Fiber and the Gut Microbiome

By: Joya Johnson, DTR, CSUN Dietetic Intern Cohort 2021-2022 

What is the gut microbiome? The cecum is a specific section of our large intestines that serves as a home to bacteria, fungi, viruses, archaea, and other microbes and together, this area is known as the gut microbiome. Bacteria and viruses are most commonly known for being associated with illnesses and diseases, but the microbiota that inhabit our gut have a prominent impact on our immune system and other metabolic functions.1

Did you know that a human being on average has more than 40 trillion microorganisms in their gut?2 We only have 30 trillion human cells; this means that our body is more microbiota than it is human.2 This microorganism population weighs about 200 grams, which is equivalent to the size of a mango.2 There is also an average of 1,000 different species of bacteria and each have a vital role in our health.1 Not only has there been significant interest in the association between the gut microbiome and health among researchers and the general population, but there is also a growing fascination on how certain factors, like dietary intake, can impact the gut microbiome. 


There are many different factors that can affect the microbiome, one of which is our diet. It is believed that certain food components, such as fiber, can modulate our microbiota.Dietary fiber is the part of the plant that is indigestible and not absorbed. Dietary fibers are fermented by the microbiome and can alter the composition of the microbiota population in our gut.3 Although certain dietary fiber can be classified as prebiotics, not all prebiotics can be classified as dietary fiber.3 Prebiotics are a source of fuel and help to promote the healthy growth of the microbiota in our gut. There are many different sources of fiber that can be derived from our diet.

Sources of Dietary Fiber and Probiotics 

Dietary sources of fiber include fresh whole fruits, vegetables, grains, legumes, and nuts and seeds. The Recommended Dietary Allowance (RDA) for fiber is 25 grams/day for women and 38 grams/day for men. Although there are fiber supplements, they tend to be low in fiber (approximately 5 grams in daily serving) and they do not contain other nutrients, such as vitamins and minerals, that are found in dietary sources. Probiotics are microorganisms that can help restore the gut microbiome.3 Sources of probiotics are fermented foods such as yogurt, sauerkraut, kimchi, kefir, tempeh, miso, and kombucha. There are probiotic supplements that contain different strains of bacteria and promote microbial diversity. Lactobacillus acidophilus and Bifidobacterium are two of the most common probiotic bacteria. L. acidophilus is known to help with diarrhea, candida, and irritable bowel syndrome.3 Bifidobacterium’s main function is assisting your body in digesting complex carbs, such as fiber.3 Incorporating both dietary sources of fiber and probiotics in our food intake can help build and maintain a healthy gut microbiome to support overall health. 

How Fiber Impacts Our Gut Microbiome 

The microbiota in our gut ferment the fiber we eat and produce short-chain fatty acids. Short-chain fatty acids (SCFA) are essential for colon health, diabetes, and heart disease.1 The short-chain fatty acid butyrate is essential in the prevention and treatment of colon cancer. According to the Center for Disease Control (CDC), colon cancer is the second leading cause of cancer-related death in the United States.4 Butyrate is the most potent SCFA that exists in the colon since lab studies have shown that butyrate can promote the growth of healthy colon cells.5 A study that fed one group of mice a high fiber diet and fed another group a low fiber diet found that the group of mice who consumed a high fiber diet (thus had a higher population of butyrate) had 75% less tumors than the mice who did not consume a high fiber diet and had less butyrate in their gut.5 A prospective study involving human participants demonstrated that the participants who consumed a higher intake of fiber derived from cereal and fruit had a reduced risk of colorectal cancer.6 Although the exact mechanism in which butyrate can reduce the development of cancer cells is not fully understood, researchers believe that butyrate can increase apoptosis (i.e., programmed cell death) in certain cells, specifically cancer cells.5 Overall, these studies have determined that individuals who consume a high intake of dietary fiber and have a higher population of butyrate have a reduced chance of developing colon cancer.  

The SCFA butyrate can also have a positive impact on people who are diagnosed with type 2 diabetes. According to the CDC, about 10% of the world’s population are diagnosed with type 2 diabetes.7 The gut microbiome can impact one’s insulin resistance and therefore affect one’s blood sugar. Studies have shown that SCFAs can increase enzyme levels in the liver, which can therefore positively impact blood sugar levels.8 Fiber cannot be broken down into sugar molecules and it can also help individuals with diabetes by slowing the absorption of sugar. According to the CDC, heart disease is the leading cause of death for both women and men in the United States.9 Butyrate can also reduce one’s risk of heart disease. A study conducted on rats analyzed the lipid panel of rats that were fed a high cholesterol diet and supplemented with butyrate.10 Although they were fed a diet high in cholesterol, the butyrate supplement was able to prevent hyperlipidemia in rats by reducing cholesterol levels.10 Researchers believe butyrate is able to interact with the genes that produce cholesterol and is thus able to reduce blood cholesterol.Overall, our microbiome plays a vital role in our health and can thus prevent many diseases and illnesses.  


Want to try a recipe rich in fiber? Check out the MMC’s Mandarin Orange, Spinach, & Quinoa Salad Recipe on the MMC’s Youtube Channel. 

Need help incorporating more fiber/probiotic rich sources into your diet? Meet with a Registered Dietitian Nutritionist at the MMC Wellness Clinic for additional nutrition advice and guidance. Visit Here. 


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