Dietary Fiber and Gut Microbiota: Surprising Key Effects

Dietary fiber and gut microbiota have a crucial impact on our health.

The relationship between dietary fiber and gut microbiota is complex.

Dietary fiber plays a significant role in maintaining overall health.

Gut microbiota is influenced by dietary fiber intake.

Understanding the connection between dietary fiber and gut microbiota is important for our well-being.

Main Findings

• Increased dietary fiber consumption positively influences metabolic health by altering gut microbiota.
• Dietary fiber affects gut microbiota composition and function, enriching certain species adapted to environmental changes in the ecosystem.
• These changes can lead to increased production of short-chain fatty acids (SCFAs), which can reduce metabolic syndrome symptoms like hyperlipidemia, hyperglycemia, hyperinsulinemia, and hypercholesterolemia.
• Further research is needed to fully understand dietary fiber–microbiome interactions, but increasing habitual fiber intake is seen as a promising and cost-effective method to reduce the burden of metabolic disease.

Introduction to Dietary Fiber

The human gastrointestinal tract (GIT) houses a complex microbial ecosystem with around 3.3 million microbial genes in the gut microbiota ¹.

Diet, antibiotics, exercise, age, and metabolic diseases like obesity, T2DM, and CVD influence microbial signatures ^{2 3 4 5}.

A low-fiber Western diet links to increased metabolic diseases ^{6 7}.

To improve health and prevent diseases, methods like fecal microbial transplants (FMT) modify gut microbes.

Dietary fiber intervention also reshapes the microbiota for better health ^{8 9}.

Evidence from studies shows that different dietary fibers and their fermentation products, like short-chain fatty acids (SCFAs), benefit metabolism ¹⁰.

SCFAs from fiber fermentation in the colon help regulate glucose and lipid metabolism ¹¹.

Understanding how specific fibers impact microbiota and their metabolic effects offers therapeutic potential for metabolic diseases.

Understanding Dietary Fiber

Dietary fiber, plant-based carbs indigestible by our enzymes, undergo anaerobic fermentation by specific gut microbes, producing short-chain fatty acids (SCFAs) ¹².

Definitions of dietary fiber vary globally; European standards accept fibers with 3 monomeric units, while others require at least 10 ^{13 14}.

Fibers with ≥10 units include cellulose, hemicellulose, gums, pectin, mucilage, inulin, psyllium, β-glucan, and resistant starch (RS) ¹⁵.

Those with 3-9 units are called resistant oligosaccharides, like GOS and FOS ¹⁶.

Despite differing definitions, all fiber types influence gut microbiota, affecting host metabolism.

Fiber’s impact on the host relies on properties like solubility, viscosity, and fermentability, which vary across types ¹⁷.

Solubility refers to water dissolution, fermentability indicates microbial metabolism, and viscosity describes gel-like formation in water.

“Microbiota-accessible carbohydrate” (MAC) excludes insoluble fibers.

Highly soluble, fermentable, and viscous fibers, such as β-glucan, gums, and pectin, are examples ¹⁸.

How Fiber Shapes Our Gut Bacteria

Dietary fiber wields significant influence over the composition, diversity, and richness of the gut microbiome by providing a range of substrates for fermentation reactions carried out by specific microbe species with the necessary enzymatic machinery ¹⁹.

The large intestine hosts numerous microbiota species specializing in fiber fermentation, and various types of dietary fiber undergo breakdown.

When dietary fiber intake increases, it alters the nutritional environment in the intestines, allowing these fiber-loving bacteria to thrive ²⁰.

Conversely, those with low fiber diets tend to exhibit reduced microbial diversity, as their diets often feature animal proteins and fats instead ²¹.

Studies from different regions and socioeconomic backgrounds consistently highlight the impact of diet on human gastrointestinal tract microbial populations ^{22 23 24 25 26 27 28 29 30}.

One common thread is the significantly higher fiber consumption among individuals in less developed and rural societies compared to those in industrialized nations ^{31 32}.

Intriguingly, industrialized nations, characterized by low fiber intake, often experience a higher prevalence of metabolic and inflammatory diseases like obesity and IBD.

For instance, De Filippo et al. (2010) ³³ found differences in gut microbiomes between Burkina Fasian and Italian children.

Burkina Fasian children displayed higher levels of Actinobacteria, Bacteroidetes, and Prevotella, whereas Italian children exhibited a higher abundance of Firmicutes and Proteobacteria ³⁴.

