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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12889-015-2569-5) contains supplementary material, which is available to authorized users.