Depression and anxiety are among the most common mental disorders that affect approximately 25% of people [1]. These two disorders that are causes of each other [3], have extensive pathophysiological overlap [4], genetic correlation [5], and comorbidities [6]. Standard medications are not effective in approximately one-third of patients [10], in addition to having side effects and increasing the risk of suicidal thoughts [11]. The stigma of mental illness also prevents from starting or continuing treatment [12]. Interest in non-pharmacological and alternative strategies for the treatment of mental illness is growing [18], and studies have focused on the consumption of foods containing prebiotics or probiotic supplements [19]. Probiotics have anti-inflammatory properties [28] and a role in the production of tryptophan (a precursor of serotonin) [26]. Studies have shown the positive effect (without serious side effects) of probiotics in improving the symptoms of depression and anxiety [30, 31]. However, some studies have suggested that there is no difference between probiotics and placebo [32]. So far, less studies have been conducted on the potential effects of probiotics on psychological disorders. Due to the lack of related study in Iran, the present study, as the first study in Iran, aims to assess the relationship of probiotic supplementation with depression and anxiety.

This is a correlational study conducted at the midst of the first peak of the COVID-19 pandemic in Iran. Participants were 300 men and women aged 20-40 years living in Tehran with at least a diploma who were selected by a convenience sampling method. The sample size was determined according to Costello [34] (10-20 samples per item). For collecting data, the Beck Depression Inventory, the Beck Anxiety Inventory, and a researcher-made Probiotic Consumption Scale (PCS) were used. The PCS was designed based on the Food Frequency Questionnaire. For having information about its validity and reliability, see Parhizgar et al. [40]. The questionnaires were provided online (due to the pandemic and observance of health protocols).
After excluding 21 samples (due to incomplete responses), statistical analysis was performed in Lisrel v.8.80 and SPSS v. 26 applications on data collected from 279 samples (76 males and 203 females). Data were described using descriptive statistics. The amount of skewness and kurtosis of all three variables was between -2 to +2 indicating the normal data distribution. Therefore, Pearson correlation test and regression analysis were used.

Most of participants (74.9%) had moderate probiotics consumption; minority of them (3.6%) had appropriate probiotic consumption. The probiotic consumption had a significance negative relationship with depression (r=-0.183, P=0.002) and anxiety (r= -0.122, P=0.041). 
Due to the value of regression coefficient (R= 0.233) and the squared regression coefficient (R2=0.054), a weak relationship was found between the criterion and predictor variables; the predictor variables played a weak role in explaining the variance of the criterion variable (Table 1).

This study was conducted considering the importance of the growing prevalence of anxiety and depression which impose economic, social, and psychological burden on society. Today, probiotics have become very important because of their beneficial effects on human health. Based on the findings of previous studies, it seems that the use of probiotics may have a beneficial effect on mood or symptoms of depression and anxiety in healthy people, but they have no significant effect in predicting anxiety disorder. 
According to the results of the present study, the prevalence of depression in our study was higher than in other related studies in Iran, while the prevalence of anxiety was lower. This difference can be attributed to the study population sample size, age and education of samples, and the study city. In this study, we found that the relationship of probiotic consumption with depression and anxiety was significant, although it was weak. These findings are explained by the fact that the gut-brain axis is bidirectional, i.e. there is a link between the central nervous system and enteric nervous system; hence, with the decrease in probiotic consumption, depression and anxiety increase or vice versa. The use of probiotics had no significant effect on anxiety, but depression could be predicted based on the use of probiotics. The findings of the present study are explained by the fact that there is a bidirectional gut-brain axis.

The study was approved by the Ethics Committee of the Tehran islamic Azad University of Medical Sciences (Code :IR.IAU.TMU.REC.1399.245). All ethical principles are considered in this article. The participants were informed about the purpose of the research. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them.

This study was extracted from the MA. Thesis of first author at the Department of Educational Science and Psychology, Islamshahr Branch, Islamic Azad University.

All authors equally contributed to preparing this article.

The authors declared no conflict of interes.

The authors would like to thank th Dr. Sedigheh Heidari and Dr. Parynaz Parhizgar for their help.
 

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