"These results are the first to demonstrate a beneficial association between blood levels of the phytoestrogen daidzein and lipoproteins in human subjects," wrote lead author Noel Bairey Merz from the University of Pittsburgh Graduate School of Public Health.
A recent scientific statement by the American Heart Association (AHA) in the journal Circulation concluded that soy had little effect on cholesterol levels, and raised doubts about health claims associated with soy.
Dr Frank Sacks, a member of the AHA panel, said in January: "It's really clear that isoflavones don't contribute anything to cardiovascular benefits."
But the new study, published in The Journal of Clinical Endocrinology and Metabolism (Vol. 91, pp. 2209-2213), reports that high blood levels of daidzein were associated with favourable lipoprotein profiles.
If its findings are reproduced in future studies, they might lead to a re-evaluation of the science that led AHA to its conclusion.
Half of the women recruited for the Women's Ischemia Syndrome Evaluation (WISE) study (483 subjects; BMI 29.4; average age 58; 79 per cent post-menopausal) were chosen for this research. All the volunteers had at least one coronary risk factor. Blood samples were taken and levels of the soy isoflavones daidzein and genistein were measured.
Blood levels of total cholesterol (TC), triglyceride (TG) and HDL-cholesterol (HDL-C) were determined by an enzymatic assay. LDL-cholesterol (LDL-C) levels were calculated using the Friedewald formula.
The researchers found that increasing levels of daidzein were associated with decreasing levels of TG and increasing HDL-C levels. Women with the highest blood daidzein levels (more than 13.2 nanograms per millilitre) had 17 per cent lower TG levels and 5 per cent higher HDL-C levels than women with the lowest levels (less than 3.2 nanograms per millilitre).
When the researchers examined the data taking into account levels of the female hormone estradiol, they found that the link between daidzein blood levels and lipid profiles was stronger in women with low estradiol levels. The relationship between daidzein and blood lipids was not significant for women with high estradiol levels.
"Our findings of the beneficial association between daidzein and lipoproteins being dominantly evident among women with low blood oestrogen levels supports the idea that daidzein may operate, in part at least, via an oestrogen receptor mechanism," said the researchers.
No relationship between blood levels of the isoflavone genistein and blood lipoprotein levels was found.
"These and prior studies suggest that cardiovascular risk reduction strategies in women should consider dietary intake of food products, such as soy, which elevate blood daidzein levels, consistent with recent recommendations," concluded Bairey Merz.
This study has several limitations, most notably that other dietary factors such as dietary fibre or fat intake were not evaluated by the researchers. Also, the design of the study prevented the researchers from proving causality.
"These results require replication in future prospective, randomised trials using a dietary source of daidzein in women with low oestrogen levels, as well as correlative dietary studies to link them to dietary habits, including supplement use," concluded the authors.
CVD causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169bn ($202bn) per year. According to the American Heart Association, 34.2 per cent of Americans (70.1m people) suffered from some form of cardiovascular disease (CVD) in 2002.