High Body Mass Index Masks Body Composition Differences in Physically Active Versus Sedentary Participants

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.

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.

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).

Using BMI to classify obesity masks body composition differences in high BMI individuals discrepant for insulin resistance and physical activity status.