The present cross-sectional study was conducted within the framework of the SEPAHAN (Study on the Epidemiology of Psychological, Alimentary Health and Nutrition) project, a cross-sectional study that investigated the prevalence of functional gastrointestinal disorders (FGIDs) and their relationship to lifestyle factors. Detailed information about data collection methods in SEPAHAN project has been published previously . In brief, this study was performed among Iranian general adults working in 50 different healthcare centers affiliated to Isfahan University of Medical Sciences (IUMS) across Isfahan province. In this project, data were collected in two main phases between April 2010 and May 2010. A questionnaire that contained information on demographic and dietary data was sent to 10,087 participants and 8691 subjects returned the completed questionnaire (response rate of 86.16%), in the first step. At the second phase, information on psychological health was collected and 6239 subjects returned the completed questionnaire. Some participants who took part in the second phase did not participate in the first phase. In addition, some participants in both phases had not written their name or national identification code on at least one of the questionnaires. This made us unable to match the data for all subjects participated in both phases. Then, 4763 questionnaires in the second phase were matched with their equivalent questionnaires in the first phase. In the current study, subjects who had total daily energy intakes outside the range of 800–4200 kcal/d were excluded due to under- or over-reporting of energy intake . In addition, those who had missing data on any relevant variable were excluded. Individuals with anti-depressant use were also excluded from the current analysis. Therefore, data from 3175 subjects, for whom complete information about both dietary intakes and psychological profile were available, were included in the current analysis. All participants provided written informed consent forms. The study protocol was ethically approved by the Regional Bioethics Committee of Isfahan University of Medical Sciences (#189069, #189082, and #189086). This cross-sectional study was reported in according to ESTROBE guideline.
Dietary intakes assessment
Dietary data were collected using a Willett-format Dish-based 106-item Semi-quantitative Food Frequency Questionnaire (DS-FFQ), which was designed and validated specifically for Iranian adults. Detailed information about the design, foods included as well as the face validity of this questionnaire has been reported elsewhere . Briefly, the questionnaire contained five categories of foods and dishes:  mixed dishes (cooked or canned, 29 items) ; carbohydrate-based foods (different types of bread, cakes, biscuits and potato, 10 items) ; dairy products (dairies, butter and cream, 9 items) ; fruits and vegetables (22 items); and  miscellaneous food items and beverages (including sweets, fast foods, nuts, desserts and beverages, 36 items). For each food item, a commonly consumed portion size was defined. Participants were asked to report their dietary intakes of foods and mixed dishes based on nine multiple choice frequency response categories varying from “never or less than once a month” to “12 or more times per day”. The frequency response categories for the food list varied from 6 to 9 choices. For foods consumed infrequently, the high-frequency categories were omitted, while for common foods with a high consumption, the number of multiple choice categories increased. For instance, the frequency response for tuna consumption included six categories, as follows: never or less than once/month, 1–3 times/month, 1 time per week, 2–4 times/week, 5–6 times/week, 1–2 times/day; and for tea consumption that is highly prevalent among Iranians, the frequency response included nine categories, as follows: never or less than 1 cup/month, 1–3 cups/month, 1–3 cups/week, 4–6 cups/week, 1 cup/day, 2–4 cups/day, 5–7 cups/day, 8–11 cups/day, ≥12 cups/day). Finally, to convert the food items into grams, the amount of each portion size was computed based on the booklet of “household measures” , and then computed the amount of intake by considering the frequency of consumption of each food item. Because the Iranian food composition table is incomplete, the United States Department of Agriculture (USDA) food composition table was used to analyze foods and beverages; however, in the dataset of USDA in the software, some traditional foods and beverages were modified based on the Iranian food composition table, which were not listed in the USDA food composition table. This means that for almost 98% of foods in the FFQ, the USDA database was used. For some food items that were not available there (for example Iranian local breads like Lavash and Barbari) and the nutrient composition of these foods were available in Iranian food composition table , these foods were added to the database of the software. Nutrient intakes for each participant was calculated using the USDA food composition database that was modified for Iranian foods. Milk, red meat, poultry, and dairy products are the major dietary sources of BCAAs . The amount of valine, leucine, and isoleucine in 100 g of these foods was calculated and then total BCAAs intake was computed by summing up the amount of valine, leucine, and isoleucine intake.
