Get to know the fascinating world of microorganisms, mood, and more.

Humans have two brains. We’re all familiar with the first – the one in our head that over the millennia has driven innovation, complex reasoning, and civilization. But you might not be as familiar with the second brain in our guts. Formally known as the enteric nervous system (ENS), this other brain runs the full length of the gastrointestinal tract and consists of more than 100 million nerve cells.

While our primary brain does the heavy lifting when it comes to cognition, the ENS is a not-so-silent partner. The brain and the ENS talk back and forth to each other, with one affecting the other. Initially scientists thought that it was one more way – mood would influence the gut and contribute to conditions such as IBD and IBS. But further research has demonstrated that this interaction is very bidirectional, with gut inflammation and irritation signaling the brain and affecting mood.

The gut-brain axis occurs through three different pathways:

• The immunoregulatory pathway describes the gut microbiota’s interaction with immune cells; this influences levels of cytokines, a group of proteins involved in cell signaling.
• The neuroendocrine pathway involves regulating the secretion of various neurotransmitters, such as serotonin and cortisol, which affect both the hypothalamic-pituitary-adrenal, or HPA axis, and the central nervous system.
• The vagus nervus pathway centers around the synaptic connections between the ENS and the vagus nervous, creating a metaphorical information superhighway.

Much of the research around the gut-brain axis has been on the interplay between the human gut microbiome and mood, anxiety, insomnia, and depressive disorders. A study led by Dr. Jeroen Raes demonstrated that people who experience clinical depression had two microorganism groups missing from their gut – Coprococcus and Dialister.  In another study published in Microbiome, T helper 17 cells (Th17), a group of T helper cells that have been implicated in both inflammatory processes and autoimmune disorders, were able to sense the gut microbiome and promote depressive behaviors. And a third study in Clinical Psychology Review showed that several species of bacteria may contribute to anxiety by communication of peripheral inflammation to the brain.

While the results are intriguing, scientists are in the early stages of determining how valuable probiotic interventions are for treating mood disorders. The research has been promising, though, and has spawned a new generation of “psychobiotics” or probiotic organisms used to improve mental health and well-being. This field explores using specific strains of probiotics, such as Lactobacillus helveticus, Mycobacterium vaccae, and Bifidobacteria infantis, to reduce the symptoms of anxiety and depression. Another area under investigation is using prebiotics as therapeutics, specifically fibers such as galactooligosaccharides (GOS) and fructooligosaccharides (FOS) to promote the growth of Lactobacillus and Bifidobacterium.

Another area under investigation is the gut’s connection to sleep. Because gut microbes are involved in our circadian clock and clock genes, it’s been investigated as one of the players in sleep quality and efficiency, as well as delayed onset sleep disorder.

Scientists continue to explore how the human gut microbiome plays into our brain, with new topics continuing to emerge such as memory, problem-solving skills, and emotional regulation. One thing is for certain – our second brain can no longer be ignored.