Has underweight increased among children and adolescents during 2012-2019, and does the prevalence depend on socioeconomic circumstances? A nationwide register-based study from Denmark.

Background: Childhood underweight, indicated by low body mass index (BMI) for age and sex, is associated with morbidity, mortality and poverty in low-income settings, but is often overlooked in high-income countries, due to unequivocal focus on overweight. This study examined the prevalence and socioeconomic characteristics of underweight among children and adolescents in Denmark during 2012-2019.

Methods: We used nationwide Danish register data on height and weight measured objectively in 4 61 041 6-7 year-olds and 3 65 312 14-15 year-olds during 2012-2019, covering 89.4% and 67.2% of all schoolchildren in these age groups. We calculated BMI z-scores and classified underweight using the International Obesity Task Force age- and sex-specific BMI cut-offs and linked with data on parental education and household income as indicators of socioeconomic circumstances.

Results: The prevalence of underweight remained relatively stable at~8.5% among 6-7-year-old girls during 2012-2019. Among 14-15-year-old girls, it was initially higher but decreased slightly (<1%-point) to 8.6% in 2019. Among boys, the prevalence increased from ~7% to just above 8% in both age groups during the period. Across all years, underweight was more common among 14-15 year-olds whose parents had long higher education (girls: 10.4%-12.1%; boys: 8.7%-10.2%) compared with primary school (girls: 5.6%-7.5%; boys: 6.1%-7.8%), with the most pronounced differences among girls. Similar differences across parental education were observed among 6-7 year-olds, although they were smaller. Less distinct but comparable differences in underweight prevalence were found across household income.

Conclusions: Underweight is prevalent but has remained relatively stable during the 2010s among Danish children and adolescents. Unlike the pattern reported in low-income settings, and that seen for overweight, underweight is associated with socioeconomic advantage in this high-income setting.