No diurnal variation is present in maximal fat oxidation during exercise in young healthy women: A cross-over study

Maximal fat oxidation during exercise (MFO) and the intensity that elicits MFO (Fat_max) seems to show a diurnal variation in men, which favours an increased performance in the afternoon than the morning. At present, it remains unknown whether the observed MFO and Fat_max diurnal variation in men is also present in women. Therefore, the current study examined the diurnal variations of MFO and Fat_max in women. Nineteen healthy women (age: 26.9 ± 8.7 years, maximum oxygen uptake: 39.8 ± 6.5 ml/kg/min) participated in the study. MFO and Fat_max were determined by a graded exercise test in cycloergometer using a cross-over design performed on two separate daytime schedules, one conducted in the morning (8am–11am) and one in the afternoon (5pm–8pm). Stoichiometric equations were used to calculate fat oxidation rates. There were no significant differences between MFO-morning and MFO-afternoon (0.24 ± 0.10 vs. 0.23 ± 0.07 g/min, respectively; P = 0.681). Similarly, there was no significant differences between Fat_max-morning and Fat_max-afternoon (41.1 ± 4.7 vs. 42.6 ± 5.5% of maximal oxygen uptake, respectively; P = 0.305). These results persisted after controlling for fat mass percentage (all P > 0.5). In summary, the main finding of the present study was that MFO and Fat_max were similar independent of the time-of-day when the exercise test is performed in healthy women. These results have important clinical implications since they suggest that, in contrast to what was found in men, MFO and Fat_max show similar rates during the course of the day in women. Highlights MFO and Fat_max were similar during the afternoon and morning in young healthy women. Our results suggest that, in women, it does not matter when endurance exercise is performed in term of fat metabolism during exercise.