Cardiorespiratory Fitness May Influence Metabolic Inflexibility During Exercise in Obese Persons

Context: We examined whether obese individuals have a reduced maximal fat oxidation (MFO) and the intensity that elicit MFO (Fat(max)) compared with normal weight and overweight persons, taking into account their level of cardiorespiratory fitness. Methods: The study subjects were 138 sedentary adults (87 women) aged 30.1 +/- 13.6 years. Based on their body mass index, subjects were categorized as being of normal weight (n=66), overweight (n=48), or obese (n=24). MFO and Fat(max) were determined for all subjects by indirect calorimetry, using a walking graded exercise test. MFO was expressed in absolute terms (g/min) and relative to whole-body lean mass (mg/kg(leanmass)/min). Cardiorespiratory fitness was assessed via a maximal treadmill test. Results: No differences in absolute MFO and Fat(max) values were seen between the obese, normal weight, and overweight subjects (all P > .0.2), although after adjusting for cardiorespiratory fitness, the obese subjects returned significantly higher values than did their normal weight and overweight counterparts (all P < 0.03). However, when expressed with respect to lean mass, the MFO of the normal weight subjects was significantly greater than that of the overweight and obese subjects, independent of age, sex, or cardiorespiratory fitness. Conclusions: Obese individuals have higher absolute MFO values when cardiorespiratory fitness is taken into account, but when expressed with respect to lean mass, normal weight individuals show a greater capacity to oxidize fat during exercise per unit of metabolically active tissue independent of age, sex, or cardiorespiratory fitness. These findings suggest that obese individuals may suffer from metabolic inflexibility during exercise.