Randomized controlled trial of the effects of soy protein containing isoflavones on vascular function in postmenopausal women.

Double-blind randomized controlled trial in postmenopausal women aged 60 to 75 years enrolled through the breast cancer screening program in Utrecht, The Netherlands. The study compared soy protein containing isoflavones with milk protein placebo over 12 months and assessed vascular function and blood pressure.

Participants in the active arm took one daily serving of 36.5 g soy protein powder containing 25.6 g isoflavone-rich soy protein, providing 52 mg genistein, 41 mg daidzein, and 6 mg glycitein (aglycone weights), for 12 months. The powder could be mixed with food or beverages and was compared with an identically looking and tasting milk protein placebo.

Soy protein containing isoflavones did not improve vascular function overall in older postmenopausal women. In intention-to-treat analyses, the change in brachial artery flow-mediated dilation was not different from placebo, 0.37 (-1.48, 2.21), P=0.69. Blood pressure findings were mixed: systolic blood pressure favored soy in some analyses, including a difference in change of 4.34 (0.25, 8.42), P=0.04 in ITT and 5.55 (1.07, 10.02), P=0.02 per protocol, but diastolic blood pressure and flow-mediated dilation were not significantly improved. Adverse events were similar between groups, with mainly gastrointestinal complaints such as constipation or heartburn.

The main vascular endpoint was null, and subgroup benefit, including possible effects in equol producers, could not be determined conclusively. Interpretation is limited by reduced analyzable sample sizes versus randomization and by restriction to older postmenopausal women from one Dutch screening population, which may limit generalizability.