There is evidence that greater body mass index (BMI) protects against depression, schizophrenia and suicide. However, there is a need for prospective studies.

We examined the association of BMI with future hospital admissions for psychoses or depression/anxiety disorders in a large prospective study of 7036 men and 8327 women. Weight and height were measured at baseline (1972–76) when participants were aged 45–64. Follow-up was for a median of 29 years.

Greater BMI and obesity were associated with a reduced risk of hospital admission for psychoses and depression/anxiety in both genders, with the magnitude of these associations being the same for males and females. With adjustment for age, sex, smoking and social class, a 1 standard deviation (s.d.) greater BMI at baseline was associated with a rate ratio of 0·91 [95% confidence interval (CI) 0·82–1·01] for psychoses and 0·87 (95% CI 0·77–0·98) for depression/anxiety. Further adjustment for baseline psychological distress and total cholesterol did not alter these associations.

Our findings add to the growing body of evidence that suggests that greater BMI is associated with a reduced risk of major psychiatric outcomes. Long-term follow-up of participants in randomized controlled trials of interventions that effectively result in weight loss and the use of genetic variants that are functionally related to obesity as instrumental variables could help to elucidate whether these associations are causal.