Body mass index trajectories in childhood and adolescence - Risk for non-affective psychosis.

Background: Underweight in early adulthood increases risk for schizophrenia, but the effect of early childhood underweight on psychosis risk is not well known.

Methods: We studied whether underweight or overweight in childhood and adolescence increases risk for non-affective psychosis or other psychiatric disorders in a population-based cohort study 'Cardiovascular Risk in Young Finns'. Body mass index (BMI) trajectories were recorded in the years 1980, 1983 and 1986 (in 3-18 years of age), before the first hospitalization due to a psychiatric disorder. BMI was categorized as underweight, normal weight or overweight, using the BMI classification for children and adolescents. We formed DSM-IV based diagnostic groups of non-affective psychosis (n = 69, including a schizophrenia subgroup, n = 41) and affective disorders (i.e. mood and anxiety disorders, n = 112) based on the Care Register for Health Care. Groups were compared with subjects with no psychiatric diagnoses (n = 3310). Sex, age, low birthweight and mother's mental disorders were included in the analyses.

Results: Underweight, but not overweight, independently predicted later development of non-affective psychosis. The risk of psychosis was over two-fold (relative risk (RR) [95% CI] 2.31 [1.2-4.4]) and of schizophrenia nearly 2.5-fold (RR 2.44 [1.03-5.8]) after underweight in childhood/adolescence. Underweight or overweight in childhood and adolescence was not associated with mood or anxiety disorders.

Conclusions: These results support the hypothesis of non-affective psychosis as a neurodevelopmental disorder with somatic manifestations throughout childhood and adolescence.