Differences in body mass index trajectories of adolescent psychiatric inpatients by sex, age, diagnosis and medication: an exploratory longitudinal, mixed effects analysis

Background: Adolescents in secure psychiatric care typically report high obesity rates. However, longitudinal research exploring the rate and extent of change is sparse. This study aimed to analyse sex differences in longitudinal body mass index (BMI) change for adolescents receiving treatment in a secure psychiatric hospital. 

Methods: The sample comprised 670 adolescents in secure psychiatric care. BMI trajectories from admission to 50 months of hospitalisation were produced using sex-stratified multilevel models. Systematic difference in mean BMI trajectories according to age at admission (14, 15, 16, or 17 years), medication (Olanzapine or Sodium Valproate), and primary diagnosis (Psychotic, non-Psychotic or Functional/behavioural disorders) were investigated.

Results: Together, males and females experienced a mean BMI increase of 2.22 m/kg² over the 50-month period. For females, BMI increased from 25.69 m/kg² to 30.31 m/kg², and for males, reduced from 25.01 m/kg² to 23.95 m/kg². From 30 to 50 months, a plateau was observed for females and a reduction in BMI observed for males. Psychotic disorders in males (β 3.87; CI 1.1–6.7) were associated with the greatest rate of BMI change. For medication, Olanzapine in females was associated with the greatest rate of change (β1.78; CI −.89–4.47). 

Conclusions: This is the first longitudinal study exploring longitudinal BMI change for adolescent inpatients. Results highlight that individual differences in adolescent inpatients result in differing levels of risk to weight gain in secure care. Specifically, males with psychotic disorders and females taking Olanzapine present the greatest risk of weight gain. This has implications for the prioritisation of interventions for those most at risk of weight gain.