BMC Public Health 2009, 9:1 http://www.biomedcentral.com/1471-2458/9/1 Background Body image disturbance is defined as a distortion of perception, behavior, or cognition related to weight or shape [1], and it is becoming a common clinical disorder [2-7]. Aesthetic standards typical of Western cultures, based on the stereotype of a lean body for women and a muscular body for men, are considered possible determinants of body image disturbance [8, 9]. While most women wish to lose weight [10] even when their anthropometric parameters were normal, a significant number of men have shown muscle dissatisfaction [11]. Some reports in the scientific literature have found an association between body image disturbance and a number of mental health outcomes including depression [12-17], anorexia [12-14], bulimia [12-14], and body dysmorphia [16]. Currently, depression is a major public health problem. There is an estimated 150 million people around the world who suffer from this disease, and it is the first and the fourth cause of morbidity among women and men, respectively [18]. Recent data from the European Community suggest that depression is responsible for 6.2% of all cases of disability-adjusted life years, making it the third cause of morbidity in this continent, surpassed only ischemic heart disease and stroke [19]. In Spain, the European Study of the Epidemiology of Mental Disorders (ESEMED) showed that depression was the most common mental disorder in the country's population, with a prevalence of 3.9% [20]. Depression has a significant socioeconomic impact on a society, because it can lead to absence from work. Furthermore, depression is strongly related to suicide, which is also a major health problem worldwide [18]. Thus, research of factors associated with depression is important in order to reduce the magnitude and consequences of this disease. Results of some studies have shown that body image disturbance is associated with depression [8, 10, 16, 17]. Other researchers suggest that depression predisposes to body image disturbance [12, 13, 15]. However, these previous studies had a cross sectional design, which does not assure the temporal sequence of the association investigated. Therefore, the aim of our study was to assess the association between body image disturbance and incidence of depression in a large prospective cohort of university graduates in Spain. Methods Subjects The SUN project (Seguimiento Universidad de Navarra) is a multi-purpose, dynamic cohort conducted in Spain, consisting of exclusively university graduates as partici- pants. The recruitment of participants began in Decem- ber 1999 and it is permanently open. Information is collected using self-administered questionnaires sent by postal mail every two years. The objectives and methods of this project have been detailed previously [21]. As of February 2008, data from 19,057 participants were coded and prepared for analysis. All participants who complete a baseline assessment (Q_0) before May 2005 were eligible for these analyses (n = 15,502). Among them, 1,852 did not answer any of the follow-up questionnaires; after five mailings, they were considered lost to follow-up. We retained 13,650 participants. Among them, 4,328 were recruited more than 6 years ago and we had data from the baseline (Q_0), the 2-year (Q_2), the 4-year (Q_4), and the 6-year (Q_6) follow-up questionnaires. The number of participants retained for the 4-year follow-up (they returned Q_2 and Q_4) and for the 2-year follow-up (they returned Q_2) were 4,265 and 5,057, respectively. The overall follow-up rate was approximately 89%. Participants who had some of the following character- istics were excluded from the analysis (n = 3,364): subjects who reported the use of antidepressants or those with a self-reported physician-made diagnosis of depres- sion at baseline (n = 1,565), those who reported extremely low or high values for total energy intake, because they were more likely to have failed to properly complete the questionnaire (less than 800 Kcal/day in men and 500 Kcal/day in women or more than 4000 Kcal/day in men and 3500 Kcal/day in women) (n = 1,184), we also excluded those without information on body size perception (n = 582), and pregnant women at baseline (n = 33). Finally, data from 10,286 participants remained available for the analysis. The study was approved by the Human Research Ethical Committee at the University of Navarra. Voluntary completion of the first questionnaire was considered to imply informed consent. Exposure assessment The information about self-reported BMI was collected in the baseline questionnaire (Q_0) and grouped into four categories using the cut-off points established by the Page 2 of 9 (page number not for citation purposes)