prebiotics and other non-digestible carbs is that prebiotics are selectively used by our microbiota. We have more than 500 species of bacteria living symbiotically in our gut; ideally containing more helpful bacteria than harmful. Unlike fermentable fiber and resistant starches, not all bacterial species are able to digest prebiotics. Rather, prebiotics are digested by certain beneficial bacteria such as those seen in probiotic supplements (Lactobacillus and Bifidobacterium). 


A Note About SIBO

Fermentable fibers, resistant starches, and prebiotics should be limited or avoided short-term by those who have SIBO or who have IBS symptoms. Feeding gut bacteria is only a good thing when the bacteria are where they’re supposed to be, in the large intestine. However, if they become overgrown elsewhere (as in SIBO), then eating these foods will feed and perpetuate the overgrowth and cause symptoms.


Short-Chain Fatty Acids

When our gut bacteria digest (ferment) fiber and resistant starches, they produce short-chain fatty acids (SCFA) inside our large intestine. The most beneficial (and most studied) SCFA is called butyrate, which can also be taken in supplement form. These fatty acids benefit our gut as well as our immune system. 

In our gut, SCFA’s are responsible for feeding the cells that line our large intestine, and maintaining the health of our intestinal barrier. In this way, SCFA’s protect our gut health and reduce gut inflammation. This leads to improvements in digestive disorders such as SIBO, IBS, ulcerative colitis, and Crohn’s disease. Furthermore, butyrate is thought to interrupt the growth of colon cancer cells and reduce the risk of this cancer.

Butyrate, more than other SCFA’s, has anti-inflammatory effects throughout the body (not just in the gut). They even play a role in the specialization of immune cells. Lowering inflammation and strengthening the immune system are key ways in protecting ourselves against chronic disease and acute infections. 


Benefits of Fiber

Increasing fiber in your diet has additional health benefits beyond those that come from butyrate and other short-chain fatty acids. Fiber can improve glucose balance, lower body weight, and reduce cholesterol levels, all of which are protective against diabetes and cardiovascular diseases. Various types of fiber can increase the feeling of fullness (satiety), reduce hunger, regulate the frequency and consistency of stool, and improve mineral absorption in the gut, particularly calcium and magnesium.   


Diversify Your Fiber Portfolio

It’s important to get your fiber from a variety of sources rather than sticking with just one or two high-fiber foods. This is because fiber foods each contain different types of fiber with different combinations of carbohydrates and starches (such as beta-glucan, inulin, or dextrin). Each type of fiber imparts unique benefits and enables our gut bacteria to thrive and maintain diverse bacterial species. 

Keep in mind that seemingly simple events in our lives, such as illness, poor diet, stress, environmental toxins, or antibiotic medication, all have a negative effect on our gut bacteria. Specifically, these insults greatly decrease the diversity of species. A person with low diversity of gut bacteria is a person at high risk for chronic disease, digestive disorders, infections, and a weakened immune system. 

A diversity of high-fiber foods creates a diversity of healthy gut flora 

High-fiber foods are found chiefly in legumes (beans), whole grains (quinoa, oats, wheat, etc), and starches (like potatoes and brown rice). They are also found in good amounts in various vegetables, fruits, and nuts/seeds. If we’re only talking about fermentable fiber, which is the food source for our gut bacteria, beans contain the highest amount. In fact, a one-cup serving of beans contains roughly half (15 grams or so) of the recommended intake of daily fiber!

Be sure to include whole grains and starches along with your vegetables and fruit in order to increase fullness and carry you to the next meal. Also, be aware that eating too much fiber, too fast, can lead to excessive gas, bloating, or abdominal discomfort. This is commonly the case with foods high in fermentable fiber or soluble fiber, especially if you aren’t used to eating a high-fiber diet. If you get digestive symptoms after eating canned beans, be sure to rinse them in a colander until all the bubbles are gone. 


Fiber is an essential part of our daily diet even though it is often overlooked. Most people do not get enough fiber as we continue to gravitate towards highly processed foods that are stripped of fiber (and other nutrients). Not only does fiber keep us regular but it is the main food source for our gut bacteria. Keeping these bacteria well fed is key to maintaining the health and function of beneficial species while harmful species are kept at bay. Not only that, but when our gut bacteria digest fiber, they produce short-chain fatty acids that play key roles in the integrity of our gut lining as well as in reducing inflammation and influencing our immune system. Fiber imparts other benefits including blood sugar control, weight loss, mineral absorption, and protective against chronic disease. Eating a variety of plant foods high in fiber will ensure the health and diversity of our gut bacteria.