No Influence of Menstrual Phase on Substrate Oxidation Following Submaximal Aerobic Exercise in the Fed State

INTRODUCTION: Recently the influence of the menstrual cycle and more importantly, fluctuations in key sex hormones across the follicular (FP), ovulation (OP) and luteal (LP) phases have on during exercise substrate oxidation has been questioned. In addition to a significant amount of work not following best practices when determining menstrual phase, the inter- and intra-individual variability in hormonal concentrations across cycles may be affecting our understanding. PURPOSE: To examine the effect of three hormonally distinct phases of the menstrual cycle (FP, OP, LP) on during and post-exercise substrate oxidation following an acute bout of submaximal aerobic exercise in the fed state while utilizing best practices to determine menstrual phase. METHODS: Thirteen females (age: 24±4 y, BMI: 24.8±5.4 kg·m -2, V̇O2max: 41.02±5.30 mL·kg·min -1) completed 30 min of submaximal aerobic exercise in the FP, OP, and LP where menstrual phase was tracked using cycle counting, oral body temperature, ovulation strips, and blood sampling. Between-phase differences in respiratory exchange ratio (RER), substrate oxidation, oxygen consumption (V̇O2), overall session O2, and post-exercise metabolism were compared using a series of linear mixed models (LMMs; timepoint x phase). RESULTS: There were no between-phase differences in V̇O2, overall session O2, post-exercise metabolism, or RER (P>0.119, R 2<0.993). There were also no between-phase differences in fat (P=0.417, R 2=0.920) or carbohydrate oxidation (P=0.235, R 2=0.845) during or post-exercise. CONCLUSIONS: These findings demonstrate no differences across menstrual phase in during and post-exercise metabolism following an acute bout of submaximal exercise.