Whole body lipid oxidation during exercise is impaired with poor insulin sensitivity but not with obesity per se

Equivocal findings regarding the influence of overweight/obesity on exercise lipid-oxidizing capacity (EX-LIPOX) might reflect inadequate control of 1) acute energy balance/macronutrient composition of diet; 2) intensity/duration of exercise; and/or 3) insulin sensitivity (IS) of participant. To assess independent/combined influences of IS and overweight/obesity with other factors controlled, we recruited sedentary adults with normal weight (NW; n = 15) or overweight/obesity (O; n = 15) subdivided into metabolically healthy (MH; n = 8) and unhealthy (MU; n = 7) groups (IS; MH > MU). Participants completed a 9-day, weight-stabilizing, controlled-feeding protocol comprising measurements of resting metabolism, body composition, oral glucose tolerance, and maximal exercise capacity. We measured EX-LIPOX during the initial 45 min of "steady state"during constant-work-rate cycling at 70% and 100% of participant gas-exchange threshold (GET). At 70%, average EX-LIPOX in absolute (0.11 ± 0.02 g min-1) and relative (2.4 ± 0.3 mg kgFFM-1 min-1) terms was lower for NW-MU than MH regardless of body composition (NW-MH, 0.19 ± 0.02 g min-1/3.9 ± 0.3 mg kgFFM-1 min-1; O-MH, 0.19 ± 0.02 g min-1/ 3.7 ± 0.3 mg kgFFM-1 min-1), whereas no difference was present for NW-MU and O-MU (0.15 ± 0.02 g min-1/2.8 ± 0.3 mg kgFFM-1 min-1). Multiple regression confirmed that with IS-controlled, overweight/obesity was not associated with decreased EX-LIPOX, whereas decreased EX-LIPOX was associated with decreased IS independent of overweight/obesity. Overweight/obesity also did not influence EX-LIPOX across MH groups or with cohort divided by body-composition classification alone (P > 0.05). Exercise lipid-oxidizing capacity is impaired with poor IS regardless of body composition, but not with overweight/obesity per se.