Hydration-related indicators and adiposity measures in children and adolescents with abdominal obesity: a clinical cross-sectional study

Background The relationship between hydration-related markers and abdominal adiposity severity in children with established abdominal obesity remains poorly understood. This study examined the associations of estimated plasma osmolality (ePosm) and urine specific gravity (USG) with waist-to-height ratio (WHtR) and BMI z-score in children and adolescents with abdominal obesity. Methods This single-center cross-sectional study included 207 children and adolescents aged 6–17 years with abdominal obesity. WHtR and BMI z-score were used to characterize central and general adiposity, respectively. ePosm was estimated from serum sodium, potassium, glucose, and urea, while USG was obtained from routine urinalysis. Linear regression models were used to evaluate associations of ePosm and USG with WHtR and BMI z-score after adjusting for potential confounders. Results The mean BMI was 29.43 ± 5.20 kg/m², mean BMI z-score was 2.44 ± 0.81, and mean WHtR was 0.62 ± 0.06. ePosm was inversely associated with WHtR in the overall cohort ( β = −0.025 per 10 mOsm/kg, 95% CI: −0.041 to −0.009; P = 0.002), and reached statistical significance among boys aged 6–12 years in subgroup analysis. The association between ePosm and BMI z-score was weaker and less consistent. USG was not significantly associated with either WHtR or BMI z-score. Conclusions In children and adolescents with abdominal obesity, ePosm was inversely associated with WHtR, whereas USG showed no consistent association with adiposity phenotypes. The subgroup findings should be considered exploratory. Larger prospective studies are needed to confirm these findings.