Soy Isoflavones—Benefits and Risks from Nature’s Selective Estrogen Receptor Modulators (SERMs