Women with high levels of daidzein in their blood had lower levels of triglycerides, higher levels of HDL-C or "good" cholesterol, and healthier ratios of total to good cholesterol levels, Dr. C. Noel Bairey Merz of Cedars-Sinai Medical Center in Los Angeles and colleagues found.
The researchers note in The Journal of Clinical Endocrinology & Metabolism that female monkeys fed a soy-rich diet show healthy changes in blood fat levels, the researchers note, but evidence of the effects of soy on cholesterol in humans has been mixed.
To investigate, the team checked blood levels of daidzein and genistein, another plant estrogen, in a group of 483 women at risk of heart disease who were participating in the Women's Ischemia Syndrome Evaluation study. About 80 percent of the women were postmenopausal.
The researchers found no association between genistein and blood fat levels. But higher blood levels of daidzein were tied to lower triglyceride levels and higher good cholesterol levels.
The effect of daidzein was strongest for women with low estrogen levels, who also showed reductions in LDL-C or "bad" cholesterol, as well as lower total cholesterol levels, suggesting that the soy component may act like the real hormone in the body to help in restoring healthier blood fat levels, the researchers suggest.
While the findings must be confirmed by additional research, they provide evidence that daidzein is responsible for soy's beneficial effects on blood lipid levels, Merz and colleagues write.
"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," they conclude.