Gut Microbiome-Associated Effects of Plant-Based Diets on Glucose Homeostasis, Body Composition, and Cognitive Function: A Scoping Review

The worldwide prevalence of noncommunicable diseases, including obesity, type 2 diabetes, and neurocognitive decline, has increased rapidly over the last decades, warranting healthy and sustainable strategies to counteract these conditions. The gut microbiome is increasingly recognized as an important factor in the development of these metabolic diseases. Current nutritional guidelines advise toward more plant-based diets for health and sustainability reasons. These diets contain relatively high quantities of dietary fibers that can be fermented by the gut microbiota in the colon. This produces short-chain fatty acids (SCFA), recognized for their beneficial effects on insulin resistance, inflammation, and satiety. In the absence of fermentable fibers, however, colonic proteolytic fermentation increases, producing metabolites that are potentially harmful to host health. Moreover, plant-based proteins are less digestible compared with animal-based proteins, potentially increasing proteolytic fermentation. However, the extent to which plant-based diets affect the microbiome, and thereby host health, is still unknown. We, therefore, conducted a scoping review investigating the effects of plant-based dietary interventions on the microbiome in relation to the effects on body composition, glucose metabolism, and neurocognition. Overall, (healthy) plant-based diets, either as a whole dietary pattern, whole-grain consumption, or plant-protein-rich products, can contribute to lower body (fat) mass, fasting glucose, improve insulin resistance and episodic memory, and reduce psychological distress and anxiety. Notably, no adverse effects of plant-based diets were reported in any of the studies. The studies that reported alterations to microbial composition after plant-based interventions generally show increased microbial diversity and abundance of SCFA-producing species such as Prevotella, Lachnospiraceae, and Bifidobacteria. However, measures on microbial functionality, based on microbial metabolites such as SCFA and branched-chain fatty acids, were often lacking or remained mostly unaffected. Furthermore, research on the long-term and individual effects of both healthy and unhealthy plant-based diets on host health and the microbiome is required.