Comorbidity, Codevelopment, and Temporal Associations Between Body Mass Index and Internalizing Symptoms From Early Childhood to Adolescence.

Importance: Obesity and internalizing mental illness begin in childhood, have common risk factors, and are leading causes of disease burden.

Objectives: To examine the comorbidity, codevelopment, and temporal precedence in body mass index (BMI) and internalizing symptoms from early childhood to midadolescence and to investigate the sex differences and socioeconomic confounding in their association.

Design, Setting, and Participants: This longitudinal study used data on individuals from the Millennium Cohort Study, a nationally representative, prospective birth cohort study in the United Kingdom of more than 19 000 individuals born from September 1, 2000, to January 11, 2002, who were assessed to date at the ages of 9 months and 3, 5, 7, 11, and 14 years. Data from 17 215 participants (88.21% of full cohort sample) with socioeconomic information in early childhood and BMI or internalizing symptoms during at least 1 assessment from 3 to 14 years of age were included in the present study. Data analysis was performed from May to November 2018.

Main Outcomes and Measures: Outcomes were BMI estimated from objectively measured height and weight and internalizing symptoms assessed using parental reports. Three measures of socioeconomic position were controlled for: parent educational level, occupational status, and equivalized family income.

Results: Among the 17 215 participants (8394 [48.8%] female), obesity and internalizing problems were not more likely to co-occur in early childhood (odds ratio [OR] at 3 years, 1.02; 95% CI, 0.69-1.50), whereas at 11 and 14 years of age, they were more likely to co-occur (OR at 11 years, 1.68; 95% CI, 1.38-2.05; OR at 14 years, 1.49; 95% CI, 1.22-1.83). Piecewise latent growth models revealed no codevelopment of BMI and internalizing symptoms from 3 to 7 years of age (r = 0.01), whereas their slopes were associated between 7 and 14 years of age (r = 0.23). Initial level and rate of change in each domain in early childhood was not associated with rate of change in the other domain at older ages. Cross-lagged models indicated no cross-domain pathways before 7 years of age, with some weak cross-domain pathways emerging between 7 and 14 years of age. Socioeconomic position attenuated some of these associations, leaving a BMI-to-internalizing symptoms pathway in later childhood and an internalizing symptoms-to-BMI pathway in early adolescence.

Conclusions and Relevance: The findings suggest that BMI and internalizing symptoms become more associated and reciprocal as children get older and that some of their temporal associations can be attributed to socioeconomic factors. The emergence of cross-domain temporal pathways in middle childhood suggests that social, physiologic, and psychological processes begin to play an increasingly important role in these health outcomes. Prevention and early intervention efforts may benefit from targeting both health outcomes in childhood.