The impact of gut fermentation of dietary fibers is a fascinating journey through our digestive system.
Soluble dietary fibers, found in oats, nuts, and beans, dissolve in water, forming a gel-like substance that aids in digestion. In contrast, insoluble fibers, present in whole grains and vegetables, add bulk to stool and are crucial for bowel health.
The fermentation of these fibers by the gastrointestinal tract microbiota produces vital short-chain fatty acids, influencing overall health.
Understanding the intricate relationship between plant cell walls, dietary fibers, and gut health is key to unlocking the myriad benefits of dietary fiber sources.
Main Findings
- Dietary fibers primarily originate from plant cell walls and are categorized as soluble and insoluble based on their water solubility.
- Soluble fibers like pectin, arabinoxylan, and others affect gut health by influencing the glycaemic response and microbiota composition.
- Insoluble fibers, such as cellulose and lignin, challenge gut bacteria due to their complex structure.
- Gut microbiota plays a critical role in fermenting dietary fibers, producing beneficial short-chain fatty acids (SCFAs).
- The diversity and complexity of the diet, including dietary fibers, positively influence gut microbiota diversity.
- The fermentation process of dietary fibers affects microbial diversity and overall health, advocating for a diet rich in varied dietary fibers.
What are Dietary Fibers and Why They Matter for Your Health
In today’s affluent societies, obesity and related chronic diseases like type 2 diabetes, cardiovascular issues, and colon cancer are on the rise 1 2.
Numerous epidemiological studies 3 4 5 have highlighted a strong connection between low dietary fiber (DF) intake and the occurrence of these ailments.
DF, sourced from fruits, vegetables, and whole grains, offer specific health benefits, including blood sugar control 6, promoting regular bowel movements 7, and lowering cholesterol levels 8.
They have also shown promise in reducing gastrointestinal tract (GIT) disorders like Crohn’s disease and ulcerative colitis 9 10 11.
This has led to a growing interest in incorporating DF into a healthy diet.
However, it’s crucial to note that despite this interest, dietary guidelines often treat “dietary fiber” as a single entity, albeit occasionally classified as soluble or insoluble.
In reality, dietary fiber is a complex group of compounds with varying biological and chemical properties, encompassing cellulosic materials, resistant starch, and non-digestible oligosaccharides.
DF constitutes a major non-digestible component in most diets, exerting physiological effects throughout the digestive tract.
It influences digesta structure (impacting satiety and food intake control), modulates digestion processes (affecting glucose and lipid levels), and serves as a primary substrate for microbial fermentation.
The focus here is on its interactions with the gut microbiota.
Types of Fibers: The Difference Between Soluble and Insoluble Fibers
Dietary fibers (DF), mostly derived from plant cell walls (PCW), play a crucial role in human health, particularly in the gut.
These fibers, composed mainly of carbohydrate polymers, are not digested in the small intestine but are fermented by bacteria in the large intestine (LI).
This fermentation process contributes to microbial diversity and function within the gastrointestinal tract (GIT), generating beneficial by-products 12 13.
Dietary fibers are classified into soluble and insoluble types 14, influencing their behavior in the GIT, including water-holding capacity and fermentation rate 15.
Soluble Dietary Fibers: Key Players in Digestive Health
Soluble dietary fibers, found in plant cell walls, play a vital role in regulating blood glucose and managing Type 2 diabetes risk.
Their ability to alter digesta viscosity impacts gastric emptying and nutrient absorption, influencing the body’s glycaemic response16 17 18 19 20
Pectin: Its Role in Gut Fermentation and Health
Pectin, a key component of plant cell walls, is predominantly found in fruits and vegetables.
It consists largely of galacturonic acid 21, with varying levels of methyl esters that determine its physical properties 22.
Pectin is categorized based on its degree of esterification (DE) into high methoxyl (DE > 50%) and low methoxyl (DE < 50%).
High DE pectin forms gels under acidic conditions or high sugar concentrations, while low DE pectin gels with metal ions like calcium 23.
Homogalacturonan, the most abundant type of pectin 24, and other complex types like rhamnogalacturonan play significant roles in dietary fiber fermentation, impacting gut health 25.
Arabinoxylan
Arabinoxylan (AX), a key hemicellulose in plant cell walls, especially in cereals like wheat and rye, consists of a β-d-xylose backbone with α-l-arabinose side-chains.
The arabinose-to-xylose ratio defines AX’s structure.
Comprising about 20% of wheat bran content 26, AX is highly fermentable in the large intestine, both in vitro and in animals like pigs 2728.
