The Gastrointestinal Tract Microbiome and Potential Link to Alzheimer’s Disease

Accumulating clinical- and scientific research-based evidence is driving our increased awareness of the significance of the human microbiome (HM) to the healthy and homeostatic operation of the human central nervous system (CNS). HM communities occupy several different but distinct microbial ecosystems on and within the human body, including nasal, oral, and otic cavities, the surface of the skin and the urogenital and the gastrointestinal (GI) tracts. The complex symbiotic inter-relationship between the GI-tract microbiome and its host is strongly influenced by diet and nutrition, and when optimized can be highly beneficial to food digestion, nutrient intake, and immune health (1–6). For example, dietary composition ultimately affects the structure, organization, function, and speciation of the HM occupying the GI tract, in part by supplying multiple substrates for microbial metabolism. Typical Western diets containing high fat–cholesterol, low amounts of soluble and insoluble fiber, and sugar- and salt-enrichment not only impart deleterious nutrition but also dietary constraints on the HM. This in turn impacts the supply of microbiome-generated molecules absorbed into the systemic circulation for transport into the extensive neurovasculature of the CNS. This short communication will focus on emerging ideas concerning the contribution of the GI-tract microbiome to human neurological disease with emphasis on Alzheimer’s disease (AD) wherever possible.