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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
WORDS: Exercise intensity, substrate utilization, obesity, lactate
threshold.
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