The trillions of organisms living in your digestive tract can literally change the way your brain works.
Scientists continue to find more and more evidence of the significant influence of gut bacteria on mental health. Studies have linked gut bacteria imbalances to a host of health issues, including depression, anxiety, autism and Alzheimer's disease, and research has also suggested that a healthy microbiome can contribute to a healthy brain and good mood.
These issues can be activated at a very young age. New research suggests that a stressful childhood might set you up for gut dysfunction and mental health issues down the road.
In a study on mice, which was published this week in the journal Nature Communications, researchers from McMaster University in Canada showed that early-life stress can lead to imbalances in the gut microbiome and contribute to the development of anxiety and depression.
"Early life stress changes the composition and metabolic activity of bacteria in the gut," the study's lead author, Dr. Premysl Bercik, a professor of gastroenterology at the university's medical school, told The Huffington Post in an email. "We postulate that this change is due to altered gut function induced by stress."
The stress-bacteria connection
For the study, the researchers subjected infant mice to stress by separating them from their mothers when they were between 3 and 21 days old.
After being subjected to maternal separation, the mice had abnormally high levels of the stress hormone corticosterone and displayed anxiety and depression-like behavior. The mice also showed imbalances in gut bacteria, which the researchers attributed to the release of acetylcholine, a neurotransmitter involved in the stress response that communicates between the body and the brain.
Then, the researchers repeated the experiment in a germ-free condition where the mice were not exposed to any bacteria. This time, mice also showed high stress-hormone levels and gut dysfunction after being subjected to stress, but they didn't show any signs of anxiety or depression.
When those same mice were colonized with bacteria, however, they began showing signs of anxiety and depression within a few weeks.
What does it all mean? Imbalanced bacteria alone wasn't enough to bring on anxiety and depression. Instead, the findings suggest that the interaction of bacteria and early-life stress may be what determines an individual's likelihood of developing anxiety and depression.
"We are starting to explain the complex mechanisms of interaction and dynamics between the gut microbiota and its host," Bercik said in a written statement. "Our data show that relatively minor changes in microbiota profiles ... can have profound effects on host behaviour in adulthood."
Happy gut, happy brain
How does it work? The brain and the gut communicate via gut-brain axis, a mode of bidirectional signaling between the digestive tract and the nervous system.
There are several central mechanisms by which gut bacteria can communicate with the brain. First, imbalances in gut bacteria can trigger inflammation by increasing the permeability of the intestinal lining, which allows toxins to seep into the bloodstream. Research has linked pro-inflammatory markers (cytokines) and increased intestinal permeability with anxiety and depression.
Secondly, bacteria can produce neurotransmitters, which are carried through the blood to the brain. Bacteria can also stimulate specific nerves in the gut that then transmit information to the brain, Bercik said.