How gut microbiome and fiber diversity shape chronic disease outcomes

By Hugo Francisco de SouzaReviewed by Susha Cheriyedath, M.Sc.Oct 13 2024

Review highlights the powerful impact of dietary fibers on the gut microbiome, showing how specific fibers may reduce risks of obesity, cancers, and heart diseases while paving the way for tailored dietary solutions.

Review: The gut microbiome and dietary fibres: implications in obesity, cardiometabolic diseases and cancer. Image Credit: Alpha Tauri 3D Graphics / Shutterstock

In a recent review published in the journal Nature Reviews Microbiology, researchers in Belgium and Ireland synthesized literature investigating the associations between specific types of dietary fibers and chronic disease risk. They elucidated the role of the gut microbiome in dietary fiber outcomes, focusing on the three most widespread non-communicable diseases: obesity, cancers, and cardiometabolic conditions.

This review contributes to an ongoing debate about the effect of dietary fiber's chemical structure and fermentability on gut microbial diversity and function by providing evidence for the relationship between varied dietary fiber types and gut microbiome composition and epidemiologically linking the increased incidence of common chronic diseases with insufficient dietary fiber intake.

The review further collates the potential mechanisms underpinning these physiological benefits, highlighting that dietary fibers may selectively promote specific bacterial taxa while emphasizing that significant inter-individual differences in gut microbial composition necessitate personalized assessments before the true therapeutic potential of the gut microbiome may be realized.

What is dietary fiber, and why should we care?

Colloquially sometimes called “roughage,” dietary fibers are compounds in plant-derived foods that resist digestion by human digestive enzymes. Dietary fibers vary significantly in their chemical structure, being composed of carbohydrates or non-starch polysaccharides. They are usually grouped by their solubility, viscosity, and fermentability by gut bacteria, which affect how fibers are processed in the body and their impact on health.

A growing body of literature highlighting the specific benefits of fermentable fibers has accelerated their popularity, coaxing pharmaceutical advances allowing for enhanced isolation of dietary fibers from plants or even the chemical synthesis of non-digestible polysaccharides or oligosaccharides.

These benefits have led to the inclusion of various types of dietary fibers across several national and international dietary recommendations. They include improved transit time, nutrient trapping, and water availability in stools, directly affecting digestion and producing short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, which modulate gut and systemic health.

Unfortunately, substantial interindividual differences in gut microbial composition have led to confounding results from most small- and medium-sized cohort trials, sparking extensive debate on the causal and functional associations between the intake of specific fibers and gut microbial health.

The present review aims to synthesize current hypotheses and epidemiological evidence on these associations and unravel their implications in chronic disease risk. Obesity, cancers (specific types), and cardiometabolic diseases are the three most prevalent chronic diseases with the highest mortality risk and comprise the focus of this review.

Effect of dietary fiber consumption on gut microbial diversity

The capacity of humans to ‘ferment’ dietary fibers can be ascribed to specific bacterial populations expressing carbohydrate-active enzymes (CAZymes). These enzymes break down different fibers based on their complexity, from simple monomers like fructans to complex polymers like pectin, which require a larger number of CAZyme families. The types of compounds that can be processed depend on the preexisting composition of gut microflora. Their output (short-chain fatty acids [SCFAs]), in turn, is hypothesized to alter the gut’s nutrient landscape, thereby influencing (and potentially expanding) future gut diversity.

Since epidemiological investigations routinely establish inverse associations between gut microbial α-diversity (richness and evenness of communities) and the risk of chronic diseases, several studies have attempted to use dietary fibers as dietary supplements.

“A wide range of activities allows the degradation of the complex mixtures of dietary fibers that are present in plant products. ‘Simple’ dietary fibers composed of mostly one monomer, such as fructan, will require the activity of one or two CAZyme families, whereas more complex dietary fibers, such as pectin, require at least seven CAZymes families. The products issued from dietary fiber degradation by CAZymes — consisting of either five-carbon or six-carbon monosaccharides — are taken up by the bacteria owing to transporter systems for glycan utilization.”

Unfortunately, these investigations have provided confounding results – while a handful of studies have observed an increase in microbial diversity, others have reported no such increases, while still others have found decreases. This variability in findings is partly explained by the individual variability in microbial composition prior to fiber consumption, suggesting that tailored dietary interventions may be necessary.

Gut, fiber, and disease

The World Health Organization (WHO) considers obesity and cancer to be the two most notable public health challenges of the current century, justified when considering that these diseases accounted for 29% (19.4 million) and 15% (9.8 million) of all human deaths in 2021.

Thankfully, obesity and several cancer types are known to be attributable to preventable factors, including diet, tobacco use, and alcohol consumption. Healthy dietary patterns, including the Mediterranean Diet (MD), have been proven to significantly reduce the disease risk across epidemiological and nutritional/gut microbial studies.

Dietary fiber and gut health are both known to improve obesity and cardiometabolic outcomes independently. While the mechanism underpinning the former outcome is well established, those determining the latter remain debated. However, increasing evidence suggests that fermentable fibers can influence metabolic pathways and inflammation through their interaction with the gut microbiome.

Similarly, dietary fibers have been shown to reduce the risk of specific cancers (colorectal and breast), but the mechanisms resulting in these observations remain unknown.

Given the gut microbiome's role in dietary fiber fermentation, its role in improving chronic disease outcomes is undeniable. However, the lack of consistent causal links between gut microbial changes and disease outcomes means further research is needed.

“Five main mechanisms involving the gut microbiota have been reported. First, fiber fermentation leads to the production of SCFAs such as butyrate, which have been largely described as antitumoural. Second, secondary bile acids (BAs), which can promote tumor formation, can be modulated by fiber intake. The pattern of BAs can be modulated by dietary fibers, both through the gut microbiota and independently of the gut microbiota. Third, lignans linked to dietary fibers can be metabolized by bacteria into enterolignans. Fourth, some dietary fibers can reduce bacterial β-glucuronidase activity, thus decreasing toxin formation and reabsorption, limiting the risk of carcinogenesis. Last, dietary fiber intake can promote antitumor immunity.”

Conclusions and future directions

This review synthesizes the present knowledge on the implications of gut microbial-dietary fiber associations against chronic diseases, namely obesity, cancers, and cardiometabolic conditions. It provides evidence for the beneficial effects of high dietary fiber consumption on disease risks (reduction) and outcomes but highlights gaps in our understanding of the mechanisms underpinning these observations. The significant interindividual variability in gut microbial composition, especially, has produced confounding outcomes, sparking substantial debate and uncertainty in dietary recommendations.

Future investigations must either adopt extensive cohort sizes, thereby accounting for gut microbial variability, or (preferably) adopt personalized assessment protocols that leverage an individual’s unique gut microbial diversity to optimally combat chronic disease. The latter approach is expensive and labor-intensive but may allow for dietary interventions or pharmacological treatments with unprecedented safety and efficacy.