Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder, characterized by abdominal pain or discomfort in association with altered bowel habits . IBS is common in the general population, the prevalence of this disease in European countries was about 20%, in recent years, the morbidity of Asian countries is rising year by year, which is nearly to that in Western countries [2-5]. Without any biological markers, the diagnosis of IBS mainly depending on the patient's symptoms. According to Rome III criteria, IBS patients were subdivided into diarrhea-predominant IBS (IBS-D), constipation-predominant IBS (IBS-C), mixed type IBS (IBS-M) and un-subtyped IBS (IBS-U) .
Although the pathophysiology of IBS is unclear, gender, diet, lifestyle, psychiatric disorders, sleep problems, early adverse life events (EALs), stress have been considered as risk factors to IBS, but there are controversial [3-5, 7-15]. Medical students are a special group, characterized by tremendous cognitive and emotional changes. At the same time, due to massively study and exams load, increasing fierce competition for jobs, undesirable living and eating habits, medical students may be an especially high-risk population for IBS . In recent years, the number of IBS research papers, relating to medical students, has increased in China and the rest of the world [3-5, 16], however, to the best of our knowledge, no comprehensive survey (including prevalence, diet, lifestyle, psychiatric disorders, sleep problems, EALs, stress) about IBS that using Rome ' criteria has been carried out in medical students in Beijing, China nowadays. Get the information about prevalence and related factors in medical students may provides targeted recommendations for the treatment of IBS, which can effectively improve the clinical symptoms of IBS patients, enhance their learning efficiency and quality of life.
So the aims of this study were: (1) to investigate the prevalence of IBS in medical students according to Rome III criteria and the related factors with IBS; (2) to evaluate the risk factors that associated with the disorder.
2. Materials and methods
This study included medical students from the first to the seven year, and we employed a multistage stratified random sample method [4, 8]. Using the following formula: n'pq / (d/t)2't2 ?? pq / d2, where t'1.96 (error of the first kind), p'20% (estimated prevalence), q'1'p, and d'15% ?? p (permissive error), the expected minimal sample size for this study was 683. According to the 90% response rate, the number of students should be 759. To ensure reliability, we proceeded to recruit 843 medical students into the study.
2.3 Criteria of exclusion
Subjects with self-reported organic gastrointestinal disorder, family history of cancer, weight loss, anemia, bloody stools, abnormal laboratory findings, other alarming signs, and those who had previously undergone gastrointestinal surgery were excluded from our study .
2.4.1 Personal general information, lifestyle and eating habits
Personal data, such as sex, age, height, weight, lifestyle and eating habits, such as exercise frequency, smoking, alcohol consumption were included in the questionnaire. With the grade, low means the students from the first to the fourth year, and the high means from the fifth to the seven year.
2.4.2 Chinese version of Rome ' criteria for dignosis of IBS.
The Chinese version of Rome ' criteria for dignosis of IBS has been widely used [1, 17-18]. The diagnosis of IBS was based on the presence of abdominal pain or discomfort had to occur at least 3 days per month for at least three months during the previous six months, with at least two or more of the following conditions: symptoms associated with a change in frequency or form of stool, pain improved after defecation. Patients with IBS were divided into diarrhea-predominant IBS (IBS-D), constipation-predominant IBS (IBS-C), mixed IBS (IBS-M) and un-subtyped IBS (IBS-U) according to the proportion of hard and lumpy stools.
2.4.3 'Severity of IBS' questionnaire
The IBS severity questionnaire includes five items: the severity of perceived pain, the presence and severity of abdominal distension, the frequency of abdominal pain or discomfort, satisfaction with bowel habits and the quantification of interference in the patient's general lifestyle by these symptoms. Each item has a 100-point scale from 20 to 100 score and the total severity score is the sum of the five items. Patients were classified as having mild IBS (IBS severity score 75-175), moderately severe IBS (score 175-300) and severe IBS (score>300) [19-20].
