Visceral Adiposity and Markers of Relative Sarcopenia in Young Adults with Normal Weight Obesity: Gender Differences

Background and Objective: Body mass index (BMI) is a worldwide screening standard but fails to distinguish between fat mass and fat-free mass. This study examines the prevalence and metabolic profile of the normal weight obesity (NWO) phenotype in a large cohort of young adults. Methods: A cross-sectional study of 4793 young adults (18-35 years) was conducted using bioelectrical impedance analysis (BIA). Participants were stratified into four phenotypes: underweight, healthy weight (HW), NWO, and obesity. Anthropometric indices, visceral fat area (VFA), and phase angle (PhA) were analyzed. Results: Within the normal BMI range (n = 2491), 40.6% (n = 1012) of patients were classified as NWO (percentage of body fat (PBF) >30% for women, >20% for men). NWO subjects showed a significantly higher VFA compared to the HW (+33.0 cm² in men and +24.3 cm² in women; p < 0.001) with a very large effect size (Cohen's d > 2.0). Furthermore, a state of relative sarcopenia was identified, characterized by significantly lower skeletal muscle mass percentage (SMM%) and PhA (p < 0.001; d = -0.82), indicating compromised cellular integrity despite a normal BMI. Conclusions: BMI misclassifies 4 out of 10 young adults with excess adiposity. NWO is a high-risk phenotype linked to visceral adiposity and early cellular frailty. Incorporating BIA in routine screenings is essential to identify this invisible risk group.