Change in Maximal Fat Oxidation in Individuals in Response to Different Regimes of High Intensity Interval Training (HIIT)

Abstract

INTRODUCTION: Increased capacity for fat oxidation (FatOx) is widely reported (Talanian et al. 2007; Burgomaster et al. 2008) in response to high intensity interval training (HIIT). However, most of these data was obtained in studies in which a non-exercising control group (CON) was not used. Based on the widespread variability in FatOx reported (Croci et al. 2014), it is unknown if these previously-reported increases in FatOx are real or a product of day-to-day variability in the measure. OBJECTIVE: To examine changes in FatOx and maximal fat oxidation (MFO) in response to 20 sessions of HIIT varying in structure in active men and women. METHODS: Thirty-nine active men and women (age and VO2max = 22.5 ± 4.4 yr and 40.0 ± 5.6 mL/kg/min) completed progressive HIIT, and 34 men and women matched for body fat and VO2max served as a non-exercising control group (CON). Ten sessions of low volume HIIT (8 – 10 1 min bouts at 90 – 110 % peak power output separated by 75 s recovery) were performed on a cycle ergometer after which subjects were randomly assigned to complete 10 additional HIIT sessions consisting of sprint interval training (SIT), high-volume HIIT, or periodized HIIT. After an overnight fast and 24 h dietary standardization, individuals underwent progressive cycling to exhaustion before, midway, and post-training to assess MFO, FatOx and carbohydrate oxidation (CHOOx). RESULTS: Results showed no effect of training on MFO (p = 0.11) although small increases in FatOx equal to an additional 4.3 g of fat were evident (p = 0.03). Individual responses in MFO occurred as 33 % of participants revealed meaningful increases in MFO from pre- to post-HIIT, and there was a significant inverse association between baseline MFO and the training-induced change, r = -0.46, p = 0.004. There was no time X training interaction for CHOOx (p = 0.31) suggesting a similar response of CHOOx to HIIT versus CON. CONCLUSIONS: Data refute previously-reported increases in MFO in response to HIIT (Astorino et al. 2013), although participants with low initial MFO tended to show meaningful increases in MFO with training. We recommend that scientists use a non-exercising control group to certify that the frequently-reported increase in FatOx attributed to HIIT is indeed a result of training and not due to marked variability in the measure.