It is possible that phytoestrogens may have a beneficial effect on vascular function by acting directly on vessel walls, perhaps via improved arterial compliance and enhanced FMD.37 A number of studies have been carried out to investigate the improvement in vascular function after treatment with soy products and isoflavones.
One trial found that dietary soy protein supplementation over three months significantly improved distal pulse wave velocity in normo-tensive male and postmenopausal female subjects, following reduction in the extent of vasoconstriction in peripheral resistance vessels. Although the trial showed that soy supplementation improved blood pressure and lipid status, it did not improve vascular function, and produced a decline in endothelial function in male subjects.38
When atherosclerotic female macaques were fed a diet rich in isoflavones, it was found that administration of acetylcholine dilated their arteries, whereas constriction was reported in those fed a low isoflavone diet. Later intravenous administration of genistein to those animals receiving the low isoflavone diet dilated previously constricted vessels.39 Infusion of genistein into the brachial artery of participants in one trial resulted in an increase in blood flow within the microcirculation of subjects' forearms.33 Another trial reported on arterial compliance in perimenopausal women following administration of 45 mg of genistein (80 mg of total isoflavonoids) over a 5-10 week period and systemic arterial compliance showed a 26% improvement.40
Consumption of soy products containing isoflavones may improve vascular function via a variety of mechanisms. Due to their structural similarities to oestrogen, it is thought that they may cause an effect by binding to oestrogen receptor (ER)P receptors present in the vascula-ture, and protect against atherosclerosis.33
After oestrogen therapy postmenopausal women have improved large artery function, enhanced brachial artery FMD and restoration of normal vasomotion.38,41 Impaired brachial artery FMD is positively associated with coronary artery endothelial dysfunction and with cardiovascular risk factors.38 Dietary soy could improve vascular function, hence reducing CVD risk, through oestrogenic mechanisms.38 Although some studies show beneficial results, others show uncertainty concerning the effects of soy isoflavones on vascular function. The effects of genistein on vascular reactivity show that it may affect development of atherosclerosis, and have some effect on angina, however further trials are necessary to confirm these findings and their possible benefits.41
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