"When you eat foods high in fiber, you feel full faster, making you less likely to overeat and more likely to have weight loss." ― Denise Austin
Introduction | What Is Fiber? | Types | Functions | The Gut | Benefits | Fiber Intake | Too Little Fiber | Final Thoughts | Takeaway Points | References | Disclaimer
Introduction
The importance of dietary fiber in human nutrition is widely recognized. It is, however, frequently associated with the notion that insoluble fiber promotes regular bowel movements. Many of us are unaware of the important role of soluble fiber and its numerous health-promoting benefits. Fiber intake has an effect on weight, immune function, gut permeability, and inflammation.
What is fiber?
The European Food Safety Authority [EFSA] defines fiber as a non-digestible carbohydrate and lignin[1]. It is found in whole foods such as vegetables, fruits, legumes, and whole grains. Fiber has a complex composition that varies depending on its source.
"Dietary fibre is principally composed of plant cell walls, but it also includes components obtained from cell walls (e.g. cellulose, pectin, and lignin), and non-starch polysaccharides (NSPs) from other sources (e.g. seaweeds and micro-organisms)."[2]
Cellulose and lignin are the most common fiber components found in most plants. Their primary function is to protect the plant from potential threats. As a result, the skins of fruits, vegetables, and whole grains contain their highest concentrations.
The types of fiber
Fiber is classified into two types: soluble and insoluble. Insoluble fibers do not dissolve in water, whereas soluble fibers do. Most plants contain both types of fiber, but the amounts vary depending on the plant.
1. The soluble fibers include:
inulin oligofructose, which is found in garlic, onion, bananas, and chicory root.
mucilage, which is found in oats, peas, berries, bananas, apples, etc.
beta-glucans, which are found in oats and barley.
pectin, which is found in apples and citrus fruit. Soluble pectin occurs in plant juices[3].
gums, which can be found as tree exudates or can be obtained from different plant parts such as seeds.
polyols, which are found in small amounts in berries, apples, and plums.
psyllium, which is obtained from the seeds of plantago plants.
resistant starch, which can be found in whole grains, green bananas, and legumes.
wheat dextrin, which is obtained through the intense processing of wheat starch. Wheat dextrin is usually added to packaged and processed foods.
2. The insoluble fibers include:
cellulose, which can be found in bran, whole grains, legumes, apple skins, seeds, etc.
hemicellulose, which can be found in whole grains, bran, legumes, flax, etc.
lignin, which can be found in vegetables, fruits, legumes, and whole grains.
pectin, which is found in green plant parts, fruits, and roots[3].
The functions of fiber in the human body
The functions of fiber in the human body vary depending on the type of fiber[4]. Soluble fibers dissolve or retain water, forming a gel-like substance that slows food digestion and nutrient absorption. This type of fiber is particularly beneficial to our gut microbiome. It provides food for all of the beneficial bacteria in our gut, allowing them to thrive. This, in turn, leads to many health benefits that are linked to a healthy and diverse gut flora.
In contrast, insoluble fibers do not dissolve in water or gastric fluids. Their primary function is to help with stool formation, maintain bowel motility, and prevent constipation. Insoluble fiber binds to waste products in the colon and aids in their elimination from the body.
Fiber and the gut
Fiber, as previously stated, is a non-digestible component of plant-based foods. The human digestive system does not absorb fiber. However, there are microbe species in the human gut that can partially metabolize the soluble complex carbohydrates via fermentation[5]. As a result, soluble fiber consumption influences the composition of the gut microbiota. It has been demonstrated that consuming inulin increases the abundance of Bifidobacterium bifidum and Faecalibacterium prausnitzii[5]. Furthermore, beta-glucan consumption increased the colonization of Bifidobacteria and Bacteroides species[5]. Finally, acacia gum fiber has been shown to increase the presence of Bifidobacteria and Lactobacilli[5].
