Maximal fat oxidation rates in endurance trained and untrained women

The aim of the present study was to examine the differences in fat oxidation between endurance trained (ET) and untrained (UT) women. Eight ET and nine UT women performed a progressive cycle ergometer test until exhaustion. The rate of fat oxidation was similar at low work rates (≤90 W) but was 80–200% higher in ET subjects at 120–180 W. When related to relative exercise intensity, the fat oxidation was similar in the low-intensity domain (≤40% VO₂max), but higher in the ET subjects both at moderate intensities (45–60% VO₂max; +22% vs. UT) and at high intensities (65–80% VO₂max; +35% vs. UT). There was no difference in the maximal fat oxidation rates between the trained and untrained women. The relative exercise intensity that elicited the highest rate of fat oxidation (Fat_max) was 56 ± 3% and 53 ± 2% VO₂max in ET and UT women, respectively (NS). In biopsies from m. vastus lateralis, the activity of the enzymes citrate synthase, β-hydroxy acyl CoA dehydrogenase (HAD), and hormone sensitive lipase was higher in the ET subjects. The HAD activity correlated significantly with fat oxidation at moderate and high intensities. We conclude that the ET women had a higher fat oxidation at moderate- and high-exercise intensities both at same relative and at absolute intensity compared with the UT women. The HAD activity and fat oxidation rates were highly correlated indicating that training-induced adaptation in muscle fat oxidative capacity is an important factor for enhanced fat oxidation. Interestingly, maximal fat oxidation occurred at the same exercise intensity.