The Role of Fitness Status and Sex on Metabolic Flexibility during a Bout of High-Intensity Interval Exercise and a High-Fat Meal Challenge

We explored whether High-Intensity Interval Exercise (HIIE) could provide a novel method to test metabolic flexibility during exercise. We also aimed to assess substrate oxidation during HIIE and in response to a high-fat meal to determine if these assessments of metabolic flexibility are influenced by fitness and sex. Fat and carbohydrate oxidation were assessed during HIIE ( n = 21) of 4x4:3-min work: recovery (0W) intervals on a cycle ergometer at a power output of halfway between ventilatory threshold and V̇ O 2peak . A high-fat meal challenge (~ 81%fat) ( n = 19) included fasted and postprandial resting metabolic rate. Participants were categorized by sex and fitness status (V̇ O 2peak : High > 40 ml•kg -1 •min -1 vs. low < 40 ml•kg -1 •min - 1 ). During HIIE, fat oxidation (g•kg -1 ) was greater in high fitness (AUC: high: 7.10 (0.04) vs. low: 7.06 (0.01), P = 0.019) and carbohydrate oxidation (g•kg -1 ) was higher in males (AUC: males: 8.27 (0.14) vs. females: 7.87 (0.15), P ≤ 0.001). There were no fitness status or sex group differences during the high-fat meal challenge for fat or carbohydrate oxidation. However, total grams of fat oxidized (g) during HIIE and the high-fat meal were related ( r = 0.631, P = 0.005). Low fit females had poorer metabolic flexibility during HIIE, lower fat oxidation during HIIE (AUC: high fit females 7.10 (0.05) vs. low fit females 7.04 (0.01), P = 0.050) and lower fat oxidation in response to the high-fat meal (AUC: high fit females: 0.59 (0.14) vs. low fit females: 0.33 (0.12), P = 0.030). HIIE may detect metabolic differences associated with training not captured by high-fat meal challenge. Low fitness status negatively impacts metabolic flexibility in response to HIIE, and this effect appears to be more prominent in females.