Many depression sufferers also suffer from irritable bowel syndrome (IBS), and a small study suggests that taking probiotics may ease both conditions.
Researchers from McMaster University in Canada conducted a randomized, placebo-controlled trial that uncovered a link between probiotics and mood improvement in 44 adults with IBS and mild to moderate depression or anxiety. The study also showed that probiotics caused changes in regions of the brain related to emotional processing. 
For the study, participants were divided into two groups and followed for 10 weeks. Half of the subjects took a daily dose of the probiotic Bifidobacterium longum, and the other half took a placebo.
After 6 weeks, 64% of those who took probiotics had decreased depression scores, compared to 32% of those who took a placebo.
Functional MRI scans revealed that improved depression scores were associated with changes in several areas of the brain involved in mood regulation. The authors wrote that those changes “support the notion that this probiotic has anti-depressive properties.” 
The findings did not indicate that those in the probiotic group experienced statistically significant independent changes in anxiety, constipation, diarrhea, or pain. But those in the probiotic group did report improvements in overall symptoms of IBS and in quality of life.
“In a placebo-controlled trial, we found that the probiotic BL reduces depression but not anxiety scores and increases quality of life in patients with IBS. These improvements were associated with changes in brain activation patterns that indicate that this probiotic reduces limbic reactivity,” the study concluded.
Read: Our Gut is Our “Second Brain” – It Affects Mood and Health More than You Know
One explanation for the improved depression scores could be that as their physical symptoms improved, their emotional symptoms followed suit. But Dr. Roger McIntyre, professor of psychiatry and pharmacology at the University of Toronto, thinks it’s more likely that the probiotics may actually be working on the brain itself.
“We know that one part of the brain, the amygdala, tends to be red hot in people with depression, and it seemed to cool down with this intervention. It provides more scientific believability that something in the brain, at a very biological level, seems to be affected by this probiotic.” 
Gastroenterologist and lead researcher Premysl Bercik, M.D., added:
“This study shows that consumption of a specific probiotic can improve both gut symptoms and psychological issues in IBS. This opens new avenues not only for the treatment of patients with functional bowel disorders but also for patients with primary psychiatric diseases.” 
Antidepressants are known to disturb “good” gut bacteria, so probiotic supplements – which produced few side effects in the study – could be a safe, natural way to treat depression.
The probiotics were manufactured and provided by Nestlé, which also funded the study. The food company reportedly was not involved in collection, analysis, or interpretation of the study data.
 Science Daily