Its fermentation varies depending on the lignin content and actual AX structure 29.
Mixed-Linkage Glucans: Health Benefits in Gut Fermentation
Mixed-linkage glucans (MLGs), found primarily in oats, rye, and barley, are unique polysaccharides within plant cell walls.
Their molecular structure, characterized by irregular β-glucosyl residue chains, enhances their water solubility and gel-forming abilities 30.
Notably, MLGs have shown multiple health benefits including lowering blood glucose and reducing cholesterol and bile acids 31.
Their fermentability has been proven in various studies, both in vitro and in vivo 32 33, involving different subjects including rats 34, pigs 35 36 37, and humans 38, illustrating their significant role in gut health and dietary fiber fermentation
Xyloglucans
Xyloglucan, a prevalent hemicellulose in the cell walls of most dicots and non-graminaceous monocots, forms a significant part of our diet through fruits and vegetables.
It is known for its unique structure, binding to cellulose in plant walls 39.
While specific health benefits of xyloglucan are not yet fully understood, its common presence in the diet hints at potential health impacts.
Recent research suggests its metabolism in the gut may influence microbial ecology, potentially impacting health, as observed with the Bacteroides ovatus species 40.
Specialized Dietary Fiber Polymers
Besides the major soluble fiber polymers in plant cell walls, there are additional fibers used as food additives, like guar galactomannan and konjac glucomannan, as well as those from algal walls (alginate, carrageenan, agar) and plant energy reserves (inulin, fructo-oligosaccharides, galacto-oligosaccharides).
Although not the focus of this review, these fibers are likely to share properties with typical plant-based soluble fibers.
Insoluble Dietary Fibers: Their Impact on Gut Bacteria
For gastrointestinal tract (GIT) bacteria, insoluble fibers like cellulose present a significant challenge.
This difficulty arises from their reduced surface area available for bacterial access and the strong hydrogen-bonding networks that hold the carbohydrate chains of these fibers together 41.
Cellulose
Cellulose, the most abundant organic polymer on earth, forms the backbone of plant cell walls 42.
It consists of β-glucosyl residues that form strong, semi-crystalline microfibrils and aggregate into fiber ribbons 43 44, contributing to the strength and rigidity of plant cell walls.
This makes cellulose highly resistant to degradation 45.
While soluble in strong alkali solutions, part of it can be hydrolyzed in acid 46.
Although extensively studied in ruminants 47 48, the interaction of cellulose with the microbiota in the monogastric gastrointestinal tract is less understood 49.
Lignin
Lignin, a complex network of phenolic compounds, is a key structural component of secondary cell walls, especially in woody tissues 50.
Unlike polysaccharides, it comprises about 40 different phenyl-propane units and is considered chemically inert 51.
Found between cellulose and hemicelluloses, its concentration varies with plant species, maturity, and cell type.
Despite its prevalence, lignin’s consumption in the human diet is minimal, estimated at less than 1 gram per day 52.
Phytonutrients
Phytochemicals, like polyphenols and carotenoids found in plants, contribute to the nutritional benefits of fruits, vegetables, and grains 53.
These compounds, including flavonoids and non-flavonoids 54 55 56, may bind to plant cell walls (PCWs) during food processing or digestion 57.
While not directly classified as dietary fibers (DF), they interact closely with DF in our diet.
Most polyphenols have low bioavailability 5859; a small percentage is absorbed in the small intestine, while the majority undergoes microbial fermentation in the large intestine 60 61.
How Microbiota Transforms Fibers into Health Benefits
The health benefits of dietary fibers (DF) are significantly influenced by gut microbes 62.
These microbes break down cell wall polymers, producing short chain fatty acids (SCFA) essential for gut health and homeostasis 63.
With advances in DNA sequencing and enzymology, research is increasingly focusing on the complex relationship between gut microbiota and plant cell walls during DF fermentation 64 65 66.
Gastro-Intestinal Tract Microbiota
The gastrointestinal tract microbiota, encompassing a diverse microbial population, plays a crucial role in digestion, immunity, and overall health 67 68 69.
While the large intestine is the primary fermentation site 70, recent studies show some fermentation occurs in the stomach and small intestine 71.
This microbial ecosystem, largely bacterial, is dynamic yet resilient to environmental changes.
It consists of major phyla including Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria 72 73 74.
Although our understanding of the microbiota’s full role is evolving, it’s clear that diet significantly influences this microbial community, impacting health in various ways 75.
Microbial Function
Gut microbiota, highly competitive and diverse, primarily perform saccharolytic metabolic activities 76.
