Opposite associations of household income with adolescent body mass index according to migrant status: Hong Kong's "children of 1997" birth cohort

Background/objectives: In economically developed settings, household income is usually inversely associated with child and adolescent adiposity, but this association may not extend to migrants. Hong Kong is a unique developed setting to study how household income and adolescent adiposity vary by migrant status given many Hong Kong-born Chinese children were born to parents who migrated from neighboring provinces of Mainland China. Subjects/methods: We examined differences between the associations of absolute household income vs. relative household income on adolescent body mass index (BMI) z-score or overweight (including obesity) status using a linear or logistic model in a Chinese birth cohort (n = 5613, 68% follow-up). We focused on whether the associations differed by mother's or father's migrant status (birthplace). Results: No association was found between absolute household income and BMI z-score among adolescents with either native or migrant mothers. However, the association of relative household income with BMI z-score varied by mother's migrant status (P-values for interaction <0.0005). In adolescents of native born mothers, greater relative household income deprivation was associated with higher BMI z-score (0.03 z-score per USD 128 difference in Yitzhaki index, 95% confidence interval (CI) 0.01 to 0.05). However, in adolescents of migrant mothers, greater relative household income deprivation was associated with lower BMI z-score (-0.05, 95% CI-0.09 to-0.01). Similar association of relative household income with overweight (including obesity) status was found in adolescents of native born mothers but not in adolescents of migrant mothers. Conclusions: Relative income (mediated by social comparisons with others in society) appears to be relevant to adolescent adiposity, but the association depends on the interplay between individual characteristics (migrant background) and societal context.