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1 Human Performance Laboratory, School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom
* To whom correspondence should be addressed. E-mail: a.e.jeukendrup{at}bham.ac.uk.
The aim of the present study was to describe fat oxidation rates over a range of exercise intensities in a large group of healthy men and women. It was hypothesised that exercise intensity is of primary importance to the regulation of fat oxidation and gender, body composition, physical activity level and training status are secondary and can only explain part of the observed inter individual variation. For this purpose, 300 healthy men and women (men n = 157, women n = 143) performed an incremental exercise test to exhaustion on a treadmill (Adapted from a previous protocol (2)). Substrate oxidation was determined using indirect calorimetry. For each individual maximal fat oxidation (MFO) and the intensity at which MFO occurred (FATmax) were determined. On average MFO was 7.8 ± 0.13mg.kg FFM-1.min-1 and occurred at 48.3 ± 0.9%VO2max equivalent to 61.5 ± 0.6%HRmax. MFO (7.4 ± 0.2 vs 8.3 ± 0.2 mg.kg.FFM-1.min-1 p < 0.01), and FATmax (45 ± 1 vs 52 ± 1 %VO2max p < 0.01) were significantly lower in men compared to women. When corrected for FFM, MFO was predicted by physical activity (SRPAL), VO2max and gender (R2 = 0.12) but not with fat mass. Men compared to women had lower rates of fat oxidation and an earlier shift to using CHO as the dominant fuel. Physical activity, VO2max and gender explained only 12% of the inter-individual variation in MFO during exercise whilst body fatness was not a predictor. The inter-individual variation in fat oxidation remains largely unexplained.
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