Effects of soy protein on endothelium-dependent vasodilatation and lipid profile in postmenopausal women with mild hypercholesterolemia.

Background: Estrogens appear to improve endothelium-dependent systemic and coronary vasodilator function, perhaps by increasing nitric oxide levels and the subsequent effect of nitric oxide on endothelial function and smooth-muscle relaxation. Because soybean foods contain phytoestrogens, we evaluated the effect of orally administered soy proteins on vascular responsiveness.

Methods: This double-blind, placebo-controlled crossover study involved 24 postmenopausal women (mean age 55 yr) who were hypercholesterolemic, who were not taking hormone replacement therapy and had not taken antioxidants or lipid-lowering agents in the preceding 2 months. The patients were randomized to receive a 25-g soy protein supplement or a mild protein placebo for 6 weeks, separated by a washout period of 4 weeks. To evaluate nitric oxide bioactivity we used ultrasonography to measure endothelium-dependent dilator responses in the brachial artery to hyperemia. We induced hyperemia by inflating a blood pressure cuff on the forearm to 250 mm Hg for 5 minutes. We also determined the effect of the supplements on various lipids.

Results: The mean (+/- standard deviation) brachial artery diameter was similar in both groups (3.94 [0.79] mm in the placebo group and 4.13 [0.74] mm in the soy protein group). In both placebo and protein groups, the serum triglyceride levels increased, and total cholesterol and low-density lipoprotein cholesterol levels fell significantly (p < 0.05). High-density lipoprotein cholesterol levels were little changed.

Interpretation: Consumption of a supplement of 25 g of soy protein daily for 6 weeks had no measurable effect on arterial diameter or vasodilatation in response to hyperemia.