These bacteria can break down complex carbohydrates, like those in dietary fibers, using enzymes such as polysaccharidases and glycosidases 77.
This process converts complex polymers into simpler molecules like sugars and amino acids, which are then fermented into short-chain fatty acids (SCFA), CO2, and other by-products 78.
This “cross-feeding” among species contributes to gut health, evaluated through indicators like SCFA production and stool quality 79.
Low Dietary Fibre Diets and Pathogenesis Associated with Microbiota
Recent studies highlight the role of probiotic bacteria like Bifidobacterium, Eubacterium, and Prevotella in maintaining gut health.
They help in outcompeting harmful bacteria, such as the Clostridiaceae family 80 81.
A lack of dietary fiber can lead to harmful shifts in gut bacteria, as shown in studies where a fiber-deficient diet caused an increase in mucus-degrading bacteria 82, leading to gut issues.
Therefore, maintaining a diverse bacterial population with a diet rich in carbohydrates is crucial for preventing conditions like colo-rectal cancers 83.
Diet and Microbial Diversity
A diverse gut microbiota, rich in different bacterial species, is key to a healthy gastrointestinal tract 84.
This diversity, resulting from a wide gene pool, makes the microbiota more adaptable and stable 85.
Diets rich in varied and complex compounds, including dietary fibers and phytonutrients like polyphenols and carotenoids, promote greater bacterial diversity 86.
Studies show Western diets, high in simple sugars and amino acids, impact microbiome composition differently compared to non-Western diets 87, which tend to have higher beneficial short-chain fatty acid production 88 89.
Reduced bacterial diversity has been linked to obesity 90, emphasizing the importance of dietary variety.
Gut fermentation of dietary fibers
Gut fermentation in monogastric systems produces both beneficial and potentially harmful by-products.
Carbohydrate fermentation leads to short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate, offering health benefits like reducing inflammation in IBD 91, providing energy for colon cells 92, and potentially preventing colon cancer.
In contrast, protein fermentation generates compounds such as amines and ammonia 93, which can be harmful if not utilized by beneficial bacteria, potentially increasing the risk of colorectal cancer and ulcerative colitis 94.
Carbohydrate Fermentation
Bacterial fermentation of carbohydrates predominantly yields beneficial short-chain fatty acids (SCFA) like acetic, propionic, and butyric acids, along with lactic acid 95.
These SCFAs have various positive effects on gastrointestinal (GIT) health 96 97.
SCFAs serve as an energy source for both humans 98 and bacteria 99, with acetic, propionic, and butyric acids being the principal products.
They promote colonic health by influencing mobility, blood flow, and pH in the colon, affecting nutrient absorption 100 101.
Additionally, SCFAs and lactic acid play a crucial role in cross-feeding among gut bacteria, influencing bacterial growth and the GIT environment 102 103.
Acetic acid, the primary SCFA in venous blood 104, has anti-cancer properties and protects against DNA oxidative damage 105 106.
Propionic acid, mainly produced from carbohydrate fermentation 107, is metabolized in the liver and used for gluconeogenesis 108.
Butyric acid serves as a significant energy source for colonocytes and may help prevent colonic cancer 109 110.
Lactic acid, produced from various dietary sources, is abundant in the small intestine but less so in the large intestine due to differences in microbial populations 111 112 113.
Protein Fermentation in the Gut
Protein fermentation occurs when bacteria break down amino acids, usually when fermentable carbohydrates are scarce.
Reduced protein fermentation benefits gut health by lowering ammonia and nitrogenous compounds in the gastrointestinal tract (GIT) 114.
Excessive protein fermentation can elevate ammonia levels, leading to potential health issues.
Ammonia is detoxified in the liver and muscles, and excess is excreted as urea through the kidneys 115.
Protein fermentation can also produce branched-chain SCFA, amines, phenols, sulfides, and thiols.
Excess of these metabolites has been associated with conditions like colorectal cancers and ulcerative colitis 116 117.
However, a constant supply of carbohydrates and sufficient saccharolytic bacteria can mitigate these negative effects 118.
Gut Fermentation on Dietary Polyphenols
Certain dietary polyphenols have gained recognition for their health benefits, particularly their antioxidant properties.
However, recent research underscores the significance of bioavailability—the ability of polyphenols to enter the bloodstream.
This process is heavily influenced by microbial transformations in the gastrointestinal tract (GIT) 119.
Polyphenols are only bioavailable once they are released from the food matrix through processes like solubilization, enzyme action, or bacterial fermentation 120.
Dietary fiber (DF) plays a crucial role in this.
