Is informal child care associated with childhood obesity? Evidence from Hong Kong's "children of 1997" birth cohort

Background: In Western populations, informal child care is associated with childhood obesity. However, informal child care and obesity share social patterning making evidence from other settings valuable. Methods: We used multivariable linear and logistic regression models to estimate the association of child care at 6 months and at 3, 5 and 11 years with body mass index (BMI) z-score and overweight (including obesity) at 11 years in a Hong Kong Chinese birth cohort. We assessed whether associations varied with sex or socio-economic position (SEP). We used multiple imputation for missing exposures and confounders. Results: Of the original 8327 cohort members, 7933 are alive, participating and living in Hong Kong. At ~11 years, 6796 had their BMI clinically assessed. Higher SEP was associated with informal care. After imputation, informal care at each of 3, 5 or 11 years was separately associated with higher BMI z-score [3 years 0.10, 95% confidence interval (CI) 0.03-0.18, 5 years 0.12, 95% CI 0.04-0.21, 11 years 0.17, 95% CI 0.04-0.31] and with the presence of overweight [odds ratio (OR) 3 years 1.19, 95% CI 1.03-1.37, 5 years OR 1.20, 95% CI 1.03-1.40, 11 years OR 1.21, 95% CI 1.02-1.45], adjusted for sex, SEP and birth weight z-score. Current informal care had the strongest association. However, informal child care at 5 years also contributed. There was no evidence of differences by sex or SEP. Conclusions: In a developed, non-Western setting, informal child care was associated with childhood obesity. Modifiable attributes of informal child care warrant investigation for obesity prevention.