ABSTRACT
In recent years there has been much interest in the potential health effects of phytoestrogens. Much of the interest has centered on the structural similarity between estrogens and plant phenolics such as the isoflavones (Figure 15.1). The topic of phytoestrogens and health has recently been reviewed in detail by Setchell.1 Although there are many different classes of plant phenolics, most of the clinical and nutritional interest has focused on the lignans that are found in high concentration in flaxseed2 and the isoflavones that are abundant in soy proteincontaining foods.3 Similarly, the effects of flaxseed phytoestrogens have been reviewed recently by Thompson.4 Flaxseed has 75 to 800 times the phytoestrogens content that of other common plant foods.5-7 Both isoflavones and lignans have been suggested to have anticancer effects.4,8,9 The levels in serum of these biologically active phytoestrogens may exceed by many orders of magnitude the levels of endogenous estrogens. Typical concentrations of soy isoflavones can exceed endogenous estradiol concentrations by 10,000-to 20,000-fold in
adults5,10-12 and infants13 and, as such, might be expected to exert biological effects at the molecular, cellular, or physiological level. However, their biological activity is low and this, combined with their affinity for estrogen receptors, makes them inhibitors by blocking the binding of mammalian (endogenous) estrogens. In this discussion the focus is the lignans of flaxseed and the isoflavones of soy that are the most concentrated sources of phytoestrogens, and their implications for different dietary patterns and incidence of cancers.
