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, η²p = 0.76) and intensity effect (p = 0.002, η²p = 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.

Graphical abstract