Objectives The substrates utilisation at high intensity exercise has been less studied and intensity at which carbohydrates (CHO) remain single energy source while fat utilisation becomes negligible (FAT_min), is debatable ⁴. Since physical activity in aerobic fat burn zone is prescribed to facilitate improvement of fat oxidation and insulin sensitivity while improving health ¹, activity equal or above FAT_min can’t be considered as beneficial. Therefore, FAT_min could become important exercise parameter in sedentary population. The aim of this study was to determine FAT_min while performing incremental treadmill test till exhaustion and to assess its correlation with anaerobic threshold (AT), which in turn can be used as an individuals performance marker or last exercise beneficial point.

Methodology Thirteen sedentary females (age: 34 ± 3 y, body mass: 62 ± 4 kg, height: 165 ± 4 cm) performed breath-by-breath test to measure maximal oxygen uptake (VO_2max;ml·kg^-1·min^-1) and substrates oxidation (g·min^-1). A Pearson correlation coefficient (r), 95% confidence interval (CI) and effect size (R² ) were used to test connexion between VO₂ at AT and at FAT_min. Alpha intervals were set at p < 0.05. Substrates oxidation was calculated using a stoichiometric equation ⁴. The AT was detected by V-slope method ².

Results VO_2max was 34.89 ± 4.34 ml·kg^-1·min^-1 (95% CI 34.02 to 35.52). FAT_min and AT occurred at 24.53 ± 4.25 ml·kg^-1·min^-1 VO₂ (95% CI 24.29 to 24.97) and 24.04 ± 4.07 ml·kg^-1·min^-1 VO₂ (95% CI 24.01 to 24.80), respectively. A high correlation was found between VO₂ at FAT_min and at AT (r = 0.91, p < 0.001, 95 % CI of 0.87 to 0.93). R² was 0.83. Absolute fat oxidation rate at FAT_min was 0.01 ± 0.01 g·min^-1 (95% CI 0.00 to 0.01).

Conclusions This is the first study to detect FAT_min and demonstrate correlation between VO₂ at FAT_min and at AT in sedentary females. High inter-subjects FAT_min variation demonstrates marked degree of individuality and importance of existing correlation should be taken into consideration when aiming to optimise exercise prescription for sedentary population.

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