The last few years have seen a plethora of books published on the microbiome and the related topic of the influence of how the gastrointestinal (GI) milieu has implication that reach far beyond one organ system. I recently finished another book in this genre, The Mind Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health by UCLA gastroenterologist and professor and Dr. Emeran Mayer MD, PhD. The focus and theme of this valuable book is on the intricate relationships the enteric nervous system (ENS) has with the brain and how alterations in this second brain are increasingly being linked to myriad conditions.
While the ENS gets much less attention than the brain and spinal cord, which together comprise the central nervous system (CNS), it is no less important and Dr. Mayer's book concretizes this fact in countless ways. Beginning with hypotheses regarding the signaling molecules that evolved between ancient bacterial species, when they were the only organisms to "talk" to on the planet, Dr. Mayer moves to the multicellular but tiny marine creature, the hydra, which is described as "a floating digestive tract". Eventually organism like the hydra were colonized with bacterial species and a symbiotic relationship evolved. Fast forward to our gut, minutely wired with nerves and teeming with neurotransmitters like serotonin, and you can understand how this system developed. As Mayer argues, as the primary brain evolved and the role of the secondary brain was relegated to the GI tract with an important connection between the two via the vagus nerve which allows constant communication and feedback. The microbes in the gut are a vital component of this system as well as their metabolites directly interact with the ENS and, consequently the brain.
To put it simply, there's a clear physiological and evolutionary reason why one gets diarrhea when nervous as a full bowel may not be the best thing to have when facing an important stressor like a lion (or the SAT exam). Taking this simple example and expanding it to reveal how perturbations in this system can lead to real pathology has led to major insight into diseases as disparate as celiac and Parkinson's disease.
There are many fascinating aspects of this book including historical observations (via a gastric fistula) made on how various emotions influence digestion rates, the role of inflammatory foods, and how psychosocial stressors (particularly early in life) can leave their mark on the gut-brain-microbiota axis. This book stands out in this ever growing field as one that provides a lot of seminal information about the ENS and the microbes it interacts with in an easily accessible manner with an emphasis on explaining real world macro-phenomena and it is for that reason I highly recommend it.