The potential heart benefits of soy isoflavones may also include reducing the early stage of artery hardening, if the results of an animal study can be translated into humans.
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 September issue of the Journal of Nutrition (Vol. 136, pp. 2384-2390), states that soy isoflavones may impact on the development of atherosclerosis, 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.
Soy and the adhesion of monocytes
The researchers, led by Shanmugam Nagarajan from the University of Arkansas for Medical Sciences, investigated the effects of soy and soy isoflavones on the adhesion of monocytes, cells in the blood that play a role in immune health, and inflamed endothelial cells (the cells that line the inner wall of arteries), as such an interaction has been reported to be an early stage in the hardening of arteries (atherosclerosis).
Atherosclerosis is the process whereby fatty substances such as cholesterol and calcium form plaque on the inner lining of an artery, causing them to harden. If enough builds up, the plaque can reduce blood flow through the artery. If it ruptures, blood clots can form, which can block the flow of blood to the heart and cause a heart attack, or stroke.
Atherosclerosis occurs naturally in humans as part of the ageing process, but certain factors including high blood cholesterol, smoking, high blood pressure, obesity and diabetes increase the risk.
The research study
Nagarajan and his colleagues, Bradford Stewart and Thomas Badger, used female Sprague-Dawley rats fed control diets (AIN-93G) containing a soy protein isolate or casein to study the impact of soy phytochemicals on a protein specific to endothelial cells (CD54).
Blood samples from the rats showed that the soy-fed rats had inhibited CD54-dependent sticking of moncytes, whereas blood from casein-fed rats did not.
To further investigate if the observations were due solely to isoflavones, the Arkansas researchers incubated monocytes with the isoflavones.
No monocyte adhesion to the CD54 protein was found. It was also found that the endothelial cells did not even produce the protein in the presence of isoflavones.
This also led to a reduction in the production of pro-inflammatory proteins that would exacerbate the development of atherosclerosis and heart disease.
“Collectively, these data suggest that the athero-protective effect of soy diets may be mediated by blocking monocyte-endothelial cell interaction,” concluded the researchers.
Further study needs to be done
Significant further study still needs to be done to investigate whether such results are applicable and reproducible in humans - particularly more studies of the same double-blind, randomised, placebo-controlled design.
Only recently, another possible benefit of soy isoflavones was reported by Chilean researchers, who said that isoflavones from soy also impacted on the gathering of platelets in the blood that is linked to blood clots, and subsequently an increased risk of heart attack and stroke. - (Decision News Media, August 2006)
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