Isoflavone Use May Improve Endothelial Function in Postmenopausal Women

Anthony J. Brown, MD

December 11, 2009

December 11, 2009 — Oral isoflavone supplementation enhances endothelial function in postmenopausal women with low levels of flow-mediated dilation (FMD) at baseline, according to a report in the November 18th online issue of the American Journal of Clinical Nutrition. In women with high levels of FMD, no benefit is seen.

"The risks of cardiovascular diseases increase with the decline in estrogen production after menopause in women," senior author Dr. Ru-Tai Hui, from the Chinese Academy of Medical Sciences and Peking Union Medical College in Beijing, China, and colleagues explain. "Isoflavone, mainly produced by soybeans, has been suggested to have estrogenic and potentially cardioprotective effects and improved endothelial dysfunction in many experimental studies."

Whether isoflavone use actually improves endothelial function in postmenopausal women, however, is controversial, the report indicates. Studies investigating this topic have been relatively small and have yielded conflicting findings. "As a result, the precise effect of isoflavone supplementation has not been established."

Dr. Hui's team set out to examine this topic in detail by conducting a meta-analysis of randomized placebo-controlled trials identified though a search of PubMed, Embase, the Cochrane Library database, and other sources. Of 561 articles identified, 9 were included in the review and meta-analysis.

The studies were published between 2000 and 2007 and typically ranged in size from 18 to 57 subjects with the exception of one study with 202 subjects.

In the overall analysis, isoflavone use significantly increased FMD (weighted mean difference, 1.75%, p = 0.0002), the report indicates.

However, further analysis showed that oral supplementation with isoflavone enhanced FMD significantly only if the age-adjusted baseline FMD level was <5.2% (weighted mean difference, 2.22%, p < 0.0001). Meta-regression analysis confirmed that age-adjusted FMD at baseline was inversely associated with effect achieved with oral isoflavone.

Regarding potential limitations of the current analysis, the authors point out that none of the studies included men, who may respond differently to isoflavone than do women. Also, just 2 of the studies included women with high cholesterol levels, subjects who might be expected to have an enhanced FMD response to isoflavone use.

Another issue is that the range of isoflavone doses used in the studies was fairly narrow, the authors point out. Thus, there is a need for future studies to better understand dose-response relationships.

Lastly, the researchers note that the sources of isoflavones were not the same in each study. In six trials, the source was soy protein, whereas in three, the sources were unclear. The source of the isoflavone might influence its effect on FMD and, therefore, additional research is needed to answer this question, the researchers state.

"The baseline endothelial profile may be an important and potential factor influencing the effect of oral isoflavone supplementation on endothelial function," the researchers conclude. "Additional high-quality rigorous studies, especially in women with cardiovascular diseases and in men, should be performed to confirm our results and explore the exact mechanisms of isoflavone in the improvement of endothelial function."

Am J Clin Nutr. Published online November 18, 2009. Abstract

Reuters Health Information 2009. © 2009 Reuters Ltd.

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