“Gut–brain axis”: Review of the role of the probiotics in anxiety and depressive disorders

Abstract Background Depressive disorders are the leading cause of disability worldwide and together with anxiety contribute to a very high burden of disease. Therefore, improving their treatment is a significant medical research target: The role of probiotics is a topic of great interest for the current research in this field. Objectives To explore the current literature about the impact of probiotics on anxious and depressive symptoms. Methods Scoping review following the PRISMA guidelines. Results The selection process yielded 23 studies. Probiotics positively affected depressive symptomatology and anxiety symptoms according to 53.83% and 43.75% of the selected studies, respectively. Among the studies assessing inflammatory biomarkers, 58.31% found they were decreased after administration of probiotics. Conclusion The results emerging from the existing literature about probiotic supplementation for depression treatment are encouraging, but further research is needed considering the shortage of clinical trials on this topic and the heterogeneity of the samples analyzed.

In recent years, several experimental works have investigated the effect of probiotics in the treatment of neuropsychiatric disorders. (Burokas, Moloney, Dinan, & Cryan, 2015).
The gut is colonized by 10 13 -10 14 microorganisms (Burokas et al., 2015), known as gastrointestinal microbiota, which plays a role in human health (Guarner & Malagelada, 2003;O'Hara & Shanahan, 2006), and contributes to the development of different diseases. Several authors focused their attention on the interaction between the gut microbiota and the central nervous system, via endocrine, neural, and immune pathways, with effects on brain function, cognition, and behavior. (Mayer, 2011) The term gut-brain axis has therefore been proposed (Burokas et al., 2015;Collins, Denou, Verdu, & Bercik, 2009) to refer to the bidirectional communication between the gastrointestinal tract and the central nervous system. (Wang & Kasper, 2014).
Besides the possible role of the gut-brain axis in the pathogenesis of depression, several studies have investigated the cytokine hypothesis of depression (Leonard, 2018;Miller & Raison, 2016), according to the finding of increased levels of pro-inflammatory cytokines in depressed patients (Duivis, Vogelzangs, Kupper, de Jonge, & Penninx, 2013;Lamers et al., 2019), and of possible improvements in depressive symptoms after anti-inflammatory treatments. A recent review showed that low-dose aspirin treatment is not only safe and well-tolerated but also potentially efficacious for "improving depressive symptoms in both unipolar and bipolar depression" (Ng et al., 2019). Furthermore, pro-inflammatory stimuli can cause depressive and anxiety symptoms. (Eisenberger et al., 2010;Harrison et al., 2009) Interestingly, probiotics can reduce pro-inflammatory cytokine levels (Ait-Belgnaoui et al., 2012;Gareau, Silva, & Perdue, 2008;Luo et al., 2014) and oxidative stress (Liu & Zhu, 2018), increase anti-inflammatory cytokine levels (Citar et al., 2015), and play an immune regulation role, silencing the inflammatory response. (Vitaliti, Pavone, Guglielmo, Spataro, & Falsaperla, 2014) Therefore, probiotic supplementations could help improve depressive and anxiety symptoms, leading to a general improvement of patients' quality of life. (Peirce & Alviña, 2019).
Briefly, probiotics are living microorganisms whose intake in adequate quantities can prove beneficial for the host's health (Food & Agriculture Organization, 2001), producing neuroactive and neuroendocrine molecules, which also act on the central nervous system et al., 2009), and acting as immunomodulators by influencing cytokine secretion. (Thomas & Versalovic, 2010).
Animal and human studies have investigated the effects of probiotics, respectively, on anxiety-like behavior and depressive-like behavior in rats, (Arseneault-Breard et al., 2012) and psychological dimensions in humans, with encouraging results. (Tillisch et al., 2013) Probiotic supplementations could be an optimal adjunct to conventional antidepressants in the treatment of depressive and anxiety symptoms. The mechanism by which probiotics achieve these effects is not completely elucidated, even though several hypotheses have been formulated. (Collins et al., 2009) Interestingly, an antimicrobial effect has been shown by antidepressants, which are widely acknowledged to act on serum cytokine levels as well. (Brunoni et al., 2014;Hannestad, DellaGioia, & Bloch, 2011;Macedo et al., 2017).
To consider probiotics as a viable option in the treatment of the major depressive disorder or other neuropsychiatric disorders, evidence from well-defined clinical trials is needed; however, only a few clinical trials investigating the influence of probiotic consumption on behavior, mood, and cognition in the general population are available. In a previous meta-analysis of ten randomized controlled trials, Ng, Peters, Ho, Lim, and Yeo (2018), Ng, Soh, Loke, Lim, and Yeo (2018), have reported that the probiotic supplementation had overall insignificant effects on mood, with only modest effects in individuals with pre-existing mood symptoms and insignificant effects in healthy, community-dwelling individuals. According to this meta-analysis, the efficacy of probiotics consumption on the improvement of depression and anxiety symptoms, quality of life, and inflammatory biomarkers still needs to be demonstrated.

