Gut – Brain Axis

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cropped-graphic-e1423682891979Volume 12, Number 9

It is nothing new to assert that activity in the central nervous system can affect the gastrointestinal (GI) system. Many persons anecdotally report that stress affects GI function, and there is scientific evidence corroborating this relationship. More novel, however, is that over the past decade there has been increasing neuroscience interest in how the gut microbiome (assemblage of microorganisms in the GI system) affects brain functioning. It is now believed that there is a complex and bi-directional relationship between the gut and the brain, even though the research is still in its infancy.

Estimates of the number of microorganisms in the human digestive system are as high as trillions. Many of these microorganisms have beneficial effects, and are known as probiotics.

Studies with rodents whose GI systems are “germ-free” (i.e., devoid of microorganisms) have found interesting behavioral manifestations of such a state, including increased anxiety. When such rodents are given selected probiotics, beneficial behavioral effects have been observed, changes that investigators characterize as parallels to reduction in human anxiety and depression.

A number of human studies have been conducted to examine mental health effects of probiotic interventions. Investigators at UCLA had healthy women eat yogurt twice a day for a month. Using functional magnetic resonance imaging (fMRI), these women were found to have less than expected activity in regions of the brain associated with the processing of emotion when shown pictures that typically evoke strong emotional responses. A study conducted among undergraduate students at the College of William and Mary found that self-reports of more fermented food consumption were associated with less social anxiety. At Tokushima University in Japan, medical students who consumed milk containing a probiotic every day for 8 weeks, compared to those consuming placebo milk, were found to have reduced indicators of physiologic stress (i.e., cortisol) one day before an exam, and higher levels of serotonin (a neurotransmitter associated with more positive mood states) two weeks after the examination.

While the potential of effects of probiotics on stress reactions, anxiety, and depression are getting research attention in studies such as those mentioned above, other scientists are looking at potential relationships between gut microbiota and autism spectrum disorders (ASD). There have been some encouraging results in several small studies looking at the behavioral effects of dietary interventions intended to affect the gut microbiome in individuals with ASD.

Two recent systematic reviews of studies looking at the effects of probiotics on central nervous system functioning point out that human studies are quite limited at this point, so it remains premature to conclude that increasing dietary probiotics has mental health benefits. But findings to date have been promising enough that investigations in this arena are continuing.

Paul J. Hershberger, Ph.D.
… is a clinical health psychologist. He is Professor of Family Medicine and Director of Behavioral Science for the Family Medicine Residency Program, Wright State University Boonshoft School of Medicine. His clinical practice includes psychotherapy, consultation, and coaching.


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