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In 2005, a group of scientists undertook a critical review of published studies that looked at isoflavone extracts and their effectiveness in reducing frequency or severity of menopausal symptoms.

Previous reviews had yielded mixed results about whether isoflavone extracts reduced menopausal symptoms, such as hot flashes. For the new research, scientists re-evaluated the findings of published studies and analyzed the results according to the type of isoflavone extract used in each study. The results showed that higher genistein supplements were effective.

The researchers narrowed the literature to 11 scientifically, well-qualified studies where the source and isoflavone identity could be determined. This provided detailed information on the isoflavone extracts used in the research.

In five of the studies, the isoflavones were from whole soybean-like extracts and provided more than 15 milligrams genistein (a soy isoflavone subtype) per day. These five studies consistently reported a statistically significant effect in reducing hot flashes. Of the remaining studies, five reported no statistically significant reduction in hot flashes.

Conclusion
Based on findings of the critical review, researchers concluded that not all soy isoflavones are equally effective in reducing menopausal hot flashes. Extracts containing higher genistein levels with a minimum genistein dose are effective in reducing hot flashes.

Researchers also called for future studies to identify and correlate the type of the isoflavones for effectiveness rather than assume all isoflavone subtypes are equally effective.