Are Phytoestrogens toxic?

Are phytoestrogens toxic to animals?

Estrogens were first found in plants in the early 1930s, but it was not known whether they would affect animals. Shortly after, it was discovered that Australian sheep grazing on clover high in phytoestrogens experienced a loss of fertility and a number of reproductive lesions. Studies of the effects of phytoestrogens on laboratory animal have found that they have effects on these species: coumestrol caused increased uterine weight and premature estrous cycles when fed to young rats, and disrupted the estrous cycle of adult female rats. In another study, genistein disrupted the secretion of luteinizing hormone by the pituitary gland.


  1. Barrett J. Phytoestrogens, friends or foes? Environmental Health Perspectives 1996;104(5):478-482.
  2. Medlock KL, Branham WS, Sheehan DM. The effects of phytoestrogens on neonatal rat uterine growth and development. Proceedings for the Society of Experimental Biology and Medicine 1995;208(3):307-13.

Are phytoestrogens toxic to humans?

Although no safety studies have been done to establish whether or not naturally-occurring plant estrogens have toxic effects in human, most food products have levels of estrogens too low to have an effect on humans. The most potent plant estrogen, coumestrol, is much lower in human diets than the isoflavones. In fact, there is evidence to support a beneficial effect of isoflavones because populations in Asia that consume a diet high in soybean products have lower rates of estrogen-dependent cancers than populations in western countries who consume fewer isoflavones.

In one human study, soy protein (high in isoflavones) was fed to volunteer women ages 20 to 29. The women on high isoflavone experienced lengthened follicular phases and delayed menstruation. However, no similar effect was seen in a study on postmenopausal women. In an Austalian study of postmenopausal women, a diet supplemented with soy or wheat flour diminished hot flashes experienced by women on that diet, compared with a control group.

There is concern about possible ill effects of phytoestrogens during prenatal and neonatal development. One researcher found that soy-based infant formulas in new Zealand contained levels of isoflavones that were higher than the level required to disrupt a woman's menstrual cycle. It is unknown whether this has any ill effects, and some speculate that early exposures to phytoestrogens may even have a beneficial effect. In one rat study, young rats that had been exposed to phytoestrogens as neonates were protected against cancer when a known carcinogen was administered.


  1. Barrett J. Phytoestrogens, friends or foes? Environmental Health Perspectives 1996;104(5):478-482.
  2. Soy Phytoestrogens: Analysis in Soybeans and Effects on Swine Reproduction
  3. Soy and Human Health World Wide Web Home Page

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Prepared Summer 1997 by Bernadene Magnuson, Ph.D.
University of Idaho, Dept. of Food Science and Toxicology - EXTOXNET FAQ Team.