A study into the effects of taking a dietary supplement containing isoflavone found it improved artery function in stroke patients.
Previous research has suggested that eating soy may help prevent breast and prostate cancer and lower cholesterol.
However, other studies have shown it could have a negative effect on the body, including reducing the quality of a man's sperm.
Isoflavone is part of the phytoestrogen family and is mainly found in soya beans, chickpeas and clovers.
For the latest study, 102 patients who had suffered a first or recurrent ischaemic stroke (caused by a clot) in the previous six months were recruited.
All had established heart disease and were split into two groups, with one group given a 12-week course of isoflavone as an 80mg daily dietary supplement, and the other given a dummy pill (placebo).
Researchers measured the the way the brachial artery (the main artery in the arm) dilated in response to an increase in blood flow.
This is known in medical terms as flow-mediated dilation (FMD), with a greater FMD suggesting a better functioning artery.
The FMD also relates to how well the cells lining the inner surfaces of blood vessels (vascular endothelium) are functioning, with dysfunction in this area implicated in cardiovascular disease.
The results showed that the prevalence of impaired FMD was similar between the two groups at the start of the study.
However, after 12 weeks, the FMD was "significantly greater" in those patients given the isoflavone supplement, suggesting a better function.
The authors added, "Furthermore, the prevalence of impaired FMD at 12 weeks became significantly lower in isoflavone-treated patients than controls."
The authors also noted that the beneficial effect of isoflavone was more pronounced in patients who were suffering the more severe endothelial dysfunction.
Patients were found to suffer no side-effects from taking the supplements. However, the researchers said further studies were needed to confirm the long-term effects.
The research was published online in the European Heart Journal and is believed to be the first randomized controlled trial to investigate the effects of isoflavone supplement on the way the brachial artery dilates in response to an increase in blood flow.
Professor Hung-Fat Tse, from the University of Hong Kong, was one of the researchers on the study.
He said, "The patients who had a lower initial FMD were found, in general, to respond with a larger absolute increase in FMD after receiving 12 weeks of isoflavone intervention, compared to patients who had a better baseline FMD in the first place.
"These findings suggest that isoflavone reverses endothelial dysfunction in this group of patients with cardiovascular disease.
"This has important clinical implications, as the benefit of the treatment is conferred to the group of patients with the highest risks for cardiovascular events, and this effect persists, even at this rather late stage of the cardiovascular continuum."
The authors said the effect of isoflavone was comparable with lifestyle changes such as endurance training or taking cholesterol-lowering statins.
However, Tse said it was too early to make clinical recommendations about the use of isoflavone supplements for stroke patients.
"Our study implied that diets with higher isoflavone contents might be beneficial in reducing cardiovascular risk in ischaemic stroke patients."
He said it could be reasonable to propose that a similar effect might be observed in other kinds of cardiovascular disease.
"However, specific response from different CVD-related conditions requires further testing.
"At this juncture, regular isoflavone supplement might not be advocated since the benefits and side-effects of long-term supplementation are still unknown.
"A balanced diet is still the top priority in promoting health.
"Diets with higher soy content might be beneficial due to the isoflavone contents. These food products also, in general, have higher contents of polyunsaturated fats, fibre, vitamins and less saturated fat."
Around 110,000 people in England have a stroke every year and 50,000 die from it. It kills more women than breast cancer.
Around 20-0% of patients die within a month of having a stroke while others are left with disabilities and 13% of survivors are discharged to institutional care.
There are currently around 300,000 people in England living with moderate to severe disabilities caused by strokes.
Dr Peter Coleman, from The Stroke Association, said, "This is an important and interesting study showing that dietary supplementation with isoflavones in people who have had a stroke may reduce their risk of further stroke or cardiovascular disease.
"Whilst this is a positive finding, it was a small study and further research is needed to discover how plant isoflavones could reduce stroke risk."
From the September 29, 2008, Prepared Foods e-Flash