Probiotics for Mental Health

Beneficial bacteria may help depression and anxiety.

Long the province of the health-conscious, probiotics — the beneficial bacteria found in supplements and fermented foods like yogurt — are increasingly the subject of intense study by researchers.

Emerging evidence suggests the bacterial community in the gut plays a role in, of all things, depression and anxiety — and that altering the balance in the gut may have a direct effect on mental health. 

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Researchers are still teasing out the exact mechanisms by which the gut can affect the brain. Much of what they know comes from animal studies. When mice are raised in sterile environments — and thus lack bacteria in the gut — they display irregularities in the function of the hypothalamic-pituitary-adrenal axis, a key driver of emotional regulation, along with other changes in the brain. They react more quickly and easily to stress than mice raised in normal environments, and produce fewer important neurotransmitters.

The implication that the gut may play a role in emotional regulation is a connection one researcher has called "surreal." Yet evidence is mounting for this unexpected connection.

Human trials of probiotics have just begun to appear. In one buzz-worthy 2014 trial from UCLA, a small group of healthy women were given a daily dose of probiotics in milk, while a control group was given milk without probiotics. After four weeks, the women took an emotional reactivity test that assessed response to negative facial expressions like fear and anger. Anxious people generally have a heightened response to the test, which activates parts of the brain involved in emotional processing. Brain scans showed that the women who'd taken probiotics had a significantly reduced emotional response to the test in comparison to those who drank non-enriched milk. 

The study was unusual because it was a carefully controlled trial that looked at actual brain activity (rather than, say, self-reported symptoms, which can be unreliable). While researchers have known for some time that bacteria affects the gut itself, it was a surprise that it also affects the brain. "To our knowledge, this is the first demonstration in humans that … a fermented milk product with probiotic can modulate brain activity," the authors wrote. 

Other small studies are suggestive. In one, probiotics reduced anxiety in people suffering from chronic fatigue syndrome, many of whom have altered gut flora. And in a 2011 French study, a probiotic cocktail reduced stress, anxiety, and depression symptoms in human participants — even among the least anxious. Most recently, an August 2015 trial, echoing the UCLA work, found probiotics reduced emotional reactivity to sad moods even in healthy, non-depressed people. The authors suggested probiotics should be investigated as a way to prevent depression.

The studies are promising, and they've generated an unusual amount of excitement in the scientific community. Yet, experts caution, much is still unknown. When researchers at Oxford tried giving probiotic-enriched yogurt to people suffering from irritable bowel syndrome, they found it helped about half of participants — nearly the same number as those who simply ate plain yogurt in the study. Perhaps yogurt alone is a mood booster, the researchers proposed.

Another recent study — this one for probiotics' ability to reduce diarrhea during a course of antibiotics for Helicobacter pylori infection — found similar results: milk alone, without probiotics, was effective in reducing the "bad" bug (which cause ulcers) in the body. This study did find some benefit for the probiotics, however: the probiotics-and-milk combination reduced the duration of antibiotic-induced diarrhea by more than half — from 10 to four days — as well as intestinal complaints.

Kirsten Tillisch, of the UCLA department of medicine, is the lead author of the UCLA study and one of the experts working at the forefront of this emerging field. Humans have been altering gut bacteria via antibiotics and probiotic supplements for many years, she notes in the study. Yet few clear associations to specific conditions have emerged. While it's increasingly clear the gut impacts the central nervous system — and thus mood — in some way, she writes, "we are far from understanding the significance of this interaction in health and disease."