The study found that the soy protein supplements would not benefit all women, but could offer benefits for women within five years of menopause.
"Take soy supplements following menopause."
Dr. Howard N. Hodis, lead study author and a physician with the University of Southern California Keck School of Medicine, said the results were consistent with research conducted over the past decade indicating there is a beneficial "window of opportunity."
He said that products that bind to the estrogen receptor including hormone-replacement therapy, soybean isoflavones or selective estrogen receptor modulators could reduce coronary heart disease if initiated within five to six years of menopause.
The study marks the largest and longest randomized clinical study to investigate the effectiveness of soy protein on the progression of clogged arteries. Researchers conducted the double-blind placebo-controlled study between 2004 and 2009.
They enrolled 350 postmenopausal women between the ages of 45 and 92 without a history of type 2 diabetes or heart disease. The participants were randomly selected to take either two daily doses of 25 grams of soy protein or a placebo for two and a half years.
About 86.5 percent of placebo patients and 91 percent of soy protein participants took the doses regularly. Compliance was measured through plasma and urine isoflavone measurements.
Investigators measured the carotid artery intima-media thickness (CIMT, a measurement of the innermost layer of the artery which is usually affected by thickening) to determine any progression in artery thickening. They found that the CIMT measurement was 16 percent lower in the group that took soy protein supplements. But among women who had experienced menopause within the last five years, consuming the soy protein was associated with a 68 percent reduction in CIMT progression as compared to the placebo group.
Researchers also found that participants who consumed the soy protein had a significant increase in good HDL cholesterol.
The clinical study was published in the November 2011 issue of Stroke.