High body mass index masks body composition differences in physically active versus sedentary participants

Background: Despite the benefits of aerobic exercise on body composition, runners with high body mass index (BMI) remain understudied. We examined body composition differences between sedentary insulin-resistant [obese insulin-resistant sedentary (OS-IR)], sedentary insulin-sensitive [obese insulin-sensitive sedentary (OS-IS)], and trained [obese trained (OT)] individuals with high BMI (≥25 kg/m²). We hypothesized that after matching for high BMI, OT individuals would have less fat mass (absolute and relative) and greater lean mass than OS-IR or OS-IS individuals. Methods: This is a retrospective analysis of OS-IR, OS-IS, and OT participants selected for similar age, sex, and BMI. Activity was self-reported. OT participants exercised at least 30 min/day (predominantly running) for 3-5 days/week. OS-IS and OS-IR participants actively exercised <0.5 hr/week. Body composition was measured by dual X-ray absorptiometry. Results: Thirty-three participants were recruited [n = 11/group, mean age 31.7 years (standard error): (0.9)], 7 females/group, overall BMI [31.6 kg/m² (0.7)]. Insulin resistance, quantified by the homeostatic model assessment for insulin resistance, was higher in the OS-IR [3.3 (0.2)] than the OS-IS [0.9 (0.2): P < 0.0001] or OT [1.6 (0.2): P < 0.0001] groups. We found the following: (i) Compared to the OS-IR group, the OT group had lower region-specific fat mass as measured by percent fat (trunk) or absolute fat mass (trunk, android region, and abdominal visceral region). (ii) OT and OS-IS groups had similar body composition. (iii) Total fat mass and percent body fat correlated with BMI, (iv) Visceral fat correlated with BMI (r = 0.80, P = 0.003) only in the OS-IR (P = 0.03 for BMI × group interaction). Conclusions: Using BMI to classify obesity masks body composition differences in high BMI individuals discrepant for insulin resistance and physical activity status.