A similar study by the same author comparing four cohorts with varying degrees of urbanization noted reduced fiber consumption among urban Italian and urban Burkina Faso children, reflected in lower Prevotella species in their microbiota ³⁵.

These microbiota differences mirrored those reported by Ou et al. (2013) ³⁶ when comparing rural South Africans with African Americans.

In 2015, a study comparing the microbiota of Papua New Guineans with Americans revealed an altered Prevotella:Bacteroides ratio in the fecal microbiota of Papua New Guineans, who consumed a fiber-rich diet.

They exhibited a high abundance of Prevotella but a low abundance of Faecalibacterium, Ruminococcus, Bifidobacterium, Bacteroides, Blautia, Bilophila, and Alistipes ³⁷.

Yatsunenko et al. (2012) ³⁸ found similar results when comparing individuals from Venezuela, Malawi, and the USA.

Another study investigated the microbiota of the Yanomami tribe in rural Venezuela, revealing a higher abundance of Prevotella and a lower abundance of Bacteroides compared to individuals from the USA ³⁹.

Additionally, the Yanomami population had lower levels of other Bacteroidetes family members, including Bacteroidales, Mollicutes, and Verrucomicrobia, and an increased abundance of the genus Phascolarctobacterium, known for SCFA production ⁴⁰.

A study comparing the Tanzanian Hadza tribe to Italians found an abundance of well-known fiber-degrading bacteria in both populations, including Firmicutes families like Lachnospiraceae, Ruminococcaceae, Veillonellaceae, Clostridiales Incertae Sedis XIV, and Clostridiaceae.

Similar to other studies, Prevotella was enriched in the Hadza microbiome, while their Italian counterparts had a lower abundance of Bacteroides ⁴¹.

Another study in the USA linked higher Prevotella levels to fiber degradation and reported correlations with Roseburia, Eubacterium rectale, Faecalibacterium prausnitzii, and increased total SCFAs concentrations in response to a plant-based diet ⁴².

These findings underscore the profound impact of dietary fiber on the microbiota.

Industrialized nations typically exhibit higher abundances of Bacteroides, Bifidobacterium, Ruminococcus, Faecalibacterium, Alistipes, Bilophila, and Blautia ^{43 44 45 46 47}.

Surprisingly, cultural and ethnic differences have minimal effects on the host microbiota, as evidenced by minimal variations in microbiota among industrialized nations that share a common Western diet ^{48 49}, high in saturated fat and low in fiber.

Prevotella species are more abundant in non-industrialized individuals with high fiber diets.

Interestingly, the microbiota of these non-industrialized populations mirrors those of Western vegans and vegetarians, all of whom consume ample dietary fiber ⁵⁰.

High fiber intake, combined with specific fiber-fermenting microbes, offers various health benefits, including substantial SCFA production.

The Connection Between Dietary Fiber and Gut Microbiota

Dietary fiber is a powerful influencer of gut microbiota composition.

It offers a rich array of substrates for fermentation reactions carried out by various microbial enzymes in the large intestine.

Different types of fiber may require multiple enzymatic steps to yield short-chain fatty acid (SCFA) products, involving numerous microbial players.

Some microbes specialize in fiber degradation and are known as primary degraders or keystone species ^{51 52}.

Meanwhile, others play minor roles, referred to as secondary fermenters or cross-feeders, benefiting from the work of primary degraders.

Remarkably, certain members of the colonic microbial community, such as Bacteroides thetaiotaomicron, exhibit flexibility, encoding multiple enzymes for the degradation of various fiber subtypes ⁵³.

Carbohydrate-active enzymes (CAZymes), including glycoside hydrolases, polysaccharide lysases, and carbohydrate esterases, play pivotal roles in fermentation reactions ^{54 55}.

Compositional differences among resistant starches can have distinct effects on the host microbiota.

In humans, RS4 consumption increased Actinobacteria and Bacteroidetes abundance while reducing Firmicutes ⁵⁶.

RS4 also led to higher levels of Parabacteroides distasonis and Bifidobacterium asolescentis.

On the other hand, RS2 had no effect at the phylum level but increased populations of Ruminococcus bromii and Eubacterium rectale at the species level ⁵⁷.

In vitro experiments demonstrated that Ruminococcus bromii is crucial for RS2 and RS3 fermentation ⁵⁸, with other colonic microbes further fermenting the products into SCFA.

Bifidobacterium breve and Bifidobacterium adolescentis were also identified as enzymes capable of degrading resistant starch ⁵⁹.

