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Lee Swanson Research Update
September 2010

Supplementation with soy isoflavones could improve blood vessel endothelial cell function and reduce cardiovascular disease risk factors, according to a new meta-analysis study.

The research, published in the journal Nutrition, Metabolism and Cardiovascular Diseases, analyzed 17 studies on the effect of isoflavones on endothelial function, finding a small but valuable benefit from isoflavone supplementation.

"Cumulative evidence from the randomized controlled trials included in this meta-analysis suggests that exposure to soy isoflavones can modestly, but significantly, improve endothelial function as measured by flow-mediated dilation," reported the researchers.

Soy foods have been shown to reduce cholesterol levels, with recent evidence suggesting that isoflavones could reduce other cardiovascular disease (CVD) risk factors, such as blood pressure and endothelial function.

Cardiovascular disease affects one in three Americans and is a leading cause of mortality globally.

Endothelial dysfunction signifies the early stages of heart disease and is a predictive marker for long-term CVD and mortality.

Flow-mediated dilation (FMD) of the brachial artery is used as an assessment tool for endothelial function. Impaired FMD response is a CVD risk factor and can precede clinically symptomatic CVD by many years.

The new research is a meta-analysis of 17 randomized controlled trials (RCT) aimed to determine whether exposure to isoflavone-containing soy products affects endothelial function.

Out of 42 trials initially found from a comprehensive literature search, 17 were selected as having sufficient data for study inclusion.

Seven of the 17 studies reported statistically significant changes in endothelial function as measured by FMD. Researchers suggested that several other studies reporting no effects may have used too small of a study population to gain significant results.

The meta-analysis found the overall change in FMD for isoflavone-containing soy product interventions to be 1.15 percent.

When the effects of separate interventions were considered, treatment effects for isolated isoflavones were 1.98 percent compared with 0.72 percent for isoflavone-containing soy protein, the study reported.

The significant improvement in FMD observed in response to isolated isoflavone exposure is within the range deemed clinically relevant, "thereby indicating that exposure to isoflavone supplements may beneficially influence vascular health," added the authors.

Researchers stated the key mechanism behind endothelial dysfunction involves the impaired release of nitric oxide (NO), causing blood vessels to constrict. Increased availability of isoflavone is suggested to increase NO production.

Though the meta-analysis finds a significant increase in endothelial function from supplementation with isoflavones, the researchers believe their results "warrant confirmation from a large, prospective, RCT."

Researchers indicated that future trials and observational studies should also attempt to detect dose-response effects: "Ideally, it would be desirable to test the hypothesis that larger doses of soy isoflavone supplementation yield greater responses in FMD."

Nutrition, Metabolism and Cardiovascular Diseases Published online ahead of print