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.