Association between body mass index and mental health among Scottish adult population: a cross-sectional study of 37272 participants

Background. The evidence is conflicting as to whether body mass index (BMI) is associated with mental health and, if so, to what extent it varies by sex and age. We studied mental health across the full spectrum of BMI among the general population, and conducted subgroup analyses by sex and age. Method. We undertook a cross-sectional study of a representative sample of the Scottish adult population. The Scottish Health Survey provided data on mental health, measured by the General Health Questionnaire-12 (GHQ), BMI, demographic and life-style information. Good mental health was defined as a GHQ score <4, and poor mental health as a GHQ score >= 4. Logistic regression models were applied. Results. Of the 37272 participants, 5739 (15.4%) had poor mental health. Overall, overweight participants had better mental health than the normal-weight group [adjusted odds ratio (OR) 0.93, 95% confidence interval (CI) 0.87-0.99, p= 0.049], and individuals who were underweight, class II or class III obese had poorer mental health (class III obese group: adjusted OR 1.26, 95% CI 1.05-1.51, p= 0.013). There were significant interactions of BMI with sex (p= 0.013) and with age (p< 0.001). Being overweight was associated with significantly better mental health in middle-aged men only. In contrast, being underweight at all ages or obese at a young age was associated with significantly poorer mental health in women only. Conclusions. The adverse associations between adiposity and mental health are specific to women. Underweight women and young women who are obese have poorer mental health. In contrast, middle-aged overweight men have better mental health.