Participants: 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 [16]. 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. To collect information about anthropometric indices, demographic and lifestyle factors, including dietary intakes and physical activity, self-administered questionnaires distributed among 10,087 subjects in the first phase, and 8,691 participants returned the completed questionnaires (response rate: 86.16%). In the second phase, data regarding gastrointestinal health were collected (response rate: 64.6%). Finally, we were able to match 4,763 questionnaires in the second phase with their corresponding questionnaires in the first phase. In the current study, we excluded subjects who had total daily energy intakes outside the range of 800-4200 kcal/d as well as those that had missing data on any relevant variable. Therefore, data from 3,363 subjects, for whom complete information about both dietary intakes and IBS 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.
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 [17]. To develop the questionnaire, a comprehensive list of foods and dishes commonly consumed by Iranian adults was constructed. Then, we chose those foods that were nutrient-rich, consumed reasonably often, or contributed to between-persons variations. This process led to the selection of the remaining 106 food items in the questionnaire. Briefly, the questionnaire contained five categories of foods and dishes: (1) mixed dishes (cooked or canned, 29 items); (2) carbohydrate-based foods (different types of bread, cakes, biscuits and potato, 10 items); (3) dairy products (dairies, butter and cream, 9 items); (4) fruits and vegetables (22 items); and (5) 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 multiplechoice 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, we omitted the high-frequency categories, while for common foods with a high consumption, the number of multiplechoice categories increased. For instance, the frequency response for tea consumption included 9 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 and ≥12 cups/day. Finally, to convert the food items into grams, we computed the amount of each portion size based on the booklet of “household measures” [18] and then computed the amount of intake by considering the frequency of consumption of each food item.
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 mo 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.
Calculation of coffee and caffeine intake: To assess coffee intake, participants were requested to report the average number of glasses of coffee they usually consume in the preceding year. They were able to choose one of these frequency response categories: “never or <1 glass/month”, “1-3 glasses/month”, “1 glass/week”, “2-4 glasses/week”, “5-6 glasses/week”, “1 glass/day”, “2-3 glasses/day”, “4-5 glasses/day” and “≥6 glasses of coffee in a day”. Total caffeine intake was estimated by summing of the caffeine participants took from all caffeine-containing foods and beverages.
Assessment of IBS: A modified Persian version of the Rome III questionnaire [19], as part of the main comprehensive questionnaire, was used for assessment of IBS. During the face validation of the questionnaire, we found that most participants were unable to distinguish between the descriptors used in the original Rome III questionnaire (never, less than 1 d a month, 1 d a month, 2-3 d a month, 1 d a week, more than 1 d a week, every day). Therefore, we modified the rating scales to consist of only four descriptors (i.e., never or rarely, sometimes, often, and always). We also decided to ask about the presence of each symptom in the previous 3 mo instead of questioning patients about the beginning of each symptom >6 mo before the evaluation, which already exists in original Rome III questionnaire. IBS was defined according to Rome III criteria as having recurrent abdominal pain or discomfort, at least sometimes, in the previous 3 mo associated with two or more of these criteria: improvement with defecation, at least sometimes, and onset associated with change in frequency or form (appearance) of stool, at least sometimes. IBS with constipation (IBS-C) was defined as having IBS and both of the following criteria: hard or lumpy stools at least sometimes and lack of loose, mushy, or watery stools. IBS with diarrhea (IBS-D) was defined as having IBS and both of the following criteria: lack of hard or lumpy stools and loose, mushy, or watery stools at least sometimes. Mixed IBS (IBS-M) was defined as having IBS and both of the following criteria: hard or lumpy stools at least sometimes and loose, mushy, or watery stools at least sometimes. The severity of IBS was examined by asking participants about the severity of their abdominal pain in the previous 3 months. They were able to choose one of these responses: mild, moderate, severe, or very severe.
Assessment of other variables: Required information on other variables including age, sex, marital status, smoking status, medication use, and disease history (diabetes and colitis) was obtained from demographic and medical history questionnaires. Data on height, weight and waist circumference (WC) were collected using a self-reported questionnaire. Physical activity was assessed using the General Practice Physical Activity Questionnaire [20], and participants were classified into two categories: physically active (≥1 h/wk) and physically inactive (<1 h/wk). Data on diet-related practices including meal regularity (often or always/never or occasionally), chewing efficiency (a lot/not a lot), breakfast skipping (skipper/non-skipper) and intra-meal fluid intake (<3 glasses/≥3 glasses) were also assessed through the use of pretested questionnaire. Dental status was also examined and subjects were categorized as “having all teeth”, “lost 1-5 teeth” and “lost >5 teeth”. 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).
Statistical analysis: General characteristics of study participants across categories of coffee and caffeine intake were presented as means ±SDs for continuous variables and percentages for categorical variables. To examine the differences across categories, we used ANOVA for continuous variables and chi-square test for categorical variables. We used binary logistic regression to estimate ORs and 95% CIs for the presence of IBS and its subtypes across categories of coffee and caffeine intake in crude and multivariable-adjusted model. The trend of ORs across categories of coffee and caffeine intake was determined by considering categories of coffee and caffeine intake as ordinal variables in the logistic regression analysis. We also used multivariable ordinal logistic regression to estimate ORs and 95% CIs for assessing IBS severity (mild/moderate/severe/very severe) across categories of coffee and caffeine intake in crude and multivariable-adjusted model. In these analyses, age, sex (male/female), energy intake (continuous), BMI (continuous), physical activity (<1 h/week/≥1 h/week), smoking status (non-smoker/former smokers and current smokers), medication use (yes/no), self-reported diabetes (yes/no) and colitis (yes/no), psychological distress (yes/no), meal regularity (often or always/never or occasionally), chewing sufficiency (a lot/not a lot), intra-meal fluid consumption (<3 glasses/≥3 glasses), breakfast skipping (skipper/non-skipper), and dental status (have all teeth/lost 1-5 teeth/lost >5 teeth) were adjusted for in the multivariable model. All statistical analyses were done using the Statistical Package for Social Sciences (version 20; SPSS Inc.). P<0.05 was considered as statistically significant.