Background: Percent body fat (%BF) is a common measurement reported on dual-energy X-ray absorptiometry (DXA) to assess for obesity. %BF is calculated as fat mass divided by total body mass, and thereby increases with either higher fat mass or reduced muscle mass. In contrast, fat mass index (FMI; kg/m²) quantifies fat mass independent of muscle mass. It is unknown whether patients classified as obese by %BF have low muscle mass, and whether individuals with sarcopenia are labeled as obese by %BF versus FMI.

Methods: We utilized whole-body DXA data from 1999-2006 NHANES in adults ≥20 years of age (N=14,850) to estimate the prevalence obesity by %BF, FMI, BMI, and waist circumference. We compared body composition across definitions and examined obesity prevalence among those with sarcopenia.

Results: More patients met criteria for obesity when defined by %BF (76.8%) compared to BMI (33.9%), FMI (35.8%), and WC (51%). Participants classified as obese by %BF had lower lean (muscle) mass, FMI, and BMI compared to participants obese by FMI (all p<0.001). Among patients with sarcopenia, 62.4% were classified as obese by %BF compared with 0.5% if BMI was used to define obesity.

Conclusions: Patients who are classified as obese by %BF had the lowest muscle mass and fat mass compared to participants who met other definitions of obesity. The majority of patients with sarcopenia are labeled as obese by %BF. We believe these findings support the use of FMI, a measurement not confounded by muscle mass, for research and clinical assessment.