title: Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners.
abstract: BACKGROUND
  Although carbohydrate is the predominant fuel source supporting high-intensity exercise workloads, the role of fat oxidation, and the degree to which it may be altered by training status, is less certain.
  
  
  METHODS
  We compared substrate oxidation rates, using indirect calorimetry, during a high-intensity interval training (HIT) session in well-trained (WT) and recreationally trained (RT) runners. Following preliminary testing, 9 WT (VO2max71±5 mL/min/kg) and 9 RT (VO2max55±5 mL/min/kg) male runners performed a self-paced HIT sequence consisting of six, 4 min work bouts separated by 2 min recovery periods on a motorised treadmill set at a 5% gradient.
  
  
  RESULTS
  WT and RT runners performed the HIT session with the same perceived effort (rating of perceived exertion (RPE) =18.3±0.7 vs 18.2±1.1, respectively), blood lactate (6.4±2.1 vs 6.2±2.5 mmol/L) and estimated carbohydrate oxidation rates (4.2±0.29 vs 4.4±0.45 g/min; effect size (ES) 90% confidence limits (CL)=-0.19±0.85). Fat oxidation (0.64±0.13 vs 0.22±0.16 g/min for WT and RT, respectively) accounted for 33±6% of the total energy expenditure in WT vs 16±6% in RT most likely very large difference in fat oxidation (ES 90% CL=1.74±0.83) runners. Higher rates of fat oxidation had a very large correlation with VO2max(r=0.86; 90% CI (0.7 to 0.94).
  
  
  CONCLUSIONS
  Despite similar RPE, blood lactate and carbohydrate oxidation rates, the better performance by the WT group was explained by their nearly threefold higher rates of fat oxidation at high intensity.
authors.affiliation: Faculty of Health and Sport Sciences , University of Agder , Kristiansand , Norway.
authors.forename: Ken J
authors.initials: KJ
authors.lastname: Hetlelid
authors.affiliation: High Performance Sport New Zealand, North Harbour, Auckland, New Zealand; Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Plews
authors.affiliation: Faculty of Health and Sport Sciences , University of Agder , Kristiansand , Norway.
authors.forename: Eva
authors.initials: E
authors.lastname: Herold
authors.affiliation: High Performance Sport New Zealand, North Harbour, Auckland, New Zealand; Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Laursen
authors.affiliation: Faculty of Health and Sport Sciences , University of Agder , Kristiansand , Norway.
authors.forename: Stephen
authors.initials: S
authors.lastname: Seiler
keywords: Carbohydrates; Fat; Metabolism; Physiology; Training