Depressive symptoms and obesity: instrumental variable analysis using mother-offspring pairs in the 1970 British Cohort Study

Background: The extent to which depression and obesity are causally related remains to be determined. We used intergenerational data on mother–offspring pairs in an instrumental variable analysis to examine the longitudinal association between adolescent depressive symptoms and body mass index (BMI) in adulthood. Methods: A total of 4733 mother–offspring pairs were identified from the 1970 British Cohort Study. Mothers completed the Malaise Inventory to assess depressive symptoms on three occasions across their offsprings' childhood/adolescence (aged 5, 10 and 16 years). Height and weight were recorded in mother and offspring (aged 16 years). Measures of height, weight and the Malaise Inventory were repeated in the participant at the age of 42 years. Results: Maternal malaise score was associated with offspring malaise score, thus confirming the validity of the chosen instrumental variable. A higher mother’s malaise score was associated with higher offspring BMI at follow-up (B=0.043; 95% confidence interval (CI): 0.013, 0.072). There was a higher risk of adulthood offspring obesity in mothers with two or three episodes of depression compared with one or none (odds ratio, 1.42; 95% CI: 1.14, 1.76). The maternal malaise–offspring BMI association remained (P=0.003) after adjustment for offspring malaise score, suggesting that maternal mental health influences offspring obesity through mechanisms other than depression. Results from standard and instrumental variable analyses did not support a causal pathway in a direction from BMI to depression. Conclusions: Our data support a causal pathway linking adolescent depressive symptoms to adiposity in adulthood over 26 years follow-up. The reverse direction, that is, adiposity to depression, was not supported.