Diagnosing Obesity in Korean Children Using BMI and Body Fat Percentage: Differences in Growth and Body Composition

Introduction: We examined how obesity classifications based on body mass index (BMI) versus body fat percentage (BFP) differentially characterize growth and body composition among Korean children, and to identify discordant phenotypes missed by BMI alone. Methods: Children aged 7 to 12 were classified into five obesity groups and a control group: BMI Obesity (BMI > +2 SDS; n = 15,719), BFP Obesity (BFP > +2 SDS; n = 8,142), BMI-and-BFP Obesity (n = 5,022), BMI-Only Obesity (BMI > +2 SDS with normal BFP; n = 10,157), BFP-Only Obesity (BFP > +2 SDS with normal BMI; n = 3,120), and Controls (BMI and BFP between 2 and +2 SDS; n = 423,745). Height, weight, BMI, BFP, body fat mass (BFM), body fat mass index (BFMI), skeletal muscle mass index (SMMI), and waist-to-hip ratio (WHR) were compared across groups and ages. Results: Children classified as BMI Obesity were taller and had higher SMMI, whereas those with BFP Obesity showed lower height and higher BFMI and BFP. These contrasts became more pronounced with age. At ages 11–12, many boys meeting BMI criteria did not meet BFP criteria, consistent with age-related increases in lean mass. In contrast, BFP-Only Obesity was more common in girls, indicating adiposity not captured by BMI. WHR distinguished children with obesity from those without but did not differentiate BMI- and BFP-defined obesity. Conclusion: BMI-based and BFP-based definitions identify distinct obesity phenotypes in late childhood. Reliance on BMI alone may lead to misclassification, particularly during early pubertal growth.