This study compared changes in substrate metabolism with high intensity interval training (HIIT) in women of different ethnicities. Twelve Caucasian (C) and ten Hispanic women (H) (age = 24 ± 5 yr) who were inactive completed nine sessions of HIIT at 85 percent peak power output (%PPO). Pre-training, changes in fat oxidation (FOx) and carbohydrate oxidation (CHOOx) during progressive cycling were measured on two days to compute the minimum difference (MD). This test was repeated after the last

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... raining session. Between baseline tests, estimates of FOx and CHOOx were not different (p > 0.05) and were highly related (intraclass correlation coefficient equal to 0.72 to 0.88), although the coefficient of variation of maximal fat oxidation (MFO) was equal to 30%. Training significantly increased MFO (p = 0.03) in C (0.19 ± 0.06 g/min to 0.21 ± 0.06 g/min, d = 0.66) and H (0.16 ± 0.03 g/min to 0.19 ± 0.03 g/min, d = 1.3) that was similar (p = 0.92) between groups. There was a significant interaction for FOx (p = 0.003) as it was only increased in H versus C, although both groups exhibited reduced CHO oxidation (p = 0.002) with training. Use of MD revealed that only 3 of 22 women show meaningful increases in MFO (>0.08 g/min). The preliminary data reveals that a small dose of low-volume HIIT does not alter fat and CHO oxidation and there is little effect of ethnicity on the response to training.