Examining The Relation Between Body Composition And Substrate Utilization Following Reduced Exertion High-Intensity Interval Training

Compared to aged-matched women, men are at a greater risk for developing cardiometabolic disease, especially among middle-aged men who often cite lack of time as a barrier to exercise. Reduced exertion high-intensity interval training (REHIT) was introduced as a time-efficient approach to improve blood glucose and potentially attenuate disease onset. Although, it is unknown whether substrate metabolism is influenced by body composition (percent body fat and lean body weight) during and after a single bout of REHIT. PURPOSE: This study aimed to examine the influence of body composition on substrate utilization during and after a single bout of REHIT in recreationally active middle-aged men. METHODS: Seventeen recreationally active middle-aged men (Age: 51 ± 8 years; percent body fat: 17.9 ± 5.2%; lean body weight: 143.6 ± 14.2 lbs; VO2max: 44.2 ± 5.9 mL·kg·min-1; fasting blood glucose: 97 ± 7 mg·dL-1) completed this study. Participants completed a control day (CON; 10 minutes quiet rest) and a REHIT day (10 minutes of unloaded cycling interspersed with two 20-second all-out sprints), each followed by 30 minutes of quiet rest and separated by at least 48 hours. Indirect calorimetry was performed to measure substrate utilization (fat oxidation, Fat Ox; carbohydrate oxidation, CHO Ox) and energy expenditure (EE) during the intervention and 30 minutes of excess post-exercise oxygen consumption. RESULTS: We observed a significant negative relationship between Fat Ox (2.5 ± 0.5 grams) and body fat percentage (r = -0.558; p < 0.05), CHO Ox (33.9 ± 7.8 grams) and body fat percentage (r = -0.499; p < 0.05), and EE (111.0 ± 15.6 kcal) and body fat percentage (r = -0.681; p < 0.01). No significance was observed between Fat Ox and lean body weight (r = 0.329; p = 0.20), although we observed a positive relationship between CHO Ox and lean body weight (r = 0.490; p < 0.05), and EE and lean body weight (r = 0.773; p < 0.01). CONCLUSION: Our findings suggest that recreationally active middle-aged men with lower levels of percent body fat utilize more fat and carbohydrates during and 30 minutes after a single bout of REHIT, ultimately leading to greater energy expenditure. However, lean body weight does not correlate with fat oxidation following REHIT exercise.