Severe Obesity Defined by Percentiles of WHO and Cardiometabolic Risk in Youth with Obesity

Background/Objectives: The pediatric definition of severe obesity (OB) depends on the body mass index (BMI) references. We evaluated different BMI-derived metrics of the World Health Organization (WHO) system to define which cut-off is associated with the highest cardiometabolic risk (CMR); Methods: In this multicentric study, data were retrieved for 3727 youths (1937 boys; 2225 children, 1502 adolescents). OB was defined as BMI > 97th percentile (BMI_97th), severe OB was defined as BMI > 99th percentile (BMI_99th), BMI ≥ 120% of the 97th percentile (120% BMI_97th), or BMI Z-score > 3 (WHO tables), or BMI ≥ the International Obesity Task Force (IOTF) value crossing a BMI of 35 kg/m² at the age of 18 (IOTF₃₅). The continuous CMR Z-score (sum of residual standardized for age and sex of waist-to-height ratio, systolic and diastolic blood pressure, triglycerides, and HDL-cholesterol x -1) and the cluster of at least two CMR factors (hypertension, high triglycerides, low HDL-cholesterol, and high waist-to-height ratio) were calculated. Results: Continuous CMR Z-score was significantly higher both in children or adolescents with severe OB defined by 120% BMI_97th compared to BMI_99th (p < 0.0001), while it was lower only in adolescents with severe OB defined by 120% BMI_97th compared to BMI Z-score >3 (p < 0.0001). Compared to 120% BMI_97th, BMI Z-score > 3 and IOTF₃₅ had higher specificity, but lower sensitivity in identifying children and adolescents with clustered CMR factors. Conclusions: The definition of severe OB based on 120% BMI_97th is superior to BMI_99th but it is inferior to BMI Z score > 3 as far as the association between severe OB and CMR factors is concerned. Pediatricians should take into consideration the implication of the use of different BMI metrics in those countries that recommend the WHO system. WHO BMI Z-score > 3 and IOTF₃₅ can be used interchangeably to predict cardiometabolic risk.