BMC Public Health 2009, 9:1 http://www.biomedcentral.com/1471-2458/9/1 WHO: BMI <= 19.99 kg/m2 for underweight subjects, 20.00 kg/m2 <= BMI <= 24.99 kg/m2 for normal weight subjects, 25.00 kg/m2 <= BMI <= 29.99 kg/m2 for overweight subjects and BMI >= 30.00 kg/m2 for obese subjects [22]. The self-reported BMI by participants of the SUN study was validated in a specific study exhibiting a high degree of validity (mean error = 1.4%) [23]. Body size perception was also assessed in the baseline questionnaire (Q_0) with the nine figure scheme (Figure 1), which was validated by Stunkard [24]. Participants were asked to select which of the nine figures most closely represent their body size. After this, they were grouped into four categories following the recommended classification: pictures 1, 2 and 3 as equivalent to underweight (BMI <= 19.99 kg/m2), pictures 4 and 5 to represent normal weight (20.00 kg/m2 <= BMI <= 24.99 kg/m2), pictures 6 and 7 representing overweight (25.00 kg/m2 <= BMI <= 29.99 kg/ m2) and pictures 8 and 9 representing obesity (BMI >= 30.00 kg/m2) [25]. The comparison between the categories of BMI and body size perception was conducted and the differences were considered as a proxy of body image disturbance. We classified individuals as overestimating their body size when they classified themselves into a higher body size category than their actual BMI. We classified individuals as underestimating their body size when they classified themselves into a lower body size category than their actual BMI. Figure 1 Options presented to select their silhouettes. F1/M1 – F3/M3 = underweight (BMI <= 19.99 kg/m2); F4/M4 – F5/M5 = normal weight (20.00 kg/m2 <= BMI <= 24.99 kg/m2); F6/M6 – F7/M7 = overweight (25.00 kg/m2 <= BMI <= 29.99 kg/m2); F8/M8 – F9/M9 = obesity (BMI >= 30.00 kg/m2). Covariate assessment The baseline questionnaire (Q_0) included 554 questions about a wide array of characteristics: socio-demographic (e.g. sex, age and marital status), anthropometric (e.g. weight and height, weight gain in the past five years), lifestyle and health-related habits (e.g. smoking status, physical activity), diet (e.g. energy and alcohol intakes), employment status (e.g. employed/unemployed and working hours/week), obstetric history for women (e.g. pregnancy) and medical history (e.g. prevalence of chronic diseases and medication use). Total energy and alcohol intakes were ascertained through a semi- quantitative food frequency questionnaire (136 food items) previously validated in Spain [26]. Physical activity was collected through a validated questionnaire that included information about 17 activities such as walking, running, cycling, swimming, judo, soccer, skiing or sailing. To quantify the volume of activity during leisure time, an activity metabolic equivalent (MET) index was computed by assigning a multiple of resting metabolic rate (MET score) to each activity [27], and the time spent in each of the activities was multiplied by the MET score specific to each activity, and then summed the overall activities obtaining a value of overall weekly MET-hours. Leisure time physical activity estimated with the questionnaire was previously validated by our group using a tri-axial accelerometer as the gold standard. Physical activity during leisure time (estimated as MET-h/week) derived from the questionnaire moderately correlated with Kcal/day assessed through the accelerometer (Spear- man's rho = 0.507, 95% CI: 0.232 – 0.707, p < 0.001) [28]. Participants were classified as having cardiovascular disease at baseline or at follow-up if they reported at least one of the following conditions: myocardial infarction, stroke, atrial fibrillation, paroxysmal tachy- cardia, coronary artery bypass grafting or other revascu- larization procedures, heart failure, aortic aneurism, pulmonary embolism, or peripheral venous thrombosis. Information regarding cancer both at baseline and during follow-up was also collected. Outcome assessment Any participant, initially free of depression and of antidepressant treatment, who positively responded to the following question in Q_2, Q_4 or Q_6: "Have you ever been diagnosed of depression by a health profes- sional?," was classified as an incident case of depression. A self-report of a physician-made diagnosis of depression has been validated in our cohort using the Structured Clinical Interview for DSM-IV (SCID-I) as gold standard. Sixty two subjects with a self-reported physician-made Page 3 of 9 (page number not for citation purposes)