Isoflavonoids

Isoflavonoids are a subclass of flavonoids, plant secondary metabolites with a characteristic C6–C3–C6 carbon framework. In isoflavonoids the B-ring attaches to the central heterocycle at the 3-position, giving the distinct isoflavone structure. They are typically present as aglycones or glycosides and vary in hydroxylation and methoxylation patterns.

These compounds occur widely in legumes, especially soybeans and soy products, but are also found in red clover, chickpeas, lentils, and alfalfa. The most studied members are genistein, daidzein, and glycitein, commonly found as glycosides such as genistin, daidzin, and glycitin. Prenylated isoflavonoids also occur in certain plants.

Isoflavonoids are synthesized in plants via the phenylpropanoid pathway, with isoflavone synthase enabling the B-ring to link at the 3-position of the C-ring to form the core isoflavone structure. This biosynthetic route leads to various isoflavones and their conjugates found in different species.

Biologically, isoflavonoids act as phytoestrogens, capable of weakly binding estrogen receptors, particularly ERβ, and can exert estrogen-like or antiestrogenic effects depending on tissue context and endogenous hormone levels. They also display antioxidant properties and can modulate cell signaling pathways. Many isoflavonoids are metabolized by gut microbiota; for example, daidzein can be converted to equol in some individuals, which may influence biological activity.

Dietary sources are dominated by soy foods such as tofu, tempeh, and soy milk, where intake influences systemic exposure to aglycones and conjugates. Research on health outcomes—menopausal symptoms, bone health, cardiovascular risk, and cancer—has yielded mixed results, and isoflavonoid effects remain an area of active study. Safety is generally good at typical dietary intakes, though high-dose supplements may affect hormone-sensitive conditions or thyroid function and should be used with medical guidance.