Substrate Utilization During High‐Intensity Interval Training as a Novel Index of Metabolic Flexibility
Authors: Olenick Alyssa , Green Edward , Person Regis , Jenkins Nathan 
Published: April 2020

Metabolic flexibility is the ability to appropriately store or utilize substrates and is traditionally assessed during the transition between fasted and feeding. However, exercise is also a robust test of one’s ability to shift between substrates to support metabolic demand. We explored whether an individual’s capacity for substrate shifting during high intensity interval exercise (HIIT) could provide a novel means of assessing metabolic flexibility. The study aim was to determine the relationship between fat oxidation during HIIT and following a single high fat meal (HFM), and the impact of aerobic fitness status and mitochondrial capacity on both HIIT‐ and HFM‐based assessments of metabolic flexibility. Recreationally trained men (n=11) and women (n=7) underwent three sessions. Women were tested during days 1–7 of their menstrual cycle. HIIT sessions were comprised of 4 × 4 min high intensity intervals performed at the power output corresponding to 50% of the difference between the ventilatory threshold and VO2 max. Intervals were separated by 3 min unloaded (0W) active recovery. Respiratory gasses were assessed throughout and for 20 minutes following exercise. Blood glucose, lactate and RPE were collected following each interval and during recovery. HFM sessions included a fasted resting metabolic assessment followed by ingestion of a high fat meal. Blood glucose, resting energy expenditure, fat (FOX) and carbohydrate oxidation (COX) measurements were taken at baseline (T0) and every 30 minutes for four hours during the post prandial period (T1–T8). The high‐fat meal consisted of 0.5 g carbohydrate, 1.3 g fat, and 0.2 g protein per kg FFM. Both testing sessions were conducted after an overnight fast. Participants were split into high aerobic fitness status (HFS; n=9; 23±4 yrs; 22.1±4.7% BF; NIRS 1.8±0.8 sec−1; VO2 max 47.5±4.4 ml/kg/min) or low aerobic fitness status (LFS; n= 10; 22±3 yrs; 25.8±5.0% BF; NIRS 1.16±0.1*sec−1; VO2 max 35.5±3.7*ml/kg/min; *p≤0.05). There was a significant correlation between total g fat oxidized (g/kg) during HIIT and HFM (R2=0.54; p=0.03). Significant correlations were observed between VO2 max vs g fat oxidized (g/kg) during HIIT (R2=0.561; p=0.016) and VO2 max vs total percent of substrate oxidized coming from fat during HIIT (R2=0.585; p=0.011), but not for the HFM. Areas under the FOX and COX (g/kg*total min) curves were greater in HFS than LFS during HIIT (p≤0.05). Differences between groups in Total FOX (g/kg) for HIIT approached significance (p=0.06) with total percent of substrate oxidized during HIIT coming from fat being significantly greater in HFS than LFS (HFS: 11.9±0.04% vs LFS: 6.1±0.02%; p≤0.01). FOX rate (g/kg/min) was significantly increased from baseline to T1 and T6‐T8 across all time points (p<0.05), with no differences between groups at any time point during the HFM. These results indicate a significant relationship between postprandial and exercise metabolic flexibility as indicated by fat oxidation. Despite the observed relationship between HIIT‐ and HFM‐based measures of metabolic flexibility, the HIIT protocol used in the current study may uncover metabolic differences induced by training not captured by the traditional HFM approach.Support or Funding InformationA. Olenick is funded by the Mary Ella Lunday Soule Scholarship.