Phytoestrogens are plant compounds that can mimic or modulate the actions of endogenous estrogens usually by binding to the estrogen receptors in a number of tissues including breast, prostate, and colon. They also possess a wide range of other biological properties including antioxidant activity and modulation of tyrosine kinase. There are two main groups of phytoestrogens—isoflavones (derived mainly from soy products) and lignans (found in many plant foods including cereals, flaxseed, fruits, and berries). Currently there is enormous research and commercial interest in these compounds because of their potential beneficial effects on cardiovascular disease, osteoporosis, and cancer—especially of the breast and prostate. The metabolism of phytoestrogens is extremely complex, involving both mammalian enzymes and the gut microflora, and there is increasing evidence that it is critically important for the biological activity of the compounds, particularly the conversion of the isoflavone daidzein to the gut flora metabolite equol. The paper is a comprehensive and critical review of the absorption, metabolism, and excretion of phytoestrogens, and it contains much new and unpublished data, so it provides a state-of-the-art summary for scientists in many different areas of phytoestrogen research. The impact and significance of the paper is increased by the fact that the co-authors, Marian Faughnan, Leane Hoey, Kristiina Wähälä, Gary Williamson, and Aedin Cassidy, are all leading experts in various aspects of phytoestrogen metabolism and activity.

It describes a lot of new and unpublished information on the absorption and metabolism of phytoestrogens, combining analytical chemistry, human studies, and animal data. It includes information on factors that have an impact on the bioavailability, e.g., dose, time, and food matrix, that are important to researchers when they are conducting experiments with the compounds.

There is increasing evidence that substances from soy and flaxseed called phytoestrogens (or plant estrogens), have potentially beneficial effects on many human diseases and conditions including breast and prostate cancer, heart disease, menopausal symptoms, and osteoporosis. When consumed, the compounds are extensively converted in the body to more active and, in some cases, less active derivatives. One such hormonally-active derivative is equol. In Europeans and Americans, only about one-third of the population can produce equol after soy consumption and it seems that this could be important in determining the beneficial effects of soy. The paper describes some of the factors that affect the way in which these phytoestrogens are taken up by the body and converted to the active derivatives.

The topic of phytoestrogens encompasses two main interests of mine—the role of diet in reducing cancer risk and the role of the gut bacterial population in metabolizing dietary compounds. The gut microflora play a crucial role in the metabolism of both isoflavones and lignans and I have been interested for some years in trying to determine the importance of the microflora metabolism in the biological activity of phytoestrogens. In this work I have had the privilege of collaborating extensively with colleagues in other European institutes, several of whom are co-authors of the bioavailability paper. The work on which the paper is based was funded largely by the European Union and the writing of the review was performed under the auspices of an EU-funded project called VENUS (Vegetal Estrogens in Nutrition and the Skeleton).