The largest and most consitent blood pressure reductions caused by consuming soy protein have been reported in studies of hypertensive adults and these findings encourages optimism about the potential beneficial effects of soy protein on blood pressure.
Rivas et al. (1) found that soymilk (1 litre a day) consumed daily for 3 months, significantly reduced systolic and diastolic blood pressure (16-18 mm Hg) while cow’s milk had much smaller effects (1-4 mm Hg).
Blood pressure reductions in hypertensives were also reported during increased intake of soy protein-rich foods and supplements.
Other published trials show impressive reductions on blood pressure. Washburn et al. found blood pressure reduction with soy protein supplements in premenopausal women (2) : 20 g soy protein (containing 34 mg isoflavones) was given in a split dose and significantly reduced diastolic blood pressure by 5 mm Hg.
Another study reported significant results of the effect of soy protein isolate on the systolic and diastolic blood pressure. 197 men and postmenopausal women received 40g/d of soy protein isolate or milk protein. After 12 weeks, systolic and diastolic blood pressure were significantly reduced in the group who received soy protein isolate (mean change in SBP/DBP= -7.5/-4.3 mm Hg) compared to those who received milk protein (-3.6 / -1.9 mm Hg) (3).
In 2005, a study investigated the relationship between soy and blood pressure in a double-blind study of 302 Chinese adults with high-normal blood pressure or mild hypertension (i.e. high blood pressure) (4). The trial group received 40 grams of soy protein daily over 12 weeks, inserted into biscuits; the control group received a placebo (i.e. biscuits without any soy).
In the soy group, both systolic and diastolic blood pressure fell considerably after 12 weeks. The reduction was also appreciably greater in people with hypertension than in people with a high-normal blood pressure. This study confirms that consumption of soy protein can reduce blood pressure. Furthermore a longitudinal study found that a higher soy consumption is correlated with lower blood pressure (5).
In a recently-published, randomized, crossover study, Welty et al (6) showed that daily consumption of soy nuts (25 g of protein, 101 mg aglycone isoflavones) reduced blood pressure to a greater extent than did the Therapeutic Lifestyle Changes (TLC) Diet alone. The TLC diet is recommended by the US National Cholesterol Education Program for cholesterol reduction and coronary heart disease reversal. Compared to the control diet, women with hypertension exhibited a 9.9% reduction in systolic blood pressure and a 6.8% reduction in diastolic blood pressure in response to soy nuts. Blood pressure reductions in normotensive women were statistically significant, albeit smaller in magnitude.
Although soy consumption is associated with lower cardiovascular disease risk in observational studies, carefully controlled clinical trials in which the soy supplement is matched for macronutrient profile with the placebo have yielded inconsistent results. Some of the most well-designed studies with high doses of soy protein (7-9) or isoflavones (10, 11) and long durations of administration have produced nonsignificant outcomes for blood pressure. The largest improvements in blood pressure have been seen in studies of adults with hypertension and in studies utilizing soy foods rather than processed supplements/isolates.
References
- Rivas M, Garay RP, Escanero JF, Cia P,Jr Cia P & Alda JO. Soymilk lowers blood pressure in men and women with mild to moderate essential hypertension. J Nutr 2002; 132: 1900-1902.
- Washburn S. Effect of soy protein supplementation on serum lipoproteins, blood pressure, and menopausal symptoms in perimenopausal women. Menopause 1999; Spring; 6(1):7-13.
- Teede HJ, Dalais FS, Kotsopoulos D, Liang YL, Davis S & McGrath,BP. Dietary soy has both beneficial and potentially adverse cardiovasculair effects: a placebo-controlled study in men and postmenopausal women: a double-blind, placebo-controlled trial. Nutr Metab Cardiovasc Dis 2001; 10: 315-322.
- He J. et al., Effect of soybean protein on blood pressure: a randomized, controlled trial. Annals of Internal Medicine 2005, July 5; 143(1), 1-9.
- Yang G. et al, Longitudinal study of soy food intake and blood pressure among middle-aged and elderly Chinese women. Am J Clin Nutr 2005, May 81(5), 1012-1017.
- Welty FK, Lee KS, Lew NS, Zhou JR. Effect of soy nuts on blood pressure and lipid levels in hypertensive, prehypertensive, and normotensive postmenopausal women. Arch Intern Med 2007;167:1060-7.
- Cuevas AM, et al. Isolated soy protein improves endothelial function in postmenopausal hypercholesterolemic women. Eur J Clin Nutr 2003;57(8):889-894.
- 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.
- Jenkins DJ, et al. Effects of high- and low-isoflavone soyfoods on blood lipids, oxidized LDL, homocysteine, and blood pressure in hyperlipidemic men and women. Am J Clin Nutr 2002;76(2):365-372.
- Colacurci N, et al. Effects of soy isoflavones on endothelial function in healthy postmenopausal women. Menopause 2005;12(3):299-307.
- Squadrito F, et al. The effect of the phytoestrogen genistein on plasma nitric oxide concentrations, endothelin-1 levels and endothelium dependent vasodilation in postmenopausal women. Atherosclerosis 2002;163(2):339-347.