The first aim of this study was to determine the exercise intensity that elicited the highest rate of fat oxidation in sedentary, obese subjects (OB; n=10 men, n=10 women) compared with endurance athletes (AT; n=10 men, n=10 women). The second aim was to investigate the relationship between VO2 at the intensity eliciting the highest rate of fat oxidation and the corresponding VO2 at the lactate threshold. Peak oxygen consumption (VO2peak) was determined in 20 AT and 20 OB using an incremental exercise protocol on a cycle ergometer. Based on their VO2peak values, subjects completed a protocol requiring them to exercise for 20 min at three different workloads (55, 65 and 75% VO2peak), randomly assigned on two separate occasions. The oxidation rates of fat and carbohydrate were measured by indirect calorimetry. The highest rates of fat oxidation were at 75 % VO2peak (AT), and at 65 % VO2peak (OB). The rate of fat oxidation was significantly higher in AT (18.2 ± 6.1) compared with OB women (10.6 ± 4.5 kJ min(-1)·kg(-1)) (p < 0.01). There was no significant difference in the rate of fat oxidation for the men (AT 19.7 ± 8.1 vs. OB 17.6 ± 8.2 kJ min(-1)·kg(-1)). AT reached LT at a significantly (p < 0.01) higher exercise intensity expressed in VO2peak than obese subjects (AT women 76.4 ± 0.1, men 77.3 ± 0.1 vs. OB women, 49.7 ± 0.1, men 49.5 ± 0.1% VO2peak). A significant correlation was found between VO2 at LT and VO2 (L·min(-1)) eliciting the maximal rate of fat oxidation in athletes (women; r = 0.67; p = 0.03; men: r = 0.75; p = 0.01) but not in the obese. In summary, we observed higher rates of fat oxidation at higher relative work rates in AT compared with OB. A significant correlation was found between LT and the exercise intensity eliciting a high rate of fat oxidation in AT (r=0.89; p < 0.01) but not in OB. Cardiorespiratory fitness, defined as VO2peak, seems to be important in defining the relationship between a high rate of fat oxidation and LT. Key PointsWithin the tested intensities of 55, 65 and 75% VO2peak athletes reached higher rates of fat oxidation at higher relative work rates compared with obese subjects.We found in obese women and men the intensity of the highest rate of fat oxidation at 65% VO2peak.Between the lactate threshold and the intensity eliciting a high rate of fat oxidation a significant correlation was found in athletes but not in obese subjects.
Keywords: Exercise intensity; lactate threshold; obesity; substrate utilization.