In another setback for the notion that depression is all in your head, recent research suggests it may be a gut reaction. Literally.
Brain has always bested bowel in the body battles for executive organ status. However, the gastrointestinal tract is moving up in the world and recently was labeled the second brain. Laboratories all over the world have been mapping a bi-directional information highway between these two organs called the gut-brain axis.
This connection has revealed the gut's influence on a wide range of things, from satiety and obesity to mood and anxiety. Even how we respond to stress is no longer the sole dominion of the brain.
The latest chapter of this story introduces the bacterial community living in our gut. The microbiome (the name given these bacteria) appears to be a key player in modulating brain chemistry and consequently mood.
Your microbiome is determined by age, genetics, geography, medication, stress and perhaps most importantly, diet. Apparently we never dine alone. When we feed ourselves, we are also serving the bacterial colonists in our gut. What they are fed defines which types of bacteria thrive. We are just beginning to appreciate the health consequences of specific microbiome profiles.
Using a variety of interventions including diet, antibiotics, probiotics and fecal transplantation, investigators have begun to understand how these gut bacteria affect mood. Two mechanisms have dominated much of this research, serotonin and inflammation.
Serotonin, a well-known chemical messenger in the brain, is the target of an entire class of antidepressants (think Prozac, Zoloft, Celexa). It so happens that the vast majority of our serotonin is located in the gut, not the brain. The body makes this critical mood regulator from tryptophan, an amino acid in food proteins. Because gut bacteria can determine how much tryptophan gets absorbed, it has a powerful effect on serotonin levels. These levels are thought to be a critical determinant of mood.
Investigators have demonstrated that diets rich in saturated fats and refined carbohydrates trigger changes in gut bacteria associated with chronic systemic inflammation. This longstanding whole-body inflammatory state has been identified as a causal factor in depression.
Certain probiotics have been shown to exert antidepressant effects in both animal models and humans. These dietary supplements, sometimes called psychobiotics, contain live microbes that may provide the next generation of antidepressants.
In an elegant series of experiments, Bravo et al. demonstrated that Lactobacilli reduced stress-induced cortisol levels and depression-related behaviors in animals. When he cut the vagus nerve, this effect was eliminated. This provided the first identification of the route such communication takes from gut to brain.
We still have much to learn. A clear determination of which microbiome profiles are consistently associated with depression and the best psychobiotic intervention has not yet been achieved. But that day is not far off.
All languages have a rich collection of body idioms. This anatomy of emotion makes it clear that science is catching up to what has been known on some level for a very long time. Of all the idioms related to the gastrointestinal tract (gutsy, butterflies in one's stomach, hard to stomach, etc.) one stands out with regard to the gut-brain axis.
"Go with your gut."
The implication is that the brain may be confused but the gut knows.
These research findings pose an ironic epistemological twist. Science, the bastion of rationality, is demonstrating what the poets have always known. If you seek the truth, feelings often trump thoughts.