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The primary aim of the current study was to determine the effect of two doses of chronic high-intensity interval training (HIT) on changes in maximal fat oxidation (MFO) and body composition.Sedentary women (N = 23, age and V˙O2max = 24.2 ± 6.2 yr and 30.3 ± 5.2 mL·kg−1·min−1, respectively) completed either high (HI) (80%–90% maximal workload) or moderate (MOD) intensity (60%–80% maximal workload) HIT on a cycle ergometer 3 d·wk−1 for 12 wk consisting of 6–10 sixty-second bouts interspersed with active recovery. Seven women of similar age and fitness level served as controls. Every 3 wk, substrate oxidation was assessed during progressive exercise via indirect calorimetry to determine MFO and minimum fat oxidation, and body composition was assessed every 6 wk. Repeated-measures ANOVA was used to examine changes in substrate oxidation in response to training, with training group used as a between-subjects variable.Results revealed improved MFO (P = 0.04, 19%–25%) and minimum fat oxidation (P = 0.001, 22–24 W) in response to HIT, yet the magnitude of improvement was similar (P > 0.05) between training paradigms. No change (P > 0.05) in body weight, percent body fat, or waist–hip circumference was revealed with training.These data suggest that 12 wk of either moderate or more strenuous interval training similarly enhance fat oxidation in sedentary women but do not alter body weight or body composition.