Soluble fiber fermentation occurs in the large intestine and results in the formation of the three main short-chain fatty acids [SCFAs]: acetate, propionate, and butyrate[6]. All of these fatty acids are important for gut health, but butyrate is especially important for maintaining a healthy mucosal barrier in the colon.
"Butyrate supports the integrity of the intestinal epithelial barrier by regulating the expression of tight junctional proteins and supporting intestinal mucus production."[7]
SCFAs are known to benefit health by improving immune function, assisting in glucose homeostasis, influencing lipid metabolism, and aiding in appetite regulation[6].
The benefits of a fiber-rich diet
A diet high in fiber can significantly improve a person's health. Now that we are aware of the significance of a diverse gut microbiome and that consuming fiber raises the number of microbes in our colon, we can look into the particular health advantages of a high-fiber diet.
A fiber-rich diet is known to:
help maintain regular bowel movements, preventing constipation[8][9][10],
strengthen the intestinal barrier, protecting against the 'leaky gut' syndrome[11][12],
aid in blood glucose regulation and improving insulin sensitivity[16][17][18],
protect against the development of metabolic syndrome[22][23][24],
reduce the risk of developing colorectal cancer[38][39][40].
Furthermore, given the effects of a fiber-rich diet on our systems, we can link its health benefits to a lower risk of diabetes[41], obesity[42], atherosclerosis[43], and cardiovascular disease[43].
How much fiber do we need?
Men, women, and children require different amounts of fiber in their daily diets.
A good rule of thumb is to include fiber from vegetables, fruit, or whole grains in every meal and snack throughout the day. The primary source of dietary fiber in our diet should be vegetables, followed by fruit and whole grains. Another important thing to remember is to include a wide variety of vegetables and fruits in our diet to expose our bodies to a wider range of fiber types. Different fibers feed different bacteria in our gut, allowing them to thrive and build our health.
The effects of eating too little fiber
The mucus layer protects the entire human digestive system and all of its components. This mucus barrier plays an important role in protecting the underlying epithilium from harmful bacteria. Its sticky consistency helps it capture and eliminate pathogens from our GI tract. Furthermore, it also serves as an energy source for both commensal [protective] and pathogenic gut bacteria[46].
The mucus layer is made up of approximately 30 proteins[47]. Mucin 2 [MCU2] glycoprotein, which contains a large number of glycan structures [mucin glycans], is its main component. These glycan structures allow the mucus layer to feed different types of bacteria while also disposing of pathogenic ones. Over time, the beneficial gut bacteria, whose presence in the gut depends on our dietary choices, rebuild and reinforce the mucus layer. Dietary fiber, particularly soluble fiber, feeds commensal bacteria, allowing them to replicate and, ultimately, restoring the mucus layer in the gut. A low-fiber diet, on the other hand, provides an inviting environment for pathogens that feed on the mucus barrier, degrading it and penetrating deeper into the epithelium.
"Microbiota dependent degradation of mucin glycans induced by a low-fiber diet increases the susceptibility to Citrobacter rodentium infection[...]"[47]
As a result, immune function declines, and inflammation develops. It also promotes the development of a variety of diseases, including ulcerative colitis, intestinal bowel disease, and even colorectal cancer.
Final Thoughts
Dietary fiber is an important component of human nutrition that has a significant impact on health. A fiber-rich diet has been shown to encourage regular bowel movements, strengthen mucus barriers, reduce inflammation, boost immune function, and protect against colorectal cancer. Soluble fiber is essential for the growth and colonization of commensal bacteria in our gut. Insoluble fiber, on the other hand, boosts gut motility and waste removal. Meeting our daily fiber requirements can change the course of our health and help us maintain good metabolic function.
Takeaway Points
Fiber is a non-digestible carbohydrate present in vegetables, fruits, and whole grains.
There are two types of fiber: soluble fiber and insoluble fiber.
Soluble fiber feeds beneficial gut bacteria.
Insoluble fiber aids in the elimination of waste products.
A fiber-rich diet has many health benefits.
A low-fiber diet can be detrimental to human health.
Men, women, and children of various ages have different fiber requirements.
References
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