Familial Confounding of Internalising Symptoms and Obesity in Adolescents and Young Adults; A Co-Twin Analysis

Abstract

Abstract Background Obesity and internalising disorders, including depression and anxiety, often co-occur. There is evidence that familial confounding contributes to the co-occurrence of internalising disorders and obesity in adults. However, its impact on this association among young people is unclear. Our study investigated the extent to which familial factors confound the association between internalising disorders and obesity in adolescents and young adults. Subjects/methods We used a matched co-twin design to investigate the impact of confounding by familial factors on associations between internalising symptoms and obesity in a sample of 4,018 twins aged 16 to 27 years. Results High levels of internalising symptoms increased the odds of obesity for the whole cohort (adjusted odds ratio [AOR]=2.7, 95% confidence interval [CI]: 1.2, 5.4), and in females (AOR=3.3, 95%CI 1.3, 8.9), but not in males (AOR=2.3 95%CI 0.6, 7.9). In the paired analysis, we found internalising disorders were associated with an increased between-pair odds of obesity (AOR 3.9, 95%CI 1.2, 13.1), but the within-pair association attenuated to the null after controlling for familial confounding. Sex-stratified analyses indicated high internalising symptoms were associated with increased between-pair odds of obesity in females (AOR 7.8, 95%CI 1.5, 40.8), but this attenuated to the null after adjusting for familial confounding. We found no evidence of between or within-pair associations for males and weak evidence that sex modified the association between internalising symptoms and obesity (likelihood ratio test p= 0.037). Conclusions: Shared early environmental and/or genetic variants among twins partially confound the association between internalising symptoms and obesity in adolescent and young adult females. Internalising symptoms and obesity were not associated in adolescent and young adult males. Prevention and treatment efforts should address individual and familial determinants of obesity, particularly targeted at female adolescents and young adults with internalising disorders and those with a family history of these disorders.