Sex differences in metabolic flexibility in transition from rest to submaximal running.

INTRODUCTION: The ability of an organism to adapt nutrient oxidation in response to energy requirements and fluctuations in substrate availability is termed metabolic flexibility. This study aimed at exploring differences in the metabolic flexibility of well-trained males and females in transition from rest to steady-state treadmill running within the moderate intensity domain. METHODS: Twenty-four participants (12 males and 12 females) completed a maximal running test on a motorized treadmill (day 1). Then, on a non-consecutive day (day 2), all participants were tested during 10 min of seated rest and 6 min of running at 55% of maximal oxygen uptake (VO2max). Participants were matched based on age and percentage achieved of predicted VO2max values (∼ 150% of predicted VO2max for both sexes). Metabolic flexibility was quantified as the β slope of the magnitude of fat oxidation increment stimulated by an increase of 1 kcal.min-1 in energy expenditure. RESULTS: Females exhibited a steeper slope of the β slope of the relationship between energy expenditure and fat oxidation (p = 0.038). CONCLUSION: These data show that females display greater metabolic flexibility than males in transition from rest to running at 55% VO2peak. Since the participants from both sexes were matched for % of predicted VO2max, these differences are unlikely to be explained by training status alone, and instead likely reflect intrinsic sex-specific metabolic regulation during submaximal exercise.