New Findings What is the central question of the study?This is the first study to examine the impact of acute hyperglycaemia on endothelial function [flow‐mediated dilatation (FMD)] in premenopausal women across the early and late follicular (EF and LF) phases of the menstrual cycle. What is the main finding and its importance?Flow‐mediated dilatation was impaired 90 min after glucose ingestion, with no significant difference between phases. This indicates that women are susceptible to acute hyperglycaemia‐induced endothelial dysfunction in both the EF and LF phases of the menstrual cycle, despite potentially vasoprotective elevations in estradiol levels during the LF phase. Abstract Acute hyperglycaemia transiently impairs endothelial function in healthy men when assessed via flow‐mediated dilatation (FMD). However, research in female participants is lacking, and the impact of menstrual phase [early follicular (EF) and late follicular (LF)] on vulnerability to acute hyperglycaemia‐induced endothelial dysfunction is unknown. Seventeen healthy, naturally menstruating women [21 ± 1 years old (mean ± SD)] participated in three visits. During two visits (EFGlucose and LFGlucose), brachial artery FMD was assessed before and 60, 90 and 120 min after an oral glucose challenge (75 g glucose). During an additional EF visit, participants ingested 300 ml of water (EFTimeControl). Blood glucose and insulin increased 30 min after glucose ingestion (P < 0.001), with no difference between phases. Flow‐mediated dilatation did not change in EFTimeControl (P = 0.748) but was reduced 90 min after glucose ingestion (Pre, 8.5 ± 2.5%; Post90, 6.6 ± 2.4%, P = 0.001; Cohen's d = 0.82), with no difference between phases (main effect of phase, P = 0.506; phase by time interaction, P = 0.391). To account for individual variability in the time course of the impact of hyperglycaemia, the maximal hyperglycaemia‐induced impairment in FMD was determined in each participant and compared between phases, revealing no significant phase differences (EFGlucose, −3.1 ± 2.8%; LFGlucose, −2.4 ± 2.1%, P = 0.181; d = 0.34). These results indicate that, similar to findings in men, acute hyperglycaemia results in FMD impairment in young women. We did not detect significant protection from acute hyperglycaemia‐induced endothelial dysfunction in the LF ‘high‐oestrogen’ phase in this sample, and further research is needed to examine the potential for a protective effect of oestrogen exposure, including oral contraceptive pills and hormone replacement therapy.

Acutely imposed oscillatory shear stress (OSS) reduces reactive hyperemia flow-mediated dilation (RH-FMD) in conduit arteries of men; however, whether a similar impairment occurs in women or with FMD in response to a controlled, sustained shear stress stimulus (SS-FMD) is unknown. The purpose of this study was to determine the impact of OSS on RH-FMD and SS-FMD in men and women. OSS was provoked in the brachial artery using a 30-min forearm cuff inflation (70 mmHg). Healthy men [ n = 16, 25 yr (SD 3)] and women [ n = 16, 21 yr (SD 2)] completed the OSS intervention twice (separate days). Brachial artery endothelial function was assessed pre- and postintervention via either RH-FMD or 6 min of handgrip SS-FMD using Duplex ultrasound. The RH-FMD stimulus was calculated as shear rate area under the curve 60 s postdeflation (SRAUC60), whereas SS-FMD shear rate was targeted to produce a similar stimulus pre- and postintervention. The OSS intervention decreased RH-FMD in both sexes [men: 6.2% (SD 3.4) to 5.2% (SD 3.0); women: 5.4% (SD 2.0) to 3.1% (SD 1.8), P < 0.001), although this was accompanied by a reduced SRAUC60. There was no significant effect of the intervention on RH-FMD with SRAUC60 as a covariate ( P = 0.310). Handgrip exercise elicited a similar stimulus before and after the intervention ( P = 0.287) in men and women ( P = 0.873). Men demonstrated blunted SS-FMD [4.8% (SD 1.9) to 3.2% (SD 1.9), P < 0.001], whereas women displayed preserved SS-FMD following the intervention [3.5% (SD 1.9) to 4.0% (SD 1.9), P = 0.061]. The lower SS-FMD in men but not women following OSS provides evidence of sex differences in the effects of OSS on conduit artery endothelial function. NEW & NOTEWORTHY Acute exposure to oscillatory shear stress induces transient endothelial dysfunction in men; however, whether women experience similar impairments is unknown. Following acutely imposed oscillatory shear stress, there was a decrease in flow-mediated dilation stimulated by a physiologically relevant sustained increase in shear stress in men but not in premenopausal women. These findings demonstrate, for the first time in humans that there are sex differences in the impact of oscillatory shear stress on endothelial function.

Prolonged sitting decreases lower limb endothelial function via sustained reductions in mean shear rate. We tested whether 30 minutes of sitting cross-legged differentially impacts superficial femoral artery shear rate pattern, flow-mediated dilation (FMD) and leg pulse-wave velocity (PWV) compared to sitting flat-footed. Sitting cross-legged attenuated the reduction in mean and antegrade shear rate and increased arterial pressure compared to sitting flat-footed. Superficial femoral artery FMD and leg PWV were unaltered following either sitting position.