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"To our knowledge, the present study is the first to study the relationship between the intake of an isoflavone supplement and platelet receptors in humans,” wrote lead researcher Argelia Garrido from the Universidad de Chile.
The research adds and expands the science behind the observed benefits that have previously been linked to the effect of isoflavones on blood cholesterol levels. However, this point is controversial and 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.
Indeed, 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.”
The new findings, published in the Elsevier journal Maturitas (Vol. 54, pp. 270-276), appear to agree with the AHA conclusions on cholesterol levels, but report that the isoflavones do impact on platelet aggregation, a result that does impact beneficially on heart health.
If these findings are reproduced in future studies, they might lead to a re-evaluation of the science that led Dr Sacks and the AHA to their conclusion.
The research study
The researchers recruited 29 post-menopausal women (average age 53,5) to take part in the double-blind, randomised, placebo-controlled study. The women were randomly assigned to receive a daily isoflavone supplement (SoyLife, Acatris) or an identical placebo for three months.
The women took two capsules daily of the soybean isoflavone extract, giving a daily dose of 100mg of isoflavones, containing 46,8mg of daidzein and 48,2mg of genistein.
Blood samples, taken before and after the intervention period, showed blood levels of daidzein and genistein did increase significantly, compared to placebo. However, no significant changes occurred in blood cholesterol levels, blood pressure, insulin levels, blood glucose levels, or sex hormones, from either the isoflavones or the placebo.
This result does not support the general hypothesis that isoflavones are good candidates for beneficial lipoprotein changes, said the researchers.
“The only unexpected finding relative to thrombotic cardiovascular disease risk factors concerned platelet TxA2 receptor density, which significantly decreased after three months of isoflavones supplementation,” said the researchers.
Platelets and heart disease
Excessive aggregation of platelets is reported to play a role in the pathogenesis of many cardiovascular disease events, such as heart attack and stroke, and several studies have shown that such events involve increases in production and density of the so-called TxA2.
“Our observations raise the possibility that the decrease in platelet TxA2 receptor density, by isoflavone supplementation, contributes to reduce the risk of thrombogenesis,” said Garrido.
In other words, by decreasing the accumulation of platelets, the risk of forming a blood clot that could lead to stroke or heart attack is reduced.
No changes in sex hormones
The researchers noted that no changes in sex hormone levels were observed, which contradicted other studies. Soy isoflavones are well-known phytoestrogens – active substances derived from plants that have a weak oestrogen-like action. This discrepancy may be due to the dose used, the length of the intervention, or the types of isoflavones present, said the researchers.
More research is needed to follow this preliminary report of isoflavones on platelet aggregation, particularly more studies of the same double-blind, randomised, placebo-controlled design.
“These findings may be helpful in elucidating how dietary isoflavones may modulate platelet function and explain how phytoestrogen-rich foods are claimed to have beneficial effects on the cardiovascular system,” concluded Garrido. - (Decision News Media, August 2006)
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