I wrote here about the relationship between the microbiome and mood disorders, such as anxiety and depression. But the connection between the gut and the brain goes much deeper than that. In fact, it appears that imbalance in the microbiome may actually be a causative factor in various types of neurodevelopmental disorders, including (but not limited to) autism.

Last week’s article discussed the association between inflammation generally and brain injury. Gut inflammation is one mechanism by which that can occur, triggering high levels of inflammatory cytokines in the brain. This connection is called the gut-brain axis.

What the Gut-Brain Axis is

The gut-brain axis describes the way in which the gut influences the brain, and vice versa. But the gut-brain axis describes a bidirectional process: imbalance in gut flora can trigger dysfunction in the brain, but dysfunction in the brain can likewise cause dysfunction in the gut.

Leaky Gut —> Leaky Brain

Overgrowth of bad bacteria, be it SIBO or some other type of dysbiotic (bad) bacteria, produces toxic byproducts that can contribute to the phenomenon known as “leaky gut syndrome”. This is when the junctions in the small intestine are “loose” enough (due to inflammation) to allow intestinal contents to leak out into the bloodstream. This can produce metabolic endotoxemia, leading to systemic inflammation, as well as weaken the junctions of the blood brain barrier.

Young children are especially susceptible to brain inflammation, as their brains are in process of development. A permeable blood brain barrier may impair the process, setting the child up for autism.

This study specifically links this process with propionic acid, produced by bacteria such as clostridia, bacteroidetes, and desulfovibrio. This study shows that propionic acid triggers neuroinflammation in animals, leading to autism-like behaviors. This study further demonstrates that propionic acid increases oxidative stress, depletes glutathione (the most powerful antioxidant in the body, critical for detoxification), and affects phospholipids, also critical for detoxification. Correspondingly, some late-stage autism seems to improve with the antibiotic vancomycin, absorbed only in the gut.

While propionic acid seems to be more associated with those types of bacteria, SIBO (overgrowth of bacteria in the small intestine) is also associated with worsening autistic symptoms.

In fact, this study posits that autism may even be partly due to underlying gut problems. This study agrees.

Leaky Brain —> Leaky Gut: It Cuts Both Ways

As it turns out, the inflammation subsequent to a traumatic brain injury also impacts the gut, which can in turn contribute to sustained inflammation in the brain. This may be in part because inflammatory cytokines can alter gut motility–and slower gut motility is a setup for dysbiosis.

Restoring the integrity of the microbiome therefore has therapeutic potential.

The Upshot

While GI distress such as gas, bloating, constipation, and diarrhea are obvious signs that the microbiome may be disrupted, we’ve long known that kids with behavioral disorders, including autism, nearly always also have gut dysfunction. We may not have realized before just how central the gut might be to the pathology in general.