Diurnal variation of fat oxidation rate and energy expenditure in an acute bout of endurance exercise by young healthy males

BACKGROUND: Few studies have reported that circadian rhythm affects fat oxidation (FOx) during exercise. Time-of-day causing greater FOx and energy expenditure (EE) during exercise would be beneficial in the prevention of physical inactivity-related health disorders such as metabolic syndrome. The study aims to compare EE and FOx rate after an endurance exercise session done in the morning and late-afternoon hours by apparently healthy young male participants. MATERIAL AND METHODS: The present crossover quasi-experimental study involved 10 moderately active but physically untrained male participants of age 18–25 years with normal body mass index. Participants did a steady-state exercise on a motorized treadmill for 30 minutes at a moderate-intensity (50 ± 2% of their heart rate reserve) on two separate occasions at 9:00–10:00 and 15:00–16:00. A similar meal followed by 2 hours of fasting was done before each trial session. During the last 2 minutes of the exercise session, the respiratory gas analysis estimated volume of oxygen (VO(2)) and volume of carbon dioxide (VCO(2)) consumed in L/min. Indirect calorimetry equations assessed FOx (mg/min), EE (Kcal/min) and respiratory exchange ratio (RER). Data of one participant were removed to adjust for extreme chronotype. Paired t-test was applied, and P ≤ 0.05 was considered significant. RESULTS: Morning versus late-afternoon variations in FOx (269 ± 110 vs 290 ± 110, P = 0.016), RER (0.86 ± 0.05 vs 0.85 ± 0.05, P = 0.040), EE (5.454 ± 0.987 vs 5.494 ± 0.959, P = 0.079) and VO2 (1.104 ± 0.201 vs 1.113 ± 0.196, P = 0.035) were present after adjustment for chronotype. CONCLUSION: In an acute bout of endurance, exercise done by young and healthy male individuals, FOx was significantly higher, while EE tends to be higher in late-afternoon than in the morning.