It can bind to polyphenols, making them resistant to release in the stomach and small intestine.
As a result, a substantial portion of dietary polyphenols reaches the large intestine, where they interact with gut microbiota 121.
The interaction between phytonutrients and GIT microbiota falls into two categories: microbial breakdown of large molecules into smaller ones, which may be absorbed, and the antimicrobial effects of certain phytonutrients on specific microbial species.
Both mechanisms can occur simultaneously 122.
Studies have shown that polyphenols can be metabolized by gut microbiota, leading to the formation of bioavailable compounds.
However, the extent of this bioconversion may vary 123.
For example, cranberry extract underwent significant metabolism by microbiota, while grape seed extract polyphenols had antimicrobial effects against specific bacteria 124.
Even the composition and diversity of gut microbiota can be influenced by polyphenols.
For instance, catechin consumption led to alterations in the gut microbial population 125.
High-flavonoid whole foods have also demonstrated antimicrobial effects, reducing the abundance of certain bacterial species 126.
Whole Foods vs. Supplements: Best Sources of Dietary Fibers
Evolution of Prebiotic Definitions
Prebiotics, initially defined as non-digestible food ingredients that selectively stimulate the growth of specific gut bacteria to improve host health, included compounds like inulin, resistant starch, and various oligosaccharides 127 128.
However, this definition has evolved.
A new perspective suggests that prebiotics are non-digestible compounds that, through microbial metabolism in the gut, modulate gut microbiota composition and activity, conferring a beneficial effect on the host 129.
This revised definition emphasizes changes in gut bacterial metabolism rather than specific bacterial species, broadening the scope to potentially include more complex or whole foods, not just purified compounds 130.
While studying purified dietary fiber allows specific associations with gut microbes, it may not accurately represent the behavior of fiber within whole plant cell walls (PCW) found in foods like grains, fruits, and vegetables 131.
Single polysaccharide fibers can ferment faster than their incorporated counterparts, affecting different GIT areas.
PCW complexity can slow fermentation by restricting accessibility, leading to changes in microbial activity.
Specific DF substrates can increase certain bacterial genera.
For example, galacturonic acid in pectin increases Bifidobacteria and Lactobacilli 132.
Despite these complexities, studying the effects of carbohydrates, whether from whole or purified sources, remains informative, especially given the impact of fermentable carbohydrates on the microbiota 133 134.
Moreover, purified DF studies are popular for potential use as prebiotics in commercial food products 135.
Whole Plant-Based Food Dietary Fibre and Microbiota
Whole plant-based foods, like whole grains, legumes, fruits, and vegetables, are essential for fostering a healthy gastrointestinal tract (GIT) microbiota.
These minimally processed ingredients have a significant impact on our gut health 136.
In studies comparing the fermentability of purified arabinoxylan (AX) with wheat bran, it was observed that purified AX ferments faster and to a greater extent than the more complex wheat bran.
This suggests that slower-fermenting materials tend to have a longer trajectory in the large intestine, influencing GIT fermentation 137 138.
Whole grains, which include all grain components in their natural proportions, provide fiber, lignans, antioxidants, phytosterols, and unsaturated fatty acids.
When compared to processed components, whole grains have been associated with a reduction in Bacteroidetes and an increase in beneficial bacteria like Bifidobacterium and Lactobacillus in the GIT microbiota 139 140 141 142.
Fruits and vegetables, although less studied, also have a positive impact on GIT microbiota.
Research has shown that consuming two apples a day can increase Bifidobacteria and Lactobacilli numbers 143.
Similarly, kiwifruit has been found to enhance microbial diversity, particularly within the Bacteroides and Bifidobacterium species 144.
Even small additions, like whole date fruits, can enrich the GIT microbiota with beneficial genera like Bifidobacteria, Lactobacillus, Enterococcus, and Bacteroides 145.
Key Takeaways for Incorporating Healthy Fibers into Your Diet
Purified dietary fiber (DF) polysaccharides have distinct effects on gut microbiota compared to whole plant-based foods.
Their simple structure causes noticeable shifts in microbial communities, sparking commercial interest in potential prebiotic properties in processed foods.
However, research comparing whole foods to their extracted DF counterparts is limited, partly due to the harsh chemical treatments required for extraction.
Additionally, the impact of phytonutrients adsorbed to plant cell walls (PCW) on gut bacteria is not well understood.
The complexity and variety of a diet, including plant-based ingredients, likely lead to a more diverse gut microbiota.
Future research, integrating microbiology, plant biology, and food technology, is crucial to understand these complex interactions and their health implications.
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