The Mind-Gut Connection

The gut and the brain are closely linked through bidirectional signaling pathways that include nerves, hormones, and inflammatory molecules. Rich sensory information generated in the gut reaches the brain (gut sensations), and the brain sends signals back to the gut to adjust its functions (gut reactions). The close interactions of these pathways play a crucial rule in the generation of emotions and in optimal gut function. The two are intricately linked.

The diversity and abundance of gut microbes vary war the lifetime of an individual. It is low during the first three years of life when a stable gut microbiome is being established, reaches its maximum during adult life, and decreases as we grow older. The early period of low diversity coincides with the vulnerability window for neurodevelopmental disorders such as autism and anxiety, while the late period of low diversity coincides with the development of neurodegenerative disorders such is Parkinson’s and Alzheimer's disease. One may speculate that these low diversity states are risk factors for developing such diseases.

Emotions are closely reflected in a person's facial expressions. A similar expression of our emotions occurs in the different regions of the gastrointestinal tract, which is influenced by nerve signals generated in the limbic system. Signals to the upper and lower GI tract can be synchronous or go in opposite directions. For example, when a person is fearful the signals go in opposite directions: downward from the upper track and upward from the lower track. When a person is angry, both signals in the tracks travel upward. Sadness makes signals in both tracks move downward.

The Caltech investigators found that the young mice exhibited changes in their gut and the gut microbiota: an imbalanced mix of gut microbes, a leakier intestine, and greater engagement of the gut-based immune system. The investigators identified a particular gut microbial metabolite that was closely related to a metabolite that had previously been identified in the urine of children with ASD. When they gave this metabolite to healthy mice born to mothers whose immune system had not been activated, those mice had the same behavioral abnormalities as mice born to mothers whose immune systems had. Most intriguing, when they transplanted the stool of the abnormal mice into germ-free mice that behaved normally, the transplanted animals behaved abnormally. This strongly suggested that transplanted stool from the affected animals produced a metabolite that could reach the brain and alter the behavior of healthy mice. Most important for people with autism spectrum disorders, they could make several (though not all) of the autism-like behaviors disappear by treating the affected mice with human intestinal bacteria called Bacteroides fragilis.

Source: The Mind-Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health (2016) by Emeran Mayer, MD