   | Phytoestrogens are plant-derived xenoestrogens functioning as the primary female sex hormone (see estrogen) not generated within the endocrine system but consumed by eating phytoestrogenic plants. Also called "dietary estrogens", they are a diverse group of naturally occurring nonsteroidal plant compounds that, because of their structural similarity with estradiol (17-β-estradiol), have the ability to cause estrogenic or/and antiestrogenic effects.
Chemical structures of the most common phytoestrogens found in plants (top and middle) compared with estrogen (bottom) found in animals.
Their name comes from the Greek phyto = plant and estrogen, the hormone which gives fertility to the female mammals. The word "estrus" -Greek οίστρος- means sexual desire and "gene" -Greek γόνο- is "to generate"). It has been proposed that plants use the phytoestrogens as part of their natural defence against the overpopulation of the herbivore animals by controlling the male fertility.
The similarities, at molecular level, of estrogens and phytoestrogens allow them to mildly mimic and sometimes act as antagonists of estrogen. Phytoestrogens were first observed in 1926, but it was unknown if they could have any effect in human or animal metabolism. In the 1940s it was noticed for the first time that red clover (a phytoestrogens-rich plant) pastures had effects on the fecundity of grazing sheep. Researchers are exploring the nutritional role of these substances in the regulation of cholesterol, and the maintenance of proper bone density post-menopause. Evidence is accruing that phytoestrogens may have protective action against diverse health disorders, such as prostate, breast, bowel, and other cancers, cardiovascular disease, brain function disorders and osteoporosis, though there is no evidence to support their use in alleviating the symptoms of menopause.
Phytoestrogens cannot be considered as nutrients, given that the lack of these in diet does not produce any characteristic deficiency syndrome, nor do they participate in any essential biological function.
Analytical methods are available to determine phytoestrogen content in plants and food.
According to a study by Canadian researchers about the content of nine common phytoestrogens in a Western diet, foods with the highest relative phytoestrogen content were nuts and oilseeds, followed by soy products, cereals and breads, legumes, meat products, and other processed foods that may contain soy, vegetables, fruits, alcoholic, and nonalcoholic beverages. Flax seed and other oilseeds contained the highest total phytoestrogen content, followed by soybeans and tofu. The highest concentrations of isoflavones are found in soybeans and soybean products followed by legumes, whereas lignans are the primary source of phytoestrogens found in nuts and oilseeds (e.g. flax) and also found in cereals, legumes, fruits and vegetables.
Phytoestrogen content varies in different foods, and may vary significantly within the same group of foods (e.g. soy beverages, tofu) depending on processing mechanisms and type of soybean used. Legumes (in particular soybeans), whole grain cereals, and some seeds are high in phytoestrogens. A more comprehensive list of foods known to contain phytoestrogens includes: soybeans and soy products, tempeh, linseed (flax), sesame seeds, wheatberries, fenugreek, oats, barley, beans, lentils, yams, rice, alfalfa, mung beans, apples, carrots, pomegranates, wheat germ, rice bran, lupin, kudzu, coffee, licorice root, mint, ginseng, hops, bourbon, beer, fennel and anise.
An epidemiological study of women in the United States found that the dietary intake of phytoestrogens in healthy post-menopausal Caucasian women is less than one milligram daily.
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