Objective: To examine the association between body size and depression in a longitudinal setting and to explore the connection between obesity and depression in young adults at the age of 31 years. Design: This study forms part of the longitudinal Northern Finland 1966 Birth Cohort Study (N ¼ 12 058). The follow-up studies were performed at 14 and 31 years. Data were collected by postal inquiry at 14 years and by postal inquiry and clinical examination at 31 years. Subjects: A total of 8451 subjects (4029 men and 4422 women) who gave a written informed consent and information on depression by three depression indicators at 31 years. Measurements: Body size at 14 (body mass index (BMI) and 31 (BMI and waist-to-hip ratio (WHR)) years and depression at 31 years by three different ways: depressive symptoms by the HSCL-25-depression questionnaire (HSCL-25), the use of antidepressants and self-reported physician-diagnosed depression. Results: Obesity at 14 years associated with depressive symptoms at 31 years; among male subjects using the cutoff point 2.01 in the HSCL-25 (adjusted odds ratio (OR) 1.97, 95% CI 1.06-3.68), among female subjects using the cutoff point 1.75 (adjusted OR 1.64, 95% CI 1.16-2.32). Female subjects who were obese both at baseline and follow-up had depressive symptoms relatively commonly (adjusted OR 1.40, 95% CI 1.06-1.85 at cutoff point 1.75); a similar association was not found among male subjects. The proportion of those who used antidepressants was 2.17-fold higher among female subjects who had gained weight compared to female subjects who had stayed normal-weighted (adjusted OR 2.17, 95% CI 1.28-3.68). In the crosssectional analyses male subjects with abdominal obesity (WHR X85th percentile) had a 1.76-fold risk of depressive symptoms using the cutoff 2.01 in the HSCL-25 (adjusted OR 1.76, 95% CI 1.08-2.88). Abdominally obese male subjects had a 2.07-fold risk for physician-diagnosed depression (adjusted OR 2.07, 95% CI 1.23-3.47) and the proportion of those who used antidepressants was 2.63-fold higher among obese male subjects than among male subjects without abdominal obesity (adjusted OR 2.63, 95% CI 1.33-5.21). Abdominal obesity did not associate with depression in female subjects. Conclusion: Obesity in adolescence may be associated with later depression in young adulthood, abdominal obesity among male subjects may be closely related to concomitant depression, and being overweight/obese both in adolescence and adulthood may be a risk for depression among female subjects.

Mental disorders and their comorbidities are distributed unevenly between sexes and age groups, are particularly associated with marital and employment status, and vary by region. There appears to be no single population subgroup at high risk for all mental disorders, but rather several different subgroups at risk for particular disorders or comorbidity patterns.

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