A new small study of healthy women who regularly consumed beneficial bacteria known as probiotics has provided the first evidence that changing the bacterial environment, or microbiota, in the gut can affect brain function in humans.

“Many of us have a container of yogurt in our refrigerator that we may eat for enjoyment, for calcium or because we think it might help our health in other ways,” said Dr Kirsten Tillisch from the University of California Los Angeles’ David Geffen School of Medicine, lead author of the study published in the journal Gastroenterology.

“Our findings indicate that some of the contents of yogurt may actually change the way our brain responds to the environment. When we consider the implications of this work, the old sayings ‘you are what you eat’ and ‘gut feelings’ take on new meaning.”

In the study, Dr Tillisch’s team divided 36 women between the ages of 18 and 55 into three groups: one group ate a specific yogurt containing a mix of several probiotics – bacteria thought to have a positive effect on the intestines – twice a day for four weeks; another group consumed a dairy product that looked and tasted like the yogurt but contained no probiotics; and a third group ate no product at all.

Functional magnetic resonance imaging scans conducted both before and after the four-week study period looked at the women’s brains in a state of rest and in response to an emotion-recognition task in which they viewed a series of pictures of people with angry or frightened faces and matched them to other faces showing the same emotions.

The researchers found that, compared with the women who didn’t consume the probiotic yogurt, those who did showed a decrease in activity in both the insula – which processes and integrates internal body sensations, like those form the gut – and the somatosensory cortex during the emotional reactivity task.

Further, in response to the task, these women had a decrease in the engagement of a widespread network in the brain that includes emotion-, cognition- and sensory-related areas. The women in the other two groups showed a stable or increased activity in this network.

During the resting brain scan, the women consuming probiotics showed greater connectivity between a key brainstem region known as the periaqueductal grey and cognition-associated areas of the prefrontal cortex. The women who ate no product at all, on the other hand, showed greater connectivity of the periaqueductal grey to emotion- and sensation-related regions, while the group consuming the non-probiotic dairy product showed results in between.

“The researchers were surprised to find that the brain effects could be seen in many areas, including those involved in sensory processing and not merely those associated with emotion,” Dr Tillisch said.

By demonstrating the brain effects of probiotics, the study also raises the question of whether repeated courses of antibiotics can affect the brain, as some have speculated. Antibiotics are used extensively in neonatal intensive care units and in childhood respiratory tract infections, and such suppression of the normal microbiota may have long-term consequences on brain development.

Finally, as the complexity of the gut flora and its effect on the brain is better understood, researchers may find ways to manipulate the intestinal contents to treat chronic pain conditions or other brain related diseases, including, potentially, Parkinson’s disease, Alzheimer’s disease and autism.

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Bibliographic information: Kirsten Tillisch et al. Consumption of Fermented Milk Product With Probiotic Modulates Brain Activity. Gastroenterology, vol. 144, no. 7, pp. 1394–1401.e4; doi: 10.1053/j.gastro.2013.02.043