| Aims of the study
The aim of this review was to identify published data from randomized controlled trials (RCTs), studying the efficacy of probiotics consumption on the improvement of depressive symptoms, anxiety symptoms, quality of life, and inflammatory biomarkers. Another aim was the identification of the population which can maximally benefit from the probiotic treatment.
Two independent reviewers (E.G. and C.G.) assessed the articles identified by the above keywords.
After removing duplicates, titles were screened first, and those not in line with the purpose of the review were excluded. Then, abstracts were assessed, and last full texts were read, eventually leading to the inclusion or exclusion of the papers. The possible disagreement between reviewers was resolved by joint discussion with a third review author (P.Z.).
The consultation of an expert in this field of research allowed the inclusion of further 13 articles related to the topic and consistent with the search strings and the purpose of the study (as reported in Figure 1).
To be included in the review, studies had to: (a) deal with depression, inflammation, and probiotic supplementation; (b) be conducted on human beings (randomized controlled clinical trials, case-control studies, and prospective studies); (c) be written in English; (d) evaluate the effects of interventions on at least one of the following outcomes: anxiety, depressive symptoms, quality of life (QoL), global functioning, social adaptation, exogenous stressors, and biomarkers. F I G U R E 1 Preferred reporting items for systematic reviews and meta-analyses extension for scoping reviews (PRISMA-ScR) checklist Animal and laboratory studies, those in a language different from English, gray literature and reviews of the literature were excluded. Data extracted from the selected studies were recorded in a datasheet using a standardized coding form, including the following categorical and numerical variables: general information about the study (author/s, year of publication, duration of the study, title, journal title, country, study type, sample size, number in the experimental group, number in the control group, and lost at follow-up), participants' information (age and diagnosis), treatment (type of probiotic), intervention information (number of weeks of assumption), outcome assessment (questionnaire used and type of biomarker), and results.
Descriptive statistics used frequencies and percentages in the case of qualitative variables and means, standard deviations (SDs), and maxima and minima in the case of quantitative variables. Group differences in categorical variables were evaluated using the chi-squared test, and group differences in continuous variables were assessed using a t test. A p < .05 was considered statistically significant. Analyses were performed using STATA 15.

| RE SULTS
As described in the PRISMA flow diagram (Figure 2), the first search identified 206 titles; according to titles, 189 records were excluded; after reading the abstract, 7 further records were excluded: One study was excluded because it was an animal experimentation, and F I G U R E 2 PRISMA flow chart six studies because they were not clinical trials. Ten full texts were fully assessed for eligibility, and seven were excluded (5 were not clinical studies, and two studies did not include a probiotic supplementation). Furthermore, 13 records were included as suggested by expert consultation and 7 records were identified from two previ- The main features of the selected studies, including data on the first Author, country and year, patients' features, probiotic treatment, outcomes and measures, and main findings, are shown in Table 1. 6 weeks in 4 studies (17.4%), (Guyonnet et al., 2007;Lorenzo-Zúñiga et al., 2014;Marcos et al., 2005;Pinto-Sanchezet al., 2017) 4 weeks in 3 studies (13%), and (Messaoudi et al., 2011;Steenbergen et al., 2015;Tillisch et al., 2013) 3 weeks in only one study (4.3%). (Benton et al., 2007) 2 studies (8.7%) lasted 24 (Malaguarnera et al., 2012) and 20 (Shinkai et al., 2013) weeks, respectively.
In all studies, a follow-up was performed. One study (4.3%) set a single follow-up visit, (Hilimire et al., 2015) while 8 studies (  The studies were published in several countries all over the world; however, they were mostly from the United States

| Participants' features
The selected studies involved different populations: 9 (39.1%) were performed on a sample from the general population, (Benton   Romijn et al., 2017) only psychotherapy was mentioned by one study (4.3%), (Romijn et al., 2017) and in the case of severe depression (Akkasheh et al., 2016) no information about treatment was provided.

| Outcomes
Studies included in the analysis used different questionnaires, either self-reported or clinician-rated, to evaluate different outcomes (Table 2). Several studies did not specify details about this information.
Seven (Akkasheh et al., 2016;Benton et al., 2007;Feher et al., 2014;Lyra et al., 2016;Messaoudi et al., 2011;Pinto-Sanchez et al., 2017;Steenbergen et al., 2015) out of these 13 studies reported a significant improvement of depressive symptoms after probiotic consump- Correlation analysis is described in Table 3. An association between probiotics efficacy in terms of reduction of depression was found only in studies where the sample did not include patients with psychiatric disorders (p = .03). No association was found among depression severity, the population involved, or type of probiotic.
No improvement of anxiety symptoms was reported by those 3 studies (Chahwan et al., 2019;Pinto-Sanchez et al., 2017;Romijn et al., 2017) which recruited a population with low-moderate depression.
No significant result emerged from the correlation analysis between the reduction of anxiety symptoms and other variables.

