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Several studies have examined the effect of soy on the function of the vascular endothelium (=single cell layer which lines the major arteries). When healthy, the endothelium releases nitric oxide, a vasodilator that reduces plaque development. One of the most common ways to measure nitric oxide response is called flow-mediated dilation (FMD) (1).                                                                                  
Ultrasound is used to detect changes in diameter of the brachial artery in response to increased blood flow, and low FMD (a small % change in artery diameter) is a predictor of coronary diseases (2).

Nestel et al. have published a series of articles showing that isoflavone supplements (containing 40-80 mg of isoflavones from either soy or red clover) significantly increased elasticity of the aorta and that enhances vascular function (3-5).

Squadrito et al. observed significant increases in FMD when 54 mg/d genistein (one of the three types of isoflavones in soy) was administered to postmenopausal women for six months (6). This treatment increased FMD from 5% to 16% compared to placebo-treated subjects who showed no change in vascular reactivity (means = 5,4% vs. 5,1%, respectively). Very convincing evidence, but other studies of soy protein (7-10) or soy isoflavones (11, 12) have reported no statistically significant effects on FMD, suggesting that more work is needed to determine which populations may benefit most from increasing soy intake. The improvements on vascular function may result from the antioxidant and lipidlowering effects of soy (13).

It can be concluded  that soy protein, more specifically the isoflavones, appear to have beneficial effects on the endothelium and vascular function. These effects appear to be independent of effects on plasma lipoprotein. Further research is still needed to confirm these results and to determine the necessary dose of soyfoods that are required, and to identify the appropriate patient populations for soy interventions.

References

  1. West SG. Effect of diet on vascular reactivity: an emerging marker for vascular risk. Curr Atheroscler Rep 2001;3(6):446-455.
  2. Shimbo D, et al. The association between endothelial dysfunction and cardiovascular outcomes in a population-based multi-ethnic cohort. Atherosclerosis 2007;192(1):197-203.
  3. Nestel PJ, Pomeroy S, Kay S, Komesaroff P, Behrsing J, Cameron JD & West L. Isoflavones from red clover improve systemic arterial compliance but not plasma lipids in menopausal women. J Clin Endocrinol Metab 1999; 84: 895-898.
  4. Nestel PJ, Yamashita T, Sasahara T, Pomeroy S, Dart A, Komesaroff P, Owen A & Abbey M. Soy isoflavones improve systemic arterial compliance but not plasma lipids in menopausal and perimenopausal women. Arterioslcerosis, Thrombosis and Vascular Biology 1997; 17: 3392-3398.
  5. Yamashita T, Sasahara T, Pomoroy S E, Collier G & Nestel PJ. Arterial compliance, blood pressure, plasma leptin, and plasma lipids in women are improved with weight reduction equally with a meat-based diet and a plant-based diet. Metabolism 1998; 47: 1308-1314.
  6. Squadrito F, Altavilla D, Morabito N, Crisafulli A, D’Anna R, Corrado F, Ruggeri P, Campo GM, Calapai G, Caputi AP &  Squadrito G. The effect of the phytoestrogen genestein on plasma nitric oxide concentrations, endothelin-1 levels and endothelium dependent vasodilatation in postmenopausal women. Atherosclerosis 2002; 263: 339-347.
  7. Hermansen K, et al. Effects of soy supplementation on blood lipids and arterial function in hypercholesterolaemic subjects. Eur J Clin Nutr 2005;59(7):843-850.
  8. Teede HJ, et al. Randomised, controlled, cross-over trial of soy protein with isoflavones on blood pressure and arterial function in hypertensive subjects. J Am Coll Nutr 2006;25(6):533-540.
  9. Evans M, et al. Effect of soy isoflavone protein and soy lecithin on endothelial function in healthy postmenopausal women. Menopause 2007;14(1):141-149.
  10. Steinberg FM, et al. Soy protein with isoflavones has favorable effects on endothelial function that are independent of lipid and antioxidant effects in healthy postmenopausal women. American Journal of Hypertension 2003;78:123-130.
  11. Hallund J, et al. Soya isoflavone-enriched cereal bars affect markers of endothelial function in postmenopausal women. Br J Nutr 2006;95(6):1120-1126.
  12. Lissin LW, et al. Isoflavones improve vascular reactivity in post-menopausal women with hypercholesterolemia. Vasc Med 2004;9(1):26-30.
  13. Ferretti G, Bacchetti T, Menanno F & Curatola G. Effect of genistein against copper-induced lipid peroxidation of human high density lipoproteins (HDL). Atherosclesrosis 2004; 172: 55-61.