Psychobiotic diet
For the second study, Dr. Berding and colleagues note that “psychobiotics” has previously achieved “promising results.”
In addition, diet is both “one of the most influential modifying factors” for the gut microbiota and an easily accessible strategy, they wrote. However, there is also a paucity of studies in this area, they added.
The researchers randomly assigned healthy volunteers with relatively poor dietary habits to either a 4-week psychobiotic diet group (n = 21) or a control group (n = 19).
Courtesy National Cancer Institute
Individuals in the psychobiotic group were told to eat a diet rich in prebiotics, such as fruit and vegetables, fiber including whole grains and legumes, and fermented foods. The control group was educated on Irish healthy-eating guidelines.
Stool and saliva samples were collected and the participants completed several self-reported mental health questionnaires, as well as a 7-day food diary. They also took the socially evaluated cold-pressor test (SECPT) to measure acute stress responses.
Results showed that total daily energy intake decreased significantly in both the diet and control groups over the study period (P = .04 for both) but did not differ significantly between the groups.
In contrast, dietary fiber intake increased significantly in the diet group (P < .001) and was significantly higher than in the control group at the end of the intervention (P = .03).
Individuals in the diet group showed significant decreases in scores on the Perceived Stress Scale (P = .002) and the Beck Depression Inventory (P = .007) during the study, an effect that was not found in the control group.
Dietary intervention
There were no significant effects of diet on the acute stress response, but both groups showed improvements in self-concept, or perceived ability to cope, on the Primary Appraisal, Secondary Appraisal index (P = .03 for the diet group, P = .04 for the control group).
The results show that a dietary intervention targeted at the microbiota “can improve subjective feelings of stress and depression in a healthy population,” the investigators wrote.
However, elucidating the “contribution of the microbiota-gut-brain axis on the signaling response to dietary interventions” will require further studies on microbiota sequencing and biological measures of stress, they added.
This will “contribute to the understanding of the benefits of a psychobiotic diet on stress and anxiety,” wrote the researchers.
Dr. Berding said in an interview that while the consumption of dietary fiber changed the most in the diet group, “it would not be the only nutrient” that had an impact on the results, with fermented foods a likely candidate.
She said the next step is to test the dietary intervention in patients with MDD; however, “doing nutritional interventions in diseased populations is always difficult.”
Dr. Berding suggested that the best approach would be to study inpatients in a clinic, as “we would be able to provide every meal and only provide foods that are part of the dietary intervention.”
Although another option would be to conduct the study in outpatients, she noted that assessing inpatients “would give us the best control over compliance.”
“Brilliant ideas”
Commenting on the findings, Sergueï Fetissov, MD, PhD, professor of physiology at Rouen University, Mont-Saint-Aignan, France, said that although both studies bring attention to a possible role for the gut microbiota in MDD, neither “provide any experimental evidence of a causative nature.”
Dr. Serguei Fetissov
Dr. Fetissov, who was not involved in either study, noted that this topic has been the subject of clinical nutritional research for many years.
However, “we still need some strong evidence to prove that some bacteria can influence the regulation of mood and anxiety and stress,” he said.
In addition, researchers currently do not know what actually causes MDD. “How we can say the gut bacteria regulates something if we don’t know what really causes the altered mood?” said Dr. Fetissov.
He noted that over the last 50 years, there have been great advances in the development of drugs that alleviate depression and anxiety by regulating dopamine, serotonin, and other neurotransmitters. However, it is still unknown whether these reflect primary or secondary aspects of mood disorders.
Furthermore, it is not clear “how probiotics to bacteria can influence these neuronal pathways,” he said.
The research by Dr. Berding and colleagues is funded by a postdoctoral fellowship grant from the Irish Research Council. The study authors and Dr. Fetissov have disclosed no relevant financial relationships.
A version of this article originally appeared on Medscape.com.