The Gut a New Therapeutic Target for Major Depression?

Liam Davenport

September 16, 2020

The gut microbiota differs significantly between patients with major depressive disorder (MDD) and healthy individuals and may be modifiable with a probiotic diet to improve stress and depression scores, two new studies suggest.

In one study, investigators compared stool samples between patients with MDD and healthy controls. They found significant differences in bacterial profiles between the two groups, as well as between patients who responded vs those who were resistant to treatment.

"This finding further supports the relevance of an altered composition of the gut microbiota in the etiopathogenesis of MDD and suggests a role in response to antidepressants," co-investigator Andrea Fontana, MSc, Fondazione IRCCS Casa Sollievo della Sofferenza, San Giovanni Rotondo, Italy, told Medscape Medical News.

Results from the second study showed significant improvements in self-reported stress, anxiety, and depression scores in healthy individuals following a "psychobiotic" diet (using probiotics or prebiotics to manipulate the microbiota to improve mental health) that was rich in fruit, vegetables, and fermented foods vs those who received dietary advice alone.

The investigators, led by Kirsten Berding, PhD, APC Microbiome Ireland, University College Cork, Ireland, now plan on testing their psychobiotic diet in patients with MDD and hope the findings could be helpful in "the development of adjuvant therapeutic opportunities" where pharmacologic treatment is not effective.

Both studies were presented at the 33rd European College of Neuropsychopharmacology (ECNP) Congress, which was held online this year because of the COVID-19 pandemic.

A "Hallmark" of Major Depression

Fontana and colleagues note that the mostly suboptimal response to pharmacologic treatments among patients with MDD is one of the factors that "contributes to the large socioeconomic burden" of the disease.

Previous research shows patients with MDD have gut dysbiosis, or an imbalance in the natural flora; that antidepressants have antimicrobial properties; and that probiotics have an antibiotic effect. However, the correlation between the composition of the gut microbiota and antidepressant response is poorly understood.

The investigators recruited 34 patients with MDD (aged 18-70 years) who were in a euthymic phase and who did not have comorbid conditions that could affect the gut microbiota.

Eight patients were treatment resistant, defined as a poor response to at least two adequate trials of different antidepressant classes, while 19 were treatment responsive and seven were treatment naive.

The researchers also recruited 20 healthy individuals via word of mouth to act as the control group. There were no significant differences between patients and the control group in terms of baseline characteristics.

Genomic sequencing of bacteria obtained from stool samples showed that it was possible to distinguish between patients with MDD and the healthy individuals, especially at the family, genus, and species levels.

In particular, there were significant differences in the Paenibacillaceae and Flavobacteriaceaea families, for the genus Fenollaria, and the species Flintibacter butyricus, Christensenella timonensis, and Eisenbergiella massiliensis, among others.

Results also showed that the phyla Proteobacteria, Tenericutes, and the family Peptostreptococcaceae were more common in patients with treatment-resistant MDD, whereas the phylum Actinobacteria was more abundant in treatment responders.

Moreover, several bacteria were found only in the microbiota of patients with treatment-resistant MDD, while others were seen only in treatment-responsive patients. This made it possible to discriminate not only between treatment-resistant and -responsive patients but also between those two patient groups and healthy controls.

"The results of our study confirm that gut dysbiosis is a hallmark of MDD, and

suggests that the gut microbiota of patients with treatment-resistant MDD significantly differs from responders to antidepressants," Fontana said.

Psychobiotic Diet

For the second study, 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 write. However, there is also a paucity of studies in this area, they add.

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).

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 write.

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 add.

This will "contribute to the understanding of the benefits of a psychobiotic diet on stress and anxiety," write the researchers.

Berding told Medscape Medical News 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."

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 for Medscape Medical News, 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."

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 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 ideas are brilliant and I support them…but we have to provide proof," Fetissov concluded.

The research by Berding and colleagues is funded by a postdoctoral fellowship grant from the Irish Research Council. The study authors and Fetissov have disclosed no relevant financial relationships.

33rd European College of Neuropsychopharmacology (ECNP) Congress: Abstracts P233 and P266. Presented September 14, 2020.

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