Background The burden of childhood thinness in the UK is poorly understood. The aim of this study was to describe the prevalence and year-on-year trends of childhood thinness in a population born between 1970 and 2006 in North East Scotland.

Methods Measurements were routinely collected by school nurses as part of school medical entry. Trends in International Obesity Task Force thinness grades 1, that is, body mass index (BMI) corresponding to adult BMI <18.5 kg/m² but ≥17 kg/m² or grade ≥2, that is, corresponding to adult BMI <17 kg/m² were analysed over time by sex and socioeconomic deprivation quintile.

Results Data were obtained for 194 391 children, 52% boys, mean age 5.6 years (SD 0.8). The prevalence of thinness grade 1 was 6.5% (95% CI 5.9% to 7.2%) and 4.8% (4.2% to 5.5%) for those born in 1970 and 2006, respectively, but between these years was variable with the fluctuations being greater for boys than girls. The prevalence of thinness grade ≥2 fell for those born between 1974 and 1985 from 6.1% (5.5% to 6.8%) to 1.3%, (1.0% to 1.6%) and remained relatively stable thereafter in boys and girls. Thinness grade ≥2 was initially less prevalent in more affluent communities, but for those born in 1990 and afterwards, prevalence was equal across deprivation quintiles. In contrast, there was no interaction between deprivation quintile and year of birth for thinness grade 1.

Conclusions Thinness has become less common in this population. While thinness was initially more prevalent among deprived communities, this association is no longer apparent.