By Hara Estroff Marano, published on July 8, 2015 - last reviewed on June 10, 2016
Your unconscious is alive and active, although not quite the way Freud envisioned it. It’s at least as much in your gut as in your head, and its primary protagonists are the intestinal bacteria known as probiotics, organisms that foster health—and especially mental health.
More than 100 trillion bacteria inhabit the gut; collectively they weigh as much as the brain. Over the past decade, researchers have been exploring their contribution to well-being in the gut and beyond. Not only does the microbiome enable normal brain development and social interaction, gut bacteria produce an array of bioactive compounds that influence behavior from anxiety to pain perception.
“To have a healthy brain and mind, you need a healthy gut,” neuroscientist John Cryan told a recent symposium. Cultivating the right bacteria, accumulating evidence suggests, may prevent and even reverse symptoms of disorders ranging from autism to stress reactivity.
Cryan, of University College Cork in Ireland, believes that the microbiome fosters neural development and exerts its greatest effect on the brain early in life. He and colleagues have recently found that animals born by cesarean section, and not exposed to the microbes accompanying vaginal birth, later had more symptoms of anxiety and depression than animals born naturally.
Other researchers have shown that in mice born with autism-like symptoms, the composition of the gut biome is markedly different from that in normal mice. Treating the animals with the probiotic Bacteroides fragilis, a common species of gut bacteria, reversed many, but not all, of the deficits, notably in communication, repetitive behaviors, and anxiety.
In other studies, Cryan has shown that animals raised in a germ-free environment have an exaggerated stress response, as indicated by high stress hormone levels. They are also deficient in BDNF, a nerve growth protein known to abet the survival of existing neurons and stimulate the creation of new ones. But when the animals are given everyday gut bacteria, the stress response normalizes.
Even in adult animals, the gut biome regulates nerve growth in the hippocampus, a region of the brain important for learning and memory and highly susceptible to damage by stress, Cryan recently reported in Biological Psychiatry.
Clinical research is gathering steam, too. Taking cues from animal studies, Timothy Dinan, also of University College Cork, recently gave healthy human volunteers Bifidobacterium longum and recorded reductions in stress levels and improvements in cognition. He aims to identify the specific bacteria—he calls them psychobiotics—that curb depression, anxiety, and other mental ills.
From birth on, maintaining the right composition of what scientists call the “downstairs brain” may be essential to the function of the “upstairs brain.”
The brain and the gut communicate in both directions, directly through the vagus nerve running between the brain and digestive tract and indirectly through various body processes.