Soy isoflavones are compounds in soybeans similar in structure to the hormone estrogen; the main ones are genistein and daidzein. As supplements they are taken mainly for menopause hot flashes and bone health.
Also known as: soy, genistein, daidzein, Glycine max.
Trials in a 2012 meta-analysis used a median of 54 mg of isoflavones a day for 6 weeks to 12 months (Taku 2012). These are study protocols, not a recommendation.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling.
What studies show, per NCCIH: soy isoflavone supplements or soy protein may reduce how often and how badly hot flashes occur, but the effect may be small and results are not fully consistent. Isoflavones probably help bone density after menopause and may lower blood pressure modestly. Soy protein in place of other protein may lower cholesterol slightly; isoflavones may not. Soy supplements have not been shown to lower breast cancer risk. A 2012 meta-analysis of 17 trials found isoflavones cut hot flash frequency by about 21% and severity by about 26% compared with placebo (Taku 2012).
Safety, per NCCIH: isoflavone extracts have been used safely in studies for up to 6 months. Side effects can include bloating, constipation, diarrhea and nausea. Some people are allergic to soy. High-tyramine soy foods (such as tofu and soy sauce) have caused dangerously high blood pressure with MAOI antidepressants. Amounts above those in foods may be unsafe in pregnancy.
We could not find a single published half-life or time to peak for soy isoflavones that applies to a supplement dose, so we do not list one.
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