IBS: The gut-brain-microbiome connections

For over 4000 years, the system of classical traditional Chinese medicine described fundamental connections of the bowel and the “lung.’  This linkage guided acupuncture and Chinese medicine practitioners ever since. Hippocrates is said to have observed, “bad digestion is at the root of all evil…death sits in the bowels.” In Western medicine the digestive system has been thought of primarily as a tube through which food passes, nutrients are extracted, and waste eliminated.

Patients are often painfully aware of the symptoms of IBS and how the intestinal tract interacts with the brain. People with IBS also tend to exhibit signs of stress, anxiety, and depression. IBS patients have a greater potential for thoughts of suicide,[i]  In the past IBS was often dismissed as “functional” or psychosomatic in origin.

Current understanding of digestion has modeled a multi-directional communication system termed “the gut-brain axis” (GBA). Put something in your mouth and the entire system is affected.Nerve connections within the various regions of the gut and between the gut and the central nervous system are beginning to unravel this complexity.

Further understanding expands the gut-brain axis to include the microbes that inhabit the alimentary canal form one end to the other. Not only are the gut and brain tightly interconnected, but the presence of gut flora influences this system such that now the GBA is extended to the gut-brain-microbiome axis (GBMA). A 2017 review[ii] provides a good summary of current concepts of the complex interactions of the GBMA.

The average adult human GI tract hosts up to 1014 organisms, over 2 pounds worth in total.  This is much larger that the number of human cells in the body.  The genetic information encoded in the microbiome is more than 100 time that in humans.  This concept is a paradigm shift in the understanding of digestion and health and brings to light ideas that originated millennia ago.

Accounting for the microbiota in health and disease provides a clinical rationale for treatments.  For example, manipulating the micro-organisms in the GI tract through the use of pre- and probiotics has been shown clinically to be effective.  With the GBMA model, we know why and how this treatment works.

Next time we’ll look at the evidence supporting pharmacologic interventions for IBS.


[i] Miller, Vivien et al. Suicidal ideation in patients with irritable bowel syndrome Clinical Gastroenterology and Hepatology , Volume 2 , Issue 12 , 1064 – 1068

[ii] Dinan, Timothy G. et al. The Microbiome-Gut-Brain Axis in Health and Disease Gastroenterology Clinics , Volume 46 , Issue 1 , 77 – 89