Inulin consumption in humans increased Bifidobacterium bifidum ^{60 61 62} and Faecalibacterium prausnitzii levels ⁶³, while lowering Enterococcus species ⁶⁴.

Cross-feeding occurred, with Eubacterium rectale breaking down inulin initially, followed by fermentation of the products by Anserostipes caccae ⁶⁵.

Wheat bran supplementation increased the Firmicutes and Bacteroidetes phyla ⁶⁶.

Obese men supplemented with wheat bran showed elevated levels of Actinobacteria at the genus level, including Prevotella, Bacteroides, Eggerthella, Lachnospiraceae, Corynebacterium, and Collinsella ⁶⁷.

In contrast, whole-grain wheat intervention in healthy individuals increased Enterococcus, Bifidobacterium, Clostridium, and Lactobacillus ⁶⁸, indicating personalized effects based on the initial microbiome.

Oat-derived β-glucan increased Bacteroidetes and decreased Firmicutes ⁶⁹.

In rats, β-glucan intervention boosted Bacteroides and Prevotella species ⁷⁰.

It also led to higher levels of Bifidobacteria ⁷¹, butyrate-producing Clostridium histolyticum ⁷², and other Clostridium family members capable of fermenting β-glucan ^{73 74 75}.

Gum acacia fiber increased Bifidobacteria and Lactobacilli ⁷⁶, with Prevotella ruminocola postulated as one of the main fermenters ^{77 78}.

Galacto-oligosaccharide (GOS) increased Bifidobacterium species ⁷⁹ and Faecalibacterium prausnitzii ⁸⁰, while fructo-oligosaccharide (FOS) fermented with Bifidobacterium and Collinsella aerofaciens ⁸¹.

Whole-grain barley intake increased Prevotella copri ⁸², whereas whole-grain rye fiber had no effect on gut microbiota in healthy Danes ⁸³.

Interestingly, rye bread supplementation in Finnish individuals with metabolic syndrome resulted in lower Bacteroidetes levels and higher Firmicutes and Actinobacteria ⁸⁴.

In essence, dietary fiber profoundly affects gut microbiota composition, involving multiple species and enzymes in fiber degradation.

This intricate symbiotic relationship starts with primary degraders and ends with cross-feeders producing SCFAs ^{85 86}.

Although cross-feeding in resistant starch fermentation is well-documented, more research is needed to understand these relationships in the breakdown of other fiber types.

To harness the full potential of dietary fiber in treating metabolic disorders, it’s essential to investigate the specific roles these microbiota play in metabolizing different fiber subtypes.

The Healthy By-products of Fiber Digestion

Dietary fiber fuels your gut’s microbial community, yielding a wealth of health benefits.

It’s linked to reduced cholesterol and improved glucose control ⁸⁷.

Fiber doesn’t just influence your gut’s composition; it also shapes the metabolites produced there ⁸⁸.

Short-chain fatty acids (SCFAs) like butyrate, acetate, and propionate are the key players.

SCFAs impact various bodily processes, especially in the gut ⁸⁹.

The type of fiber and the mix of gut microbes determine the quantity and variety of SCFAs produced ^{90 91}.

SCFAs are usually absorbed in the colon, playing a pivotal role in supporting your health and metabolism.

Reduced fiber intake can lead to lower microbial diversity and fewer SCFAs ⁹².

How Fiber Benefits Your Body

Dietary fiber, a key player in gut health, unleashes short-chain fatty acids (SCFAs), especially butyrate, which activate essential receptors in the digestive tract.

These receptors, including GPR41, GPR43, and GPR109A, influence vital metabolic functions like glucose and lipid regulation, as well as appetite control ⁹³.

Butyrate and propionate, when produced by fiber fermentation, stimulate gluconeogenesis genes, promoting a feeling of fullness by enhancing hepatic portal vein glucose sensing.

Acetate, another SCFA, aids in satiety through hypothalamic signaling.

Moreover, SCFAs trigger the release of satiety-inducing peptides, PYY and GLP-1, from colon cells, helping manage appetite and digestion ^{94 95 96 97}.

Dietary fiber, through SCFAs, indirectly affects metabolism by curbing calorie intake and promoting fullness.

Fiber’s Role in Managing Blood Sugar and Fats

Dietary Fiber’s Influence on Glucose Regulation:

Research has shown that dietary fiber positively affects glucose metabolism by producing short-chain fatty acids (SCFAs) in the colon ⁹⁸.

