Investigating the intra-individual consistency of follicular changes in endothelial function across two menstrual cycles
Research has shown that estrogen has vasoprotective properties, however there is conflict in the literature regarding whether vascular endothelial function is lower in the low estrogen, early follicular (EF) phase and higher in the high estrogen, late follicular phase (LF) of the menstrual cycle. This may be due to inter-individual variability in responses to cyclic changes in estrogen because, although this has not been systematically examined, studies have shown that individuals may exhibit higher, lower or unchanged endothelial function in LF vs. the EF phase. It is unknown whether this individual variability reflects important, stable, inter-individual differences in responses to estrogen, as research to date has only followed women over one cycle. It is hypothesized that women will respond consistently over multiple cycles such that the phase changes in conduit and microvascular endothelial function during one cycle, will predict the phase change during a subsequent cycle. Fourteen healthy, naturally cycling women (22 +/- 3 years [mean +/- SD]) participated in four visits over two menstrual cycles. During each visit (EF Visit 1, LF Visit 2, EF Visit 3 and LF Visit 4), conduit and microvascular endothelial function were assessed. Blood hormone analysis was used to confirm phase. Data are [mean +/- SD]. There was no impact of phase or cycle on conduit or microvascular endothelial function at the group level. Individual responses varied in each cycle, however the phase changes in endothelial function were inconsistent between cycles, such that phase change in endothelial function during cycle 1 did not predict phase change in cycle 2 (FMD: r = 0.08, p = 0.78, AUC Change LVC: r = 0.002, p = 0.99). This study demonstrated for the first time that at the individual level, phases changes in endothelial function were inconsistent across two phases. Future research is required to investigate potential mechanisms that influence, and the functional relevance of variability, in endothelial function changes with menstrual phase.