Disadvantaged children at greater relative risk of thinness (as well as obesity): a secondary data analysis of the England National Child Measurement Programme and the UK Millennium Cohort Study

INTRODUCTION: Young children living in more disadvantaged socio-economic circumstances (SECs) are at an increased risk of overweight and obesity. However, there is scant research examining the prevalence and social distribution of thinness in early childhood, despite potential negative consequences for health and development across the life-course. METHODS: We examined the social gradient in thinness (and overweight and obesity for comparison) for 2,620,422 four-to-five year olds attending state maintained primary schools from 2007/8 to 2011/12, in the England National Child Measurement Programme (NCMP), and 16,715 children from the UK Millennium Cohort Study (MCS), born in 2000–2002, and measured at ages of three, five and seven. Children were classified as being thin, healthy weight (and, for completeness, overweight or obese) using international age and sex adjusted cut-offs for body mass index (BMI). Prevalences (and 95 % confidence intervals (CIs)) were estimated, overall, and according to SECs: area deprivation (NCMP, MCS); household income, and maternal social class and education (MCS only). Relative Risk Ratios (RRRs) and CIs for thinness, overweight and obesity were estimated in multinomial models by SECs (baseline healthy weight). In the MCS, standard errors were estimated using clustered sandwich estimators to account for repeated measures, and, for thinness, RRRs by SECs were also estimated adjusting for a range of early life characteristics. RESULTS: In 2007/8 to 2011/12, 5.20 % of four-to-five year old girls (n = 66,584) and 5.88 % of boys (78,934) in the NCMP were thin. In the MCS, the prevalence of thinness was 4.59 % (693) at three, 4.21 % (702) at five, and 5.84 % (804) at seven years. In both studies, and for all measures of SECs, children from the most disadvantaged groups were more likely to be thin than those from the most advantaged groups. For example, MCS children whose mothers had no educational qualifications were fifty percent more likely to be thin (RRR 1.5 (CI: 1.24, 1.8)) than those whose mothers had a degree. These patterns were attenuated but remained after adjusting for early life characteristics. CONCLUSIONS: Children from more disadvantaged backgrounds are at elevated relative risk of thinness as well as obesity. Researchers and policymakers should consider environmental influences on thinness in addition to overweight and obesity. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12939-015-0187-6) contains supplementary material, which is available to authorized users.