Exercise Priming: Effect Of Morning Exercise On Fat Oxidation During Afternoon Walking

Abstract

Reduced fat oxidation is emerging as a predictor for obesity-related diseases; therefore, interventions that increase fat oxidation may have clinically important health benefits. Previous research suggests that exercise priming may provide a novel solution by augmenting fat oxidation during subsequent exercise. PURPOSE: To investigate the effects of morning Reduced-Exertion High-intensity Interval Training (REHIT) on fat oxidation during afternoon Low-intensity Steady State (LISS) walking. METHODS: Twelve sedentary or recreationally active university students (7 male, 5 female) participated in 2 randomly assigned morning interventions, REHIT (two 20-second sprints on a cycle ergometer separated by 4-minutes of active recovery) or rest, on two separate days. On both occasions, participants returned to the lab in the afternoon (4 hours later) to complete a 45-minute treadmill walk at 5.6 km/h. Participants remained fasted for both morning and afternoon trials. Blood and gas samples were collected pre-, during, and post-LISS exercise. RESULTS: After 45 minutes of LISS walking, plasma non-esterified fatty acids (NEFA) was greater in the REHIT trial compared to control (REHIT, 1.03 ± 0.39 mmol/L; Control, 0.75 ± 0.43 mmol/L; p = 0.03; Cohen’s d = 0.67). However, lipid oxidation was not concomitantly elevated (p = 0.35). Furthermore, plasma glucose, carbohydrate oxidation, respiratory exchange ratio (RER) and energy expenditure remained consistent between the trials across all time points (p = 0.40; p = 0.98; p = 0.92; p = 0.81, respectively). CONCLUSION: Exercise priming may not influence fat oxidation during subsequent bouts of LISS exercise; however, plasma NEFA may be greater suggesting changes in substrate availability. Relatively long rest intervals between the morning and afternoon trials and insufficient exercise workload could explain similarities in substrate oxidation. Isotopic research is required to understand the metabolic fate of the elevated substrate concentrations.