2.4.4 Hospital anxiety and depression scale (HADS)
HADS was widely used in China [17-18]. It has 14 questions (with two 7-item subscales), and was used in estimating the emotional disorders of anxiety and depression status in patients. For each subscale, the result fell into 3 grades as follows: normal cases (0-7), borderline cases (8-10) and severe cases (over 11) [17-18].
2.4.5 Pittsburgh sleep quality index (PSQI)
PSQI is a 19-item, self-administered questionnaire . It has been widely used to assess the sleep quality of subjects . The contents of the PSQI including seven items: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleep inducers, and daytime dysfunction. Each item uses a three-point response scale, the total score was used to calculated the PSQI global index (ranging from 0-21), a total score'5 means poor sleepers, when the total score is up to or greater than 8, it means a severe sleep disorder.
2.4.6 Child trauma questionnaire (CTQ)
CTQ, which has 28 items, was developed by Bernstein D and Fink L . CTQ consists five subscales (emotional abuse, physical abuse, sexual abuse, emotional neglect and physical neglect, each subscale has five items) to evaluate the degree of child trauma (one type of EALs) and three items for validity evaluation. Each item has a 5-point scale from 1 to 5 score, the total score range from25 to 125 .
2.4.7 Student-life Stress Inventory (SLSI)
SLSI was designed by Gadzella BM . SLSI has 51 items with five stress categories (frustration, conflicts, pressures, changes, and self-imposed) and four categories of reactions to stressors. Each item using a 5-point scale, the total score is the sum of the nine categories. The purpose of the SLSI was to assess the mean responses to different stressors and reactions to stressors among students .
2.5 Statistical analysis
Statistical analysis was performed using the SPSS version 19.0 (IBM Corporation., NY., USA). Categorical variables were analyzed by Pearson's ??2 or Fisher's exact test. The t test was used to compare group differences for continuous variables, data is presented as mean??SD. Logistic regression analysis was used to assess the possible risk factors. The odds ratio (OR) with a 95% confidence interval (CI) was calculated. All calculated P-values were two-tailed and P< 0.05 was considered statistically significant.
3.1 Response rate, overall characteristics, lifestyle and eating factors of participants
Of the 843 medical students, 767 (91.0%) completed the survey, among these subjects, no people following the exclusion criteria, and they were all included in the final analyses. The age of the medical students ranged from 18 to 27 years, with a mean age of 23.26??2.88 years, 196 (25.6%) were males and 571 (74.4%) were females. The demographic characteristics, lifestyle and eating factors of participants are listed in Table 1 and Table 2. It is apparent from Table 2 that significant difference was found between IBS and control groups in seafood diet, IBS patients had a lower frequency in this item.
3.2 Prevalence of IBS
Of the 767 medical students, 255 fulfilled the criteria for having IBS, the prevalence of IBS was 33.3%, for females, the prevalence was 36.1% (206/571), and for males, 25.0% (49/196) cases were found to be positive. At the same time, we found that, for women, the students with low grade had a higher prevalence of IBS (45.9%, 67/146), and the high grade was 32.8% (139/425) (P=0.015), for men, no statistical difference was found (23.1%, 24/104 vs 27.2%, 25/92 P'0.514). Among the IBS patients, 112 cases were IBS-M (43.9%), 79 cases were IBS-D (31.0%), 49 cases were IBS-U (19.2%), and 15 cases were IBS-C (5.9%). Female predominance was IBS-M (45.1%), and male was IBS-D (44.9%). Of the 255 IBS patients, 20.4% were mild IBS, 77.6% had moderately severe IBS and 2.0% were severe cases.
3.3 HADS and PSQI scores in IBS patients and controls
The anxiety and depression scores in IBS patients were 4.89??2.87 and 4.34??2.77, for control group, there were 4.58??2.95 and 4.23??2.94, no statistical differences were found (Table 3). However, for females, IBS patients had a higher score than healthy controls in anxiety (4.83??2.67 vs 4.23??2.85, P=0.015).
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