Body mass index and mental disorders: A Danish cohort study.

Aims: Low and high body weight present significant global health challenges and may be linked to specific mental disorders. Thus, we aimed to examine the associations between body mass index (BMI) and specific mental disorders.

Materials and methods: In this Danish, population-based cohort study using questionnaire and registry data, we included participants (N = 290 468, 18-79 years of age, 49.8% male) in the Danish National Health Survey (2010/2013). BMI was modelled as a cubic spline and a categorical variable (underweight <18.5 kg/m², normal weight 18.5 to <25 kg/m² [reference], overweight 25 to <30 kg/m², and obesity ≥30 kg/m²). Participants were followed up until December 2018 and the median follow-up time was 5 years. Incidence rates per 1000 person-years and adjusted hazard ratios (aHRs) were estimated for specific mental disorders using cause-specific Cox proportional hazard regression.

Results: We found U-shaped associations of BMI with schizophrenia, mood disorders, anxiety, stress-related, and somatoform mental disorders, eating disorders, and personality disorders. For people with low but not high BMI (≥25 kg/m²), we observed associations with organic and substance use disorders. All associations were more prominent among women than men, and most associations attenuated with advancing age.

Conclusions: Both underweight and obesity were associated with elevated risk of mental disorders, although the patterns of specific mental disorders differed. Our findings underscore the importance of monitoring mental health in people with underweight and obesity.