Lay Summary

We used genetically informed methods to investigate i) the directionality between weight and depressive symptoms, and ii) the phenotypic and aetiological relationships between different indictors of weight and depression. A bidirectional relationship between BMI and depression exists between ages 12 and 16 years, with a stronger directional effect between earlier BMI and later depression. Obesity and depression are highly prevalent among children and adults. Understanding the timing and directionality of these assocations highlight opportunities for early intervention.

Background

Emotional symptoms such as anxiety and depression are associated with different indicators of weight and adiposity. However, it remains unclear whether this association reflects causal relationships. This study aims to implemented genetically informed methods to (i) investigate the directionality and etiology between BMI and depression and (ii) explore the magnitude of genetic and environmental influences on different types of adiposity indicators, anxiety, depression, and their associations.

Methods

Data from 4833 twin pairs from the Twins Early Development Study (TEDS), and 707 twin pairs from TwinsUK were used. Measures included self-report depressive symptoms and height and weight at ages 12, 16 and 21 years from TEDS, and self-report emotional symptoms and measured height, weight and adiposity indicators were used from TwinsUK. A genetically informative cross-lagged model was applied to TEDS data to estimate the phenotypic longitudinal effect and etiology between measures, whilst adjusting for the concurrent genetic and environmental associations between them. A multivariate ACE model was used to estimate the genetic and environmental influences on the covariance between anxiety, depression, and weight and adiposity indicators in TwinsUK.

Results

Results from TEDS indicate a bidirectional phenotypic association between BMI and depression between ages 12 to 16, but not between ages 16 to 21 years. Results from TwinsUK show a small but significant genetic correlation between adiposity indicators and depression. These findings will be discussed considering the growing literature on the weight, adiposity, and mental health.

Conclusion

A bidirectional relationship between BMI and depression exists between ages 12 and 16 years, with a stronger directional effect between earlier BMI and later depression. The size of this association was too modest to decompose, resulting in non-significant genetic and environmental correlations. These findings will be discussed considering the growing literature on the weight, adiposity, and mental health.