Chapter 49 - The impact of the gut microbiome in Alzheimer's disease: cause or consequence?

Abstract

Alzheimer's disease is a devastating neurodegenerative disease clearly characterized by loss of synapses, changes in metabolism, and neuronal death in brain tissue. The impact of nutrition and other lifestyle factors on pathomechanisms has been extensively investigated in recent decades. However, one main issue has been out of focus during these research efforts—the human body is not a single entity but provides an ecological niche for a huge number of other organisms, its microbial flora. These microorganisms outnumber the host's genetic pool indisputably. While the human encodes for 30,000 genes, the microbiome of the gastrointestinal tract inherits a multitude of commensal microbes accounting for about 4×106 genes. Not all metabolites arising from this community—or from pathological invaders—will have an impact on human health despite a direct effect on the gut. However, it is intriguing to consider the impact of the gut's microbiome on aspects of pathology of the brain. The brain and the gut are interconnected via several routes such as the vagus nerve and share at least some properties, because the gut contains the body's hugest neuronal network outside the central nervous system. Within this review we focused on Alzheimer's disease as the most prevalent form of dementia and report on (1) what is known about changes in gut physiology in Alzheimer's disease; (2) how the microbiome might be affected in Alzheimer's disease; (3) the consequences of those changes on the host; and (4) the potential therapeutic value of this knowledge.