These SCFAs, particularly propionate and acetate, activate receptors like GPR43 and stimulate the release of GLP-1, a hormone that promotes insulin secretion and inhibits glucagon ^{99 100 101}.

This interaction enhances insulin sensitivity and glucose tolerance.

Butyrate, another SCFA, independently induces glucose synthesis in the intestines, further improving glucose metabolism ^{102 103 104}.

Additionally, SCFAs can reduce hepatic gluconeogenesis ¹⁰⁵, which is linked to insulin resistance and chronic gut diseases ^{106 107}.

Dietary Fiber’s Impact on Lipid Metabolism:

SCFAs from fiber fermentation also influence lipid metabolism.

They activate receptors like GPR41 and GPR43 ¹⁰⁸ and even the key regulator PPAR-γ ¹⁰⁹.

Acetate can be used in the liver for lipogenesis ¹¹⁰, while propionate enhances adipose tissue lipoprotein lipase activity ^{111 112}.

Studies have shown that SCFA administration improves metabolic parameters, including fat oxidation and energy expenditure¹¹³.

Resistant Starch (RS) and Glucose Regulation:

Studies on RS, a type of dietary fiber, have yielded mixed results ^{114 115}.

While some research showed improvements in insulin sensitivity and reductions in visceral fat and cholesterol levels ¹¹⁶, other studies found no significant effects ¹¹⁷.

Arabinoxylan, another fiber type, has been linked to lower postprandial glucose and insulin levels, especially in patients with type 2 diabetes ^{118 119 120}.

Gum Fibers and Glucose Control:

Gum fibers have demonstrated the ability to reduce fasting blood glucose levels and insulin release ¹²¹.

Their viscosity helps bind bile acids, reducing glucose absorption in the intestines ^{122 123}.

Gum guar, for instance, reduced plasma fatty acids, HbA1c, and waist circumference in type 2 diabetes patients ¹²⁴.

Galactooligosaccharides (GOS), Fructooligosaccharides (FOS), and Inulin:

These fiber types have shown potential in altering glucose and lipid metabolism ^{125 126}.

However, the results vary depending on the study design ^{127 128}.

Inulin, in particular, has been associated with reduced circulating triacylglycerides ¹²⁹ and improved glucose metabolism ¹³⁰.

Beta-Glucans and Their Impact:

Beta-glucans are known for their benefits in obesity, metabolic syndrome, type 2 diabetes, and cardiovascular disease ¹³¹.

They have consistently been shown to reduce cholesterol and triacylglycerides ^132133134135.

Oat-derived beta-glucans, with their bile acid-binding capacity, also improve glucose metabolism ^{136 137}.

Rye Fiber’s Positive Effects:

Rye-based products have been found to reduce hunger ^{138 139 140}, postprandial glucose, insulin levels ¹⁴¹, and cholesterol ¹⁴².

They have a role in managing metabolic syndrome and improving cardiovascular health.

Fiber, Obesity, and Diabetes

The Impact of Dietary Fiber on Obesity:

High dietary fiber intake correlates with increased gut microbiota diversity and lower long-term weight gain ^{143 144}.

Reduced microbial diversity is associated with obesity, often reflecting lower fiber consumption ¹⁴⁵.

Dietary fiber demonstrates anti-obesity effects ^{146 147}, making it a vital component in the battle against this condition.

Studies reveal that gut microbial changes play a significant role in obesity ^{148 149}.

For instance, obese individuals exhibit lower proportions of Bacteroidetes phyla compared to lean individuals.

However, weight loss through low-calorie diets can increase Bacteroidetes abundance ¹⁵⁰.

Research also demonstrates that gut microbes contribute to increased nutrient utilization and adiposity, further underscoring their role in obesity development ¹⁵¹.

The transfer of gut microbiota from healthy to germ-free mice increases body weight and promotes insulin resistance ¹⁵², highlighting the influence of the human gut microbiome on metabolic outcomes.

These findings are reinforced by experiments showing that colonizing mice with “obese microbiota” significantly increases body fat compared to those colonized with “lean microbiota.”

Differences in the core microbiota between obese and lean individuals underscore the role of gut microbiota alterations in obesity ¹⁵³.

Specific Microbes and Obesity:

At the species level, certain microbes have been associated with obesity and metabolic syndrome.

For instance, Lactobacillus has been linked to obesity, with obese individuals having a higher abundance of this bacterial genus ¹⁵⁴.

