A Summary of the article on “Going with the grain: Fiber, cognition, and the microbiota-gut-brain-axis”
Background
The minireview article, “Going with the grain: Fiber, cognition, and the microbiota-gut-brain-axis” authored by Berding K, Carbia C, and Cryan J (2021), from APC Microbiome Ireland and University College Cork, was published in Experimental Biology and Medicine in April 2021. It focuses on exploring the underexplored connection between dietary fibre and cognitive function in humans. While the importance of a healthy diet for overall health and mental well-being has long been recognised, and the role of dietary fibre in gastrointestinal and metabolic health is well established, the evidence supporting its direct impact on cognitive function is not yet conclusive enough for health claims.
Results
The authors highlight the microbiota-gut-brain axis as a key pathway through which nutrition, especially fibre, can influence the brain. Dietary fibre is defined as “carbohydrate polymers with ten or more monomeric units which are not hydrolysed by the endogenous enzymes in the small intestine of humans”. It encompasses various types, such as soluble fibres (e.g., pectins, β-glucans) and insoluble fibres (e.g., cellulose), found in foods like whole grains, legumes, fruits, and vegetables. Despite the well-documented health benefits, most people in Western countries do not meet the recommended daily intake of fibre.
Dietary fibre serves as a major nutrient source for the gut microbiota, a vast community of microbes inhabiting the gastrointestinal tract. A high-fibre diet is typically associated with a “healthy” microbiota, characterised by increased microbial diversity and a bloom in beneficial microbes like Bifidobacterium and Lactobacillus. This microbial fermentation of fibre leads to the production of important metabolites, most notably short-chain fatty acids (SCFAs).
Human observational studies have begun to shed light on the association between dietary fibre and cognition. For example, higher insoluble and total fibre intake was linked to better cognitive control in children. In adults, consuming more fibre or multigrain products was associated with improved cognitive performance. Furthermore, a prospective study indicated that higher fibre intake in young adulthood correlated with better verbal learning, memory, psychomotor speed, sustained attention, and working memory in middle age. However, some studies noted gender-specific associations and a systematic review highlighted low evidence for a direct link between childhood fibre intake and cognitive function.
Interventional studies using either isolated fibres/prebiotics or high-fibre foods have yielded inconsistent results. Some studies showed improvements in recognition memory, cognitive flexibility, sustained attention, and emotional information processing following prebiotic supplementation. Conversely, a recent meta-analysis concluded that current evidence does not robustly support the use of prebiotics to influence cognitive measures. Studies with whole high-fibre foods also presented mixed findings, with some showing cognitive improvements but often making it difficult to attribute benefits solely to fibre due to the presence of other beneficial nutrients like polyphenols.
The article proposes several candidate mechanisms for fibre’s impact on cognition, both microbiota-dependent and independent. Microbiota-dependent mechanisms are considered more relevant to brain-modulating properties and include:
- Short-chain fatty acids (SCFAs): As end-products of fibre fermentation, SCFAs can support intestinal barrier integrity, protect against neuroinflammation, enhance blood–brain barrier integrity, modulate neurotransmitters and neurotrophic factors, and influence the hypothalamus-pituitary-adrenal (HPA) axis.
- Neurotransmitters and neurotrophic factors: The gut microbiota can influence neurochemistry and brain structure, with beneficial microbes responding to dietary fibre and potentially modulating brain-derived neurotrophic factor (BDNF).
- Gut hormones: Fibre-responsive microbes and SCFAs can stimulate the secretion of gut hormones (e.g., GLP-1, PYY, ghrelin), which can affect synaptic activity and cognitive functions.
- Immune system: Dietary fibre’s anti-inflammatory potential, mediated by the gut microbiota, could alleviate cognitive dysfunction.
- Kynurenine pathway: The microbiota can regulate this pathway, which is implicated in neurobiological functions including cognition.
- Vagus nerve: Fibre-responsive gut microbes and SCFAs can activate the vagus nerve, a direct communication route between the gut and brain, potentially enhancing cognition and neurogenesis.
Conclusions
In conclusion, while animal studies suggest an influence of dietary fibre on cognitive abilities, more robust human data are needed to establish definite evidence and understand the long-term implications of low fibre consumption. Future research should focus on diverse age groups, optimal fibre amounts and types, and delve deeper into underlying mechanisms, particularly the role of the gut microbiota, to develop evidence-based health claims and therapeutic interventions.
References
Berding, K., Carbia, C., & Cryan, J. F. (2021). Going with the grain: Fiber, cognition, and the microbiota-gut-brain-axis. Experimental Biology and Medicine, 246(7), 796–811. https://doi.org/10.1177/1535370221995785
This post is based on Open Access research and is for informational purposes only.
