Timing matters: diurnal variation of maximal fat oxidation and substrate oxidation rates in metabolic syndrome-a randomized crossover study.

Purpose: The aim of this study was to investigate if diurnal oscillation in maximal fat oxidation (MFO) and substrate oxidation rates during exercise exists in subjects with metabolic syndrome (MetS).

Methods: In a randomized crossover design, 14 MetS patients were assigned to two graded exercise tests conditions performed in the morning (between 7:00 and 9:00 a.m) and in the afternoon (between 4:00 and 5:00 p.m). MFO was defined as the highest absolute value of fat oxidation obtained from the average of last 2-min stages during an indirect calorimetry test.

Results: MFO increased by 20.6% from morning to afternoon (p = 0.0002, Cohen's d = 0.52). There was a significant time of day, (p < 0.0001, ηp = 0.76) and intensity effect (p = 0.002, ηp = 0.32) in fat oxidation (Fatox) rates indicating that Fatox was higher in the afternoon than in the morning.

Conclusion: Our study extends previous findings on the existence of diurnal variation in maximal fat oxidation to MetS patients, highlighting the afternoon as a more favorable time for fat utilization during exercise. These findings have practical implications for optimizing training timing in MetS patients.

Trial Registration Number: PACTR202306776991260.