| Global functioning
Five (Guyonnet et al., 2007;Herranen et al., 2003;Hilimire et al., 2015;Romijn et al., 2017;Vaghef-Mehrabany et al., 2014) of the studies included in this review analyzed the improvement of global functioning in the population; 2 studies (Guyonnet et al., 2007;Hilimire et al., 2015) demonstrated a significant effect, associated with a reduction of anxiety symptoms, but none of them included a depressed population.

| D ISCUSS I ON
In the current literature, the number of clinical studies evaluating the impact of probiotic supplementation on anxiety and depressive symptoms, QoL, and inflammatory biomarkers remain limited. Furthermore, these studies do not follow a standardized methodology.
Only in the last years probiotic integration caught the attention of the scientific community; hence, the effects of the alteration of the intestinal microbiota and the mechanisms underlying its role in various medical disorders still need to be clarified.
Previous studies have highlighted that in IBS are present subclinical inflammation at the gut mucosa level as well as the involvement of psychosocial factors. (Ng, Soh, et al., 2018) Probiotics could be potentially useful in this setting as it has alleged anti-inflammatory and immunomodulatory effects.
Various questionnaires, both self-administered and clinician-rated, were used for the assessment of outcomes and clinical variables: They considered different items (Julian, 2011) and had different psychometric properties. The present review did not apply restrictions on the questionnaires in order not to excessively limit the number of the studies included.
Regarding inflammatory biomarkers, the studies selected for this review showed variability in those assessed and also in the biological samples collected. Moreover, the difference in sample size across studies could influence the possibility to compare their results.
The probiotic supplementation in the various studies presented two further elements of variability: the duration of administration (from several weeks to several months) and the composition.
This could be relevant in the comparison of the results since it is acknowledged the species specificity of the effects of probiotics in the treatment of different medical conditions. ( Bercik et al., 2010) The current literature is not well equipped to answer questions on the safety of probiotic interventions with confidence as there appears to be a lack of systematic reporting of adverse events. (Gwee et al., 2018). The available evidence does not indicate an increased risk, but there are anecdotal reports that probiotics may worsen outcomes, for example, in patients receiving radiotherapy (Hempel et al., 2011).
In the current scoping review, all the studies reported that probiotic treatment was well tolerated, with no relevant side effects.
It is important to underline that not all probiotics are equal. The Human Microbiome project revealed the microbial taxa complexity in the human gut, and also highlighted the highly individualized microbiome composition due to inheritance, diet, and environmental factors. Every effort should be made to report specific probiotic strains or mixture of strains when analyzing the efficacy and safety of probiotics (McFarland, Evans, & Goldstein, 2018).
It is also important to highlight that there are still existing gaps in knowledge regarding the interaction between the microbiome and the host in vivo-and the pathway of its metabolites-and how their metabolites influence the microenvironment. Further mechanistic studies involving "omics" technologies, as adapted from previous studies (Wang et al., 2018), might help shed light on these questions.

| Depression and anxiety
The impact of probiotic supplementation was described as effective in reducing depressive symptoms and anxiety by 53.83% and 43.75% of the studies, and in improving QoL and global functioning any further comorbidity, and only one study (Akkasheh et al., 2016) has involved patients with a MDD diagnosis. Even in this case, anyway, no comparison with populations affected by subthreshold depression or Healthy Controls (HCs) was made; however, in all the populations examined, data concerning the improvement of QoL and depressive and anxious symptoms were analyzed.

| Biomarkers
Significant results have been reported by 58.31% of studies evaluating changes in inflammatory biomarkers, which is encouraging.
Considering the few studies that included a population with a diagnosis of depression, (Akkasheh et al., 2016;Chahwan et al., 2019;Pinto-Sanchez et al., 2017;Romijn et al., 2017) inflammatory biomarkers were significantly reduced only in the study that considered a population with MDD: (Akkasheh et al., 2016) This result is consistent with the inflammatory hypothesis of depression.

| Quality of life and global functioning
In the literature, it has been shown that the microbiota can influence

| Strengths and limitations
The current review could add to the existing literature on the use of probiotic supplementation in the treatment of mood and anxiety disorders or symptoms, which is still lacking methodologically sound clinical studies and systematic reviews.

| CON CLUS ION
Our review found that available literature on this topic is very heterogeneous regarding type of probiotic used and duration of treatment, type of sample, methodology, assessment tools, and outcomes. Therefore, it is still difficult to draw clear conclusions about the effectiveness of probiotic supplementation in patients with depression and anxiety symptoms. . Furthermore, the studies included are heterogeneous regarding the type of probiotic, the methods used to test symptoms and inflammatory status, and study outcomes; for these reasons, the possibility to analyze and generalize the emerging results is limited.

ACK N OWLED G M ENTS
Not applicable.

CO N FLI C T O F I NTE R E S T S
The authors declare that they have no competing interests. Zeppegno revised it critically for important intellectual content.

E TH I C S A PPROVA L A N D CO N S E NT TO PA RTI CI PATE
Approval for the research and informed consent are not necessary for this type of work.

PEER R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1002/brb3.1803.

DATA AVA I L A B I L I T Y S TAT E M E N T
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.