Internalising problems and self-reported BMI/physical health: Correlated genetic and environmental influences versus probable causal mechanisms

Internalising problems (depressive and anxiety symptoms) are associated with poor physical health indices. This may reflect causal mechanisms or shared genetic and environmental factors but this has not been previously tested. We tested whether indirect relationships between internalising problems and physical health indices though health behaviours and poor sleep quality were better explained by genetic and environmental correlations. The sample comprised participants in the UK Twins Early Development Study cohort at ≈22 years (9697 and 8718 participants of whom 38.2 % were male, 55.6 % from low socioeconomic backgrounds and 95.5 % were white). Participants were assessed for internalising symptoms, health behaviours, sleep quality, BMI and self-rated health. We compared three twin genetic models to determine whether genetic and environmental correlations versus mediation were a better explanation for phenotypic relationships; and for the best genetic model, we tested differences by sex, socioeconomic status and high versus normal BMI. Although, health behaviours and sleep quality appeared to mediate the phenotypic associations between internalising problems and physical health, genetic and environmental correlations emerged a better explanation for observed associations; and these correlations were stronger in those with high BMI. We concluded that poor health behaviours and sleep quality are relevant to understanding the aetiological links between internalising problems and elevated BMI, especially among those who are overweight or obese. Causal mechanisms alone appear insufficient to explain the links between internalising problems and physical health outcomes. Future research should incorporate genetic information in investigating these relationships.