However, in obese children, Faecalibacterium prausnitzii was found to be more abundant [ 210].

Meanwhile, germ-free mice colonized with Bacteroides thetaiotaomicron exhibited a 23% increase in body fat composition ¹⁵⁵.

Other studies have identified correlations between specific microbial populations and obesity risk, with variations in Clostridium histolyticum, Eubacterium rectale, Clostridium coccoides, and the Bacteroides/Prevotella group among obese individuals ¹⁵⁶.

Lipopolysaccharide (LPS)-producing microbes have also been implicated in obesity, with obese individuals showing significantly higher circulating LPS concentrations ^{157 158}.

The Role of Dietary Fiber in Shaping Microbiota:

Dietary fiber interventions may offer a promising strategy to address obesity.

Differences in gut microbiota composition between obese and lean individuals ¹⁵⁹ are similar to those observed between individuals consuming high-fiber and low-fiber diets ¹⁶⁰.

This correlation strengthens the hypothesis that dietary fiber can influence obesity through its impact on gut microbiota.

Obesity, T2DM, and Gut Microbiota:

T2DM is closely associated with obesity and metabolic syndrome.

Over 80% of T2DM individuals are overweight or obese, and increased weight gain is a major risk factor for T2DM development ¹⁶¹.

Studies have revealed significant alterations in the gut microbiota of individuals with T2DM.

These alterations include a lower abundance of beneficial butyrate-producing microbes and a higher prevalence of pathogenic bacteria ¹⁶².

The impact of metformin, a commonly prescribed T2DM medication, on gut microbiota further underscores the role of gut microbes in T2DM ^{163 164}.

While the exact mechanisms remain unclear ¹⁶⁵, it appears that metformin’s effects are mediated through the gut microbiota.

Individuals with obesity and T2DM experience shifts in their gut microbiota, contributing to their disease phenotype ^{166 167}.

Dietary Fiber as a Therapeutic Approach:

Dietary fiber has the potential to modulate the gut microbiota, creating an environment conducive to the growth of beneficial short-chain fatty acid (SCFA)-producing microbes while improving glucose and lipid metabolism.

This underscores its importance as a potential therapeutic approach for individuals grappling with metabolic challenges.

Conclusion

Historical Dietary Changes and Metabolic Health:

In recent centuries, dietary shifts contributed to differing metabolic disease rates in developed and developing nations.

The Western diet, rich in high-glycemic load, low-fiber foods, contrasts with our ancestors’ 100-gram daily fiber intake ¹⁶⁸.

Nowadays, non-industrialized nations average 50 grams ¹⁶⁹, while Western industrialized nations typically consume only 12–18 grams daily ^{170 171 172}.

This fiber gap, along with high protein and fat intake, correlates with gut microbiota differences ^{173 174 175}.

Western countries’ microbiomes exhibit minimal variation ¹⁷⁶, but vegan/vegetarian diets resemble non-industrialized populations with higher fiber intake and fewer metabolic diseases ¹⁷⁷.

Current fiber recommendations are 30-35 grams for males and 25-32 grams for females ¹⁷⁸, but Western societies fall short.

Dietary Fiber and Metabolic Health:

Dietary fiber plays a pivotal role in improving metabolic health in individuals with obesity, metabolic syndrome, and gut-related disorders ^{179 180 181}.

Numerous studies link dietary fiber intake to overall health ^{182 183}, with some inconsistencies ¹⁸⁴.

Varying fiber amounts among studies result in diverse gut microbiota and metabolic changes, compounded by individual microbiome variations.

The highly personalized human gut microbiome complicates interventions, as some individuals lack the necessary microbiota for specific fiber breakdown.

The Need for Personalization:

A personalized approach may be necessary for addressing global metabolic health issues, potentially requiring higher fiber doses as seen in non-industrialized nations (50 grams) ¹⁸⁵.

Specific fiber types like resistant starch, arabinoxylan, and gum acacia show better tolerance at higher doses.

Gradual fiber intake increases may enhance adaptability ¹⁸⁶.

Challenges and Potential Solutions:

Dietary fiber interventions alone may not suffice to reverse metabolic issues.

Studies with germ-free mice suggest that certain beneficial microbiota may not be hereditary ¹⁸⁷.

A low-fiber diet can alter the microbiome within three generations, potentially irreversibly.

Combining dietary fiber with fecal microbial transplantation could introduce efficient fiber-digesting bacteria for more effective treatment.

Review date not set.