Season of birth is associated with adult body mass index in patients with bipolar disorder.

Cardiovascular risk factors, such as abdominal obesity and obesity in general, are very prevalent among patients with bipolar disorder (BD). Although long-term use of psychotropic medications is an important determinant of these risk factors, other evidence suggests that early development may interact with the mood disorder diathesis to exponentially increase the risk of obesity. The goal of our study was to test whether season of birth is associated with adult body mass index (BMI) and abdominal obesity in individuals with bipolar disorder. We compared season of birth effects on BMI in 375 adult patients with bipolar disorder and 196 adult patients with unipolar major depression. We found a significant season of birth effect on BMI in patients with bipolar disorder, but not unipolar. In patients with bipolar disorder, season of birth was also associated with waist circumference, with a stronger effect in males. Season of birth affects adult BMI and waist circumference in patients with bipolar disorder, but not in patients with unipolar depression. Our results suggest that early environmental factors, yet to be identified, interact with specific neurobiological features of bipolar disorder to determine stable traits and disease risk factors in adult life.