Abstract
PURPOSE: Global trends generally show declining thinness alongside rising obesity; however, some countries face a dual burden of malnutrition, with both underweight and overweight/obesity coexisting. Iran represents one such setting, yet evidence on the prevalence of underweight in school-aged children remains scarce, with inconclusive findings regarding socioeconomic inequalities. METHODS: Data were obtained from four waves of the national CASPIAN surveys (2003-2016), including 76,949 children and adolescents aged 6-18 years. Underweight was defined as a body mass index (BMI) below the 5th percentile for age and sex. Socioeconomic status (SES) was derived through principal component analysis of parental education, occupation, and household assets. Determinants and SES-related inequalities were examined using multivariable logistic regression and Oaxaca decomposition analyses. RESULTS: The prevalence of underweight rose from 9.3% (95% CI: 9.1% - 9.7%) in 2003-2004 to 16.0% (95%CI: 15.5% - 16.7%) in 2015-2016, with projections estimating 24.2% (95% CI: 22.3% - 26.3%) by 2030. Initially, underweight was more common in high-SES groups, but later waves revealed a reversal, with underweight becoming more common among low-SES groups. Rural residence, larger family size, and family history of underweight were significant risk factors, while older age and female sex were protective. Oaxaca analysis indicated that SES disparities were partly explained by demographic characteristics, including urban residence and family size. CONCLUSION: The rising prevalence of underweight among Iranian youth, particularly in low-SES and rural populations, highlights a distinct manifestation of the double burden of malnutrition. Urgent, multifaceted interventions are needed to address both undernutrition and overnutrition, with attention to sex-specific and context-sensitive strategies to reduce these inequalities. CLINICAL TRIAL NUMBER: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s40200-025-01742-1.