Growing evidence suggests an association between attention-deficit/hyperactivity disorder (ADHD) and obesity, although very little is understood about the nature of this link. The aims of this thesis were to examine the following aspects of the ADHD-obesity association: (1) the directionality of the link from childhood to adolescence, (2) behavioural mediators during childhood and adolescence, and (3) prenatal risk factors common for both disorders.

Participants were from the Northern Finland Birth Cohort (NFBC) 1986 (N=9479). Data were obtained on pregnancy and birth factors, and child/adolescent mental health, obesity, and lifestyle factors.

Regression analyses showed that ADHD symptoms significantly predicted obesity, rather than in the opposite direction, from childhood to adolescence. Mediation analyses examined potential underlying behavioural factors – physical activity and binge-eating, and showed that physical inactivity mediated the longitudinal ADHD symptom-obesity association. Further, there was a bidirectional, longitudinal association between physical inactivity and ADHD symptoms.

ADHD and obesity may share common prenatal risk factors, including prenatal exposure to cortisol. This was studied using a quasi-experimental approach by examining the impact of prenatal exposure to synthetic glucocorticoids (sGC). Results from propensity-score and mixed-effects methods showed that prenatal sGC increased the risk for general psychiatric disturbance and inattention symptoms, but not obesity, in childhood. Placental size may represent another common prenatal contributing factor; placental size was positively associated with behaviour problems, including ADHD symptoms, in child and adolescent boys, but was not associated with obesity.

This thesis addresses important unexplored aspects of the association between ADHD and obesity, and provides insight into risk factors for both disorders. The direction of the association was driven from ADHD symptoms to obesity, and physical inactivity was a behavioural mediator underlying the link. Although there was no evidence that both disorders share common prenatal risk, prenatal sGC and placental size were positively associated with ADHD symptoms.