Fat Utilization During High-Intensity Exercise: When Does It End?

Background: This study examined substrate oxidation at high-intensity exercise and aimed to determine when fat oxidation ends (FAT(min)). We hypothesized the existence of a connection between the anaerobic threshold (AnT) and FAT(min) point. Methods: Breath-by-breath data obtained from indirect calorimetry during a graded treadmill test were used to measure substrate oxidation and maximal oxygen uptake (VO2max) on 47 males (30 athletes (ATL) and 17 non-athletes (NATL)). Pearson correlation coefficient (r) and effect size (R-2) were used to test correlations between VO2 at AnT and at FAT(min). Results: Maximal oxygen uptake (VO2max) was 56.17 +/- 4.95 and 46.04 +/- 3.25 ml kg(-1) min(-1) in ATL and NATL, respectively. In ATL, AnT was observed at 87.57 +/- 1.30 % of VO2max and FAT(min) was observed at 87.60 +/- 1.60 % of VO2max. In NATL, AnT and FAT(min) were at 84.64 +/- 1.10 % of VO2max and 85.25 +/- 1.10 % of VO2max, respectively. Our data show large correlations between VO2 at AnT and VO2 at FAT(min) for ATL (r = 0.99, p < 0.01, 95 % CI 0.99 to 1.00) and NATL (r = 0.97, p < 0.01, 95 % CI 0.91 to 0.98). The effect size of correlations for ATL and NATL were 0.98 and 0.94, respectively. Conclusions: Our results show high correlation between AnT and FAT(min) in both ATL and NATL with equal substrate oxidation rates at AnT.