Trends in the associations between family income, height and body mass index in US children and adolescents: 1971-1980 and 1999-2008

Aim: This paper evaluates trends in the childhood and adolescent associations between family income, height and body mass index (BMI) between the periods 1971-1980 and 1999-2008. Subjects and methods: US-born children (aged 2-11) and adolescents (aged 12-19) in the NHANES I, NHANES II and 1999-2008 NHANES are evaluated for BMI and height using flexibly-estimated structured additive regression models. Results: Three trends are identified: BMI gains have been greater in lower-income relative to higher-income children and adolescents; height has increased more in lower-income relative to higher-income children (but not in adolescents); and BMI has increased more in taller children than shorter children (but not in adolescents). Following from these trends is that contemporary children exhibit a negative height-income interaction effect on BMI such that the inverse association between income and BMI is larger in taller children. Similar results hold when categorical obesity is considered but with modest height-related changes and income-height interaction effect in adolescents. Race-sex differences are explored and descriptive evidence on the potential role of changes in caloric intake is presented. Conclusion: The relationship between the economic environment and growth in US youth has changed in recent decades. Contemporary taller and lower-income children exhibit greater body mass and obesity.