I recently attended the 2nd annual Gut Health, the Microbiota and Probiotics through the Lifespan Research Symposium held by Harvard University’s School of Nutrition. In recognition of the importance of gut flora for human health, this Symposium is an opportunity for researchers from around the world to present their current findings.
Two fascinating trials were reported on at the Symposium that offer insight into depression. Depression affects over 14 million people a year, and is notoriously resistant to pharmaceutical treatments. The studies gave some very helpful clues about why that may be. In one trial, reported by Ted Dinan, PhD, the GI tract of depressed rodents demonstrated less diversity overall than the flora from the GI tract of normal rodents. But guts are complicated right? Maybe they had different lunches, trauma in their past or it was a genetic influence, right? That is exactly what we assume in humans.
The next step of the experiment was to erase the flora in the healthy rodent. They do this with an antibiotic cocktail that creates a “sterile gut”. Then, they implant the non-depressed gut with the flora of the depressed rodent. Voila. Depression.
There was not change in social climate, stressors, diet, atmosphere or physical contact. Simply replacing the non-depressed flora with flora from a depressed animal manifested the symptoms. Obviously a question that came up was whether a fecal transplant could be used to reverse depression. The research is on-going, but this study really speaks to the power of the GI tract to lead changes in the experience of mental health.
Evolving research was presented by Elaine Hsiao, PhD, who reported on how bacterial species affect serotonin synthesis. Our bodies make serotonin, or they should. The entire philosophy behind a major class of antidepressant medications, the SSRIs or Selective Serotonin Reuptake Inhibitors is that the body makes serotonin, but it doesn’t stay in the brain long enough. The SSRI helps trap the serotonin in the open spaces in the brain so it is readily accessed when needed.
Dr. Hsiao and her team discovered that tryptophan tends to be high in sterile guts. Tryptophan is an amino acid precursor to serotonin, which means it is the stuff from which Serotonin is made. It is abundant in certain foods, and sometimes recommended in a form called 5-HTP to help the body build its own serotonin reserves. To make serotonin from tryptophan, the body has to have an enzyme called tryptophan hydroxylase. In guts that weren’t sterile, there seemed to be more serotonin than tryptophan. Certain types of bacteria seemed to be present when lots of serotonin was being made, but it was hard to say whether the bacteria was making the serotonin from the tryptophan or whether it was nudging the body into making serotonin. The results of the work of Dr. Hsiao and her team showed that the right types of bacteria prompt the body to increase its production of serotonin. Without those bacteria present, the body held onto a reservoir of tryptophan, but it didn’t yield the serotonin an SSRI would need to work.
Human health is more complicated that rodent health, but these insights are a major leap forward in our understanding of depression. What we learned from this fascinating work is that, when cognitive behavioral therapy and pharmaceutical treatments are not yielding the expected result, there is great reason to expect better results from proper support of the GI tract.