The Impact of Gut Microbial Metabolites Circulation provides an important opportunity for gut-brain axis crosstalk. The gut absorbs nutrients and releases them into the bloodstream, and some of these nutrients can eventually get into the brain. For example, short-chain fatty acids (SCFAs) are a type of metabolite produced by the microorganisms that influence CNS processes through many of the already-mentioned pathways, including the circulatory and immune systems. Many probiotic strains produce these metabolites.  SCFAs are notable because they play an important role in gut health by maintaining intestinal barrier integrity. They can also help to maintain the blood-brain barrier and support maturation of brain immune cells. Gut microbes can also affect a person’s metabolism, which can influence the types and levels of nutrients in the blood. This could then affect the nutrients available to the brain, and also has the ability to influence the permeability of the blood-brain barrier. This connection means the gut can have an indirect influence over the brain’s production of serotonin, dopamine, or hormones that are connected to mood. 

  For example, tryptophan, an essential amino acid, is used by the brain to make serotonin and melatonin and must be supplied by your diet. Turkey famously contains tryptophan (thanks to the amino acid’s connection to melatonin, people tend to blame turkey at Thanksgiving for making them sleepy), as does chicken, whole milk, tuna, oats, and more. Gut microbes can affect tryptophan metabolism in the gut, which impacts the amount of tryptophan in the blood, and this may impact how much of the amino acid is available to the brain to make serotonin and melatonin.

 The Immune System & the Gut-Brain Axis

While the intestine digests food, it also houses a large part of the immune system. Immune cells within the intestine support food tolerance and friendly bacteria, but can also stimulate allergic reactions and food sensitivities, as well as defend against undesirable bacteria.  The intestinal immune system can sense bacteria within the gut and communicate with the brain. Dendritic cells are immune cells with arms (“dendrites”), that can sense intestinal contents, including gut bacteria and probiotics you take by mouth. After sensing specific types of gut bacteria, these immune cells then migrate to the lymphatic system. Their signaling process results in the release of molecules called cytokines that can pass the blood brain barrier.  These cytokines can interact with receptors on brain neurons to change brain activity. Interestingly, certain cytokines are associated with mood and appetite. Some immune activity may impact serotonin receptors and transporters, and could impact mood as well. Make a Gut-Brain Connection As you can tell, the gut-brain axis is complicated – and not yet fully understood. Hopefully, this left you with a greater understanding of the many ways the gut and brain communicate and work together.