Going with the grain: Fiber, cognition, and the microbiota-gut-brain-axis

Abstract

Healthy dietary intake has been acknowledged for decades as one of the main contributors to health. More recently, the field of nutritional psychiatry has progressed our understanding regarding the importance of nutrition in supporting mental health and cognitive function. Thereby, individual nutrients, including omega-3 fatty acids and polyphenols, have been recognized to be key drivers in this relationship. With the progress in appreciating the influence of dietary fiber on health, increasingly research is focusing on deciphering its role in brain processes. However, while the importance of dietary fiber in gastrointestinal and metabolic health is well established, leading to the development of associated health claims, the evidence is not conclusive enough to support similar claims regarding cognitive function. Albeit the increasing knowledge of the impact of dietary fiber on mental health, only a few human studies have begun to shed light onto the underexplored connection between dietary fiber and cognition. Moreover, the microbiota-gut-brain axis has emerged as a key conduit for the effects of nutrition on the brain, especially fibers, that are acted on by specific bacteria to produce a variety of health-promoting metabolites. These metabolites (including short chain fatty acids) as well as the vagus nerve, the immune system, gut hormones, or the kynurenine pathway have been proposed as underlying mechanisms of the microbiota-brain crosstalk. In this minireview, we summarize the evidence available from human studies on the association between dietary fiber intake and cognitive function. We provide an overview of potential underlying mechanisms and discuss remaining questions that need to be answered in future studies. While this field is moving at a fast pace and holds promise for future important discoveries, especially data from human cohorts are required to further our understanding and drive the development of public health recommendations regarding dietary fiber in brain health.

Keywords: Fiber; cognition; microbiota-gut-brain axis; nutrition.

Conflict of interest statement

DECLARATION OF CONFLICTING INTERESTS: The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: JFC has been an invited speaker at meetings organized by Mead Johnson, Friesland, Neuropharmex, Yakult, and Alkermes, has been a consultant for Nestle, and has received research funding from Mead Johnson, Cremo, Nutricia, and DuPont. All other authors report no conflict of interest.

Figures

Figure 1.

Candidate mechanisms for the impact of dietary fiber on cognitive functions. While the mechanisms underlying the benefit of fiber on cognition have not been established, several microbiota-dependent pathways could be proposed. Certain microbes can degrade dietary fiber, supporting their growth and the production of metabolites (e.g. short chain fatty acids (SCFAs)). These microbes themselves and their metabolites can regulate neurochemistry relevant to cognitive processes, stimulate neurotransmitter production, and influence neuronal function. Likewise, gut hormones (e.g. ghrelin, peptide YY) with neuroactive potential can be affected by microbes and microbial metabolites. Another important pathway whereby dietary fiber exerts influence on cognition could be immune mediated, through the kynurenine pathway or the vagus nerve.