The validity of DS-FFQ was examined in a subgroup of 200 randomly selected participants of the SEPAHAN project. All participants in the validation study completed the DS-FFQ at study baseline and 6 months later. During this validation study, participants provided three detailed dietary records that were used as the gold standard. As shown in earlier studies, it seems that this questionnaire provides reasonably valid measures of long-term dietary intakes .
Assessment of the psychological profile
The Iranian validated version of Hospital Anxiety and Depression Scale (HADS) was used to screen for anxiety and depression . HADS is a brief and useful questionnaire to assess psychological disorders and symptom severity of depression and anxiety disorders. The HADS contains 14 items and include two subscales: anxiety and depression. Each item includes a four-point scale; higher scores indicate an elevated level of anxious and depressive symptomatology. Maximum score is 21 for anxiety and depression. Scores of 8 or more on either subscale were considered as psychological disorders and scores of 0–7 were defined as “normal” in the current study. The convergent validation of translated version of HADS questionnaire was examined in 167 Iranian adults using the correlation of each item with its hypothesized scale. Pearson’s correlation coefficients varied from 0.47 to 0.83 (P < 0.0001) for anxiety subscale and from 0.48 to 0.86 (P < 0.0001) for depression subscale, indicating that the questionnaire provides relatively valid measures of psychological health . The Iranian validated version of General Health Questionnaire (GHQ) with 12-items was used to assess psychological distress . GHQ-12 is a brief, simple, easy-to-complete instrument for measuring current and primary mental health that asks the respondents whether they have experienced a particular symptom of psychological distress or a change in their behavior recently. Each item consists of a four-point scale (less than usual, no more than usual, rather more than usual, or much more than usual). In this study, the bimodal (0–0–1-1) scoring method was used. This gives scores ranging from 0 to 12. Higher scores indicate a greater degree of psychological distress. In the current study, the score of 4 or more was defined as having psychological distress . The convergent validity of GHQ-12 was examined in 748 Iranian young people. Significant inverse correlation was seen between the GHQ-12 and global quality of life scores (r = − 0.56, P < 0.0001) .
Assessment of other variables
Required information on other variables including age, sex, marital status, smoking status, education, and chronic conditions (diabetes, asthma, colitis, stroke, myocardial infarction, heart failure, and cancers) and antidepressant and supplements (vitamins, minerals, calcium and iron) was obtained from demographic and medical history questionnaires. Physical activity was assessed using the General Practice Physical Activity Questionnaire (GPPAQ) , and participants were classified into two categories: physically active (≥1 h/week) and physically inactive (< 1 h/week). Anthropometric measures including weight, height, and waist circumference were assessed using a self-administered questionnaire. The validity of self-reported values of weight, height, and waist circumferences (WC) was examined in a pilot study on 200 participants from the same population. In the validation study, self-reported values of anthropometric indices were compared with actually measured values. The correlation coefficients for self-reported weight, height, and WC versus corresponding measured values were 0.95 (P < 0.001), 0.83 (P < 0.001), and 0.60 (P < 0.001), respectively. Body Mass Index (BMI) was calculated by dividing weight (kg) to height (m2). The correlation coefficient for computed BMI from self-reported values, and the one from measured values was 0.70 (P < 0.001) .
General characteristics of study participants across tertiles of BCAAs were expressed as means ±SDs for continuous variables and percentages for categorical variables. Dietary intakes of study participants across tertiles of BCAAs were compared using analysis of variance (ANOVA). Binary logistic regression was used to estimate ORs and 95% CIs for the presence of psychological disorders across tertiles of BCAAs in crude and multivariable-adjusted models. Age (continuous), sex (male/female), total energy intake (continuous), marital status (married/single), education (diploma or under-diploma/university graduate), vitamin supplements use (yes/no), smoking (non-smoker/former smokers and current smokers), physical activity (< 1 h/week/≥1 h/week), presence of chronic conditions (yes/no), dietary intakes of omega 3, fiber, group B vitamins, fruits, vegetables, and BMI were controlled for in the multivariable-adjusted model. P for trends was determined by considering tertiles of BCAAs intake as ordinal variables in the logistic regression analysis. All statistical analyses were done using the Statistical Package for Social Sciences (version 20; SPSS Inc.). P < 0.05 was considered as statistically significant.