Phytoestrogens

General information

Phytoestrogens are naturally occurring, polyphenolic, non-steroidal plant compounds that are structurally similar to 17β-estradiol and have estrogenic and/or antiestrogenic effects. These effects are mediated by binding to estrogen receptors, by alterations in the concentrations of endogenous estrogens, and by binding to or stimulation of the synthesis of sex hormone binding globulin.

There are four main groups of phytoestrogens: isoflavonoids, flavonoids, stilbenes, and lignans [ ]. Of these the most commonly occurring are the flavonoids (of which the coumestans, prenylated flavonoids, and isoflavones have the greatest estrogenic effects) and the lignans. The isoflavonoids include genistein, daidzein, coumestrol, and equol; the prenylated flavonoids include 8-prenylnaringenin, the stilbenes include resveratrol, and the lignans include enterodiol and enterolactone. Mycoestrogens (mycotoxins) are metabolites of Fusarium , a fungus that is often found in pastures and in alfalfa and clover.

A wide range of plants and the products derived from them, such as soya milk and tofu, contain estrogens (isoflavones or lignans) with some chemical affinity for estradiol; the concentrations of isoflavone are particularly high in soya. On the other hand, the isoflavones are of very low potency, estimated by one manufacturer at between 1:10 000 and 1:140 000 of the activity of estradiol itself. The sources of phytoestrogens are summarized in Table 1 [ ]. For example, Humulus lupulus (hop) contains a variety of phytoestrogens and phytoestrogen precursors [ ]; some species of Polygonum (knotweed) contain stilbene phytoestrogens, including resveratrol [ ]. Trifolium pratense (red clover) contains phytoestrogens, as well as flavones (isorhamnetin, pratensein), phenolic coumarins, and demethylpterocarpin [ ].

In a study of nine common phytoestrogens, the foods with the highest phytoestrogen contents were nuts and oilseeds, followed by soy products, cereals and breads, legumes, meat products, processed foods that contain soy, vegetables, fruits, and alcoholic and non-alcoholic beverages [ ]. Flax seed and other oilseeds contained the highest total phytoestrogen content, followed by soy bean and tofu. The highest concentrations of isoflavones were in soy bean and soy bean products (for example. tofu) followed by legumes, whereas lignans were the primary source of phytoestrogens found in nuts and oilseeds (for example flax seed) and also in cereals, legumes, fruits, and vegetables.

Drug studies