Individual variation of follicular phase changes in endothelial function across two menstrual cycles.

NEW FINDINGS What is the central question of this study? The purpose of this study was to determine intra-individual reproducibility of follicular phase changes in endothelial function [flow-mediated dilation (FMD)] over two menstrual cycles, in healthy, premenopausal women. What is the main finding and its importance? Phase changes in endothelial function were not consistent at the individual level across two menstrual cycles, which challenges the utility of interpreting individual responses over one cycle. ABSTRACT Evidence regarding the impact of menstrual phase on endothelial function is conflicting and studies to date have only examined responses over a single cycle. It is unknown whether the observed inter-individual variability of phase changes in endothelial function reflects stable, inter-individual differences in responses to estrogen (E2 , a primary female sex hormone). The purpose of this study was to examine endothelial function changes from the early follicular (EF, low E2 ) phase to the late follicular (LF, high E2 ) phase over two consecutive cycles. Fourteen healthy, regularly menstruating women [22±3 years (mean ± SD)] participated in four visits (EFVisit 1 , LFVisit 2 , EFVisit 3 and LFVisit 4 ) over two cycles. Ovulation testing was used to determine the time between the LF visit and ovulation. During each visit endothelial function [brachial artery flow-mediated dilation (FMD)], E2 and progesterone were assessed. At the group level, there was no impact of phase or cycle on FMD (p = 0.48, p = 0.65, respectively). Phase change in FMD in cycle 1 did not predict phase change in cycle 2 (r = 0.03, p = 0.92). Using threshold based classification (2 x Typical Error threshold), 4 out of 14 participants (29%) exhibited directionally consistent phase changes in FMD across cycles. E2 was not correlated between cycles and this may have contributed to FMD response variability. The intra-individual variability in follicular fluctuation in FMD between menstrual cycles challenges the utility of interpreting individual responses to phase over a single menstrual cycle. This article is protected by copyright. All rights reserved.