Previous research has shown that mice fed a probiotic bacteria that's often found in yogurt experienced changes in brain activity and reduced behaviors associated with stress, anxiety, and depression.
But no one has ever looked at whether changes in gut bacteria can change human behavior, Tillisch says.
In the new study, 45 women ages 18 to 50 without psychiatric or medical illnesses were divided into three groups. One group was assigned to eat a probiotic yogurt, one got non-fermented yogurt (with no probiotic bacteria), and the third got no product. Those in the first two groups ate one cup of yogurt twice a day for four weeks.
After the four weeks, everyone was shown a series of pictures of frightened and anxious faces -- images design to evoke an emotional response.
They had functional MRI scans to capture brain activity before and after looking at the pictures.
The people in the probiotic group showed a muted response in brain areas involved in processing and sensation, compared with the other two groups.
On the flip side, people who didn't eat any yogurt had more activity in the sensory and emotional regions of the brain.
A long-term goal is to determine whether eating probiotic products or taking probiotic supplements regularly can change the balance of bacteria in the gut and alter emotional response to stress and other negative stimuli, Tillisch says.
John Petrini, MD, a gastroenterologist at the Sansum Clinic in Santa Barbara, Calif., says the findings suggest that there is "some kind of interaction" between bacteria in the gut, activity in the brain, and emotions, but that the work is "very preliminary."
A shortcoming of the study is that the researchers looked at activity in areas of the brain that they hypothesized would be involved in such an interaction, Petrini says. "There could be other [areas of the brain] that are more important."
"I wouldn't run out and buy a lot of yogurt in hopes of becoming less emotional or less stressed," he tells WebMD.
The study was funded by Danone Research, makers of Dannon Yogurt.
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.