Association between parental socioeconomic status with underweight and obesity in children from two Spanish birth cohorts: a changing relationship.

BACKGROUND: Our objective was twofold: to estimate the prevalence of underweight, overweight, and obesity in two birth cohorts (1999-2000 and 2007-2008) from Castilla-La Mancha, Spain; and to examine the association between parental socioeconomic status (SES) and weight status in these two cohorts. METHODS: Cross-sectional analysis of baseline measurements was utilised in two cluster randomised trials. Using population-based samples of children from Castilla-La Mancha, Spain, 1158 children with a mean age of 9.5 years, born in the years 1999-2000 and 1588 children with a mean age of 5.3 years born in the years 2007-2008 participated. Children were classified according to the body mass index cut-offs proposed by the International Obesity Task Force criteria. An index of SES was calculated using questions regarding parental education and occupation levels. RESULTS: Prevalence of underweight was higher in the 2007-2008 birth cohort (20.5%, 95 % CI: 18.5, 22.5) than in the 1999-2000 birth cohort (8.1%, 95% CI: 6.5, 9.7), and the overweight/obesity prevalence was 20.4% (95% CI: 18.4, 22.5) and 35.5% (95% CI: 32.7, 38.3) respectively. In the lower SES stratum, in the 2007-2008 birth cohort, the prevalence of underweight and overweight/obesity was 36.7% (95% CI: 22.2, 51.2) and 16.3% (95% CI: 4.9, 27.7) respectively, and 22.2% (95% CI: 2.8, 60.0) and 55.5% (95% CI: 21.2, 86.3) in the 1999-2000 cohort. The ratio between underweight:overweight/obesity showed higher values for all SES categories in 2007-2008 cohort, but particularly in the lower SES group (0.4 in the 1999-2000 cohort and 2.2 in the 2007-2008 cohort). CONCLUSION: Underweight prevalence was lower in the cohort of children born in 1999-2000, and the prevalence of overweight and obesity was lower in the cohort of children born in 2007-2008. Furthermore, while in the 1999-2000 children's cohort underweight was more frequent amongst children from high SES families and overweight/obesity was more frequent in children from low SES families, in the 2008-2009 children's cohort the opposite was true.