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 (FATmin). We hypothesized the existence of a connection between the anaerobic threshold (AnT) and FATmin 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 FATmin. 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 FATmin was observed at 87.60 ± 1.60 % of VO2max. In NATL, AnT and FATmin 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 FATmin 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 FATmin in both ATL and NATL with equal substrate oxidation rates at AnT.