Can fiber change your emotions?

With the exception of the March issue, each Study Deep Dives so far has included at least one study that involves the gut microbiome. This is by no means surprising. After all, science is slowly discovering the complexity of the gut microbiome (as evidenced by Figure 1) and its potent, far-reaching health effects. One area the Study Deep Dives has yet to discuss is the microbiome’s possible role in stress.

Figure 1: Microbiome is so hot right now

The stress response is regulated by a network called the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis is ultimately responsible for the release of cortisol, and its effects spread to every corner of the body, affecting digestion, immune system function, energy storage and expenditure, and emotions. For instance, it has been suggested^[1] that HPA-axis communication is disrupted in people with depression and that greater cortisol dysregulation predicts increased symptom severity.

Our understanding of the interactions between the microbiome and the stress response is in its infancy. The “gut-brain” axis is a relatively recent focus of research, and the health consequences of its dysregulation may range from visceral pain, obesity, and cardiovascular disease to autism spectrum disorders, anxiety, depression, and multiple sclerosis. It has been previously demonstrated in rodents that the lack of a microbiome exacerbates^[2] the neuroendocrine and behavioral responses to stress, possibly because cortisol can alter^[3] gut permeability and barrier function, and that recolonization of the gut bacteria reverses these effects.

Identifying the beneficial and harmful bacteria responsible for these effects is a long way off, however. This should come as no surprise considering that our intestinal tract is home to upward of one hundred trillion microorganisms—more than ten times the number of human cells in our bodies—that contain 150 times as many genes as our genome. Rather than using probiotics, which would supplement only a handful of species, the current study explored the effects of two types of common prebiotic fibers, which may help multiple healthy species flourish. Researchers investigated the processing of emotional information and HPA-axis activity in healthy human volunteers.

The brain and gut microbiome communicate through multiple pathways. The HPA axis that regulates stress and how we respond to it is one such pathway that may be affected, for better or worse, by changes in the microbiome.