Over the past year, a number of interesting studies have shed light on the hitherto unknown functions of the bacteria that live in our gut.  Researchers at UCLA found that women who regularly consumed beneficial bacteria through yogurt showed altered brain function, both while in a resting state and in response to an emotion-recognition task.  Commenting on the study published in Gastroenterology, lead author Dr. Kirsten Tillisch noted that “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.”  An article linked here provides further insight into the study and its significance.

Researchers have known that the brain sends signals to the gut, which is why stress and other emotions can contribute to gastrointestinal symptoms. This study shows what has been suspected but until now had been proved only in animal studies: that signals travel the opposite way as well.

“Time and time again, we hear from patients that they never felt depressed or anxious until they started experiencing problems with their gut,” Tillisch said. “Our study shows that the gut–brain connection is a two-way street.”

The small study involved 36 women between the ages of 18 and 55. Researchers divided the women 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 (fMRI) 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. This task, designed to measure the engagement of affective and cognitive brain regions in response to a visual stimulus, was chosen because previous research in animals had linked changes in gut flora to changes in affective behaviors.

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 from 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, Tillisch said.

The knowledge that signals are sent from the intestine to the brain and that they can be modulated by a dietary change is likely to lead to an expansion of research aimed at finding new strategies to prevent or treat digestive, mental and neurological disorders.

So how do we change the bacteria in our gut?  A recent study published in Nature showed that changing the food you eat rapidly changes the composition of your gut bacteria.  In the study, researchers had nine volunteers go on two extreme diets for five days each.  In the first diet, volunteers ate eggs and bacon for breakfast, ribs and briskets for lunch, and an assortment of pork meat and cheeses for dinners.  Then after a break, the nine volunteers ate a diet that consisted almost entirely of plants. They had granola cereal for breakfast, and for lunch and dinner a combination of jasmine rice, cooked onions, tomatoes, squash, garlic, peas, lentils, bananas and mangoes.

Researchers analyzed the volunteers’ microbiomes before, during and after each diet.  Lead researcher Lawrence David noted that “the relative abundance of various bacteria species looked like it shifted within a day after the food hit the gut.”  After the volunteers had spent about three days on each diet, the genes which the bacteria expressed began to change.  The bacteria found in people’s guts after they ate an animal-rich diet have previously been associated with higher levels of inflammation in the body.  An NPR article on the study is linked here.

Overall, studies like these show that what we eat affects our health in ways that we have not previously appreciated.