Isoflavone
Japanese: イソフラボン
A type of polyphenol abundant in soybeans. Fermentation converts it into the aglycone form, which is more easily absorbed by the body.
Isoflavones are polyphenols found in soybeans. Structurally similar to estrogen, they are also called 'phytoestrogens' and are believed to help maintain bone density and alleviate menopausal symptoms.
Most isoflavones in soybeans exist as glycosides—bound to sugars, making them poorly absorbed. During fermentation, β-glucosidase from koji mold cleaves these sugars, converting them to the 'aglycone' form. The better absorption of isoflavones in miso and natto compared to raw soybeans is thanks to this enzymatic reaction.
This is a prime example of how fermentation changes not the amount of a nutrient but its bioavailability. The same soybeans offer different benefits to the body after fermentation.