A growing group of researchers around the world are investigating how the microbiome, as this bacterial ecosystem is known, regulates brain function.
By now, the idea that gut bacteria affects a person’s health is not revolutionary. Many people know that these microbes influence digestion, allergies, and metabolism. The trend has become almost commonplace: New books appear regularly detailing precisely which diet will lead to optimum bacterial health.
But these microbes’ reach may extend much further, into the human brains. A growing group of researchers around the world are investigating how the microbiome, as this bacterial ecosystem is known, regulates how people think and feel. Scientists have found evidence that this assemblage could play a crucial role in autism, anxiety, depression, and other disorders.
Some of the most intriguing work has been done on autism. For decades, doctors, parents, and researchers have noted that about three-quarters of people with autism also have some gastrointestinal abnormality, like digestive issues, food allergies, or gluten sensitivity.
Scientists have also gathered evidence that gut bacteria can influence anxiety and depression. Stephen Collins, a gastroenterology researcher at McMaster University in Hamilton, Ontario, has found that strains of two bacteria, lactobacillus and bifidobacterium, reduce anxiety-like behavior in mice. Humans also carry strains of these bacteria in their guts.
Now, new research by investigators at Brigham and Women's Hospital (BWH) suggests that bacteria living in the gut may also remotely influence the activity of cells in the brain that are involved in controlling inflammation and neurodegeneration. Using pre-clinical models for multiple sclerosis (MS) and samples from MS patients, the team found evidence that changes in diet and gut flora may influence astrocytes- characteristic star-shaped glial cells in the brain and spinal cord- and, consequently, neurodegeneration, pointing to potential therapeutic targets.
To explore this, Quintana and colleagues performed genome-wide transcriptional analyses on astrocytes in a mouse model of MS, identifying a molecular pathway involved in inflammation. They found that molecules derived from dietary tryptophan (an amino acid famously found in turkey and other foods) act on this pathway, and that when more of these molecules are present, astrocytes are able to limit brain inflammation. In blood samples from MS patients, the team found decreased levels of the tryptophan-derived molecules.
“For the first time, we’ve been able to identify that food has some sort of remote control over central nervous system inflammation,” said Francisco Quintana, an investigator in the Ann Romney Center for Neurologic Diseases at BWH and corresponding author on the study. “What we eat influences the ability of bacteria in our gut to produce small molecules, some of which are capable of traveling all the way to the brain. This opens up an area that’s largely been unknown until now: how the gut controls brain inflammation.”
The research team plans to investigate this pathway and the role of diet in future studies to determine if the new findings can be translated into targets for therapeutic intervention and biomarkers for diagnosing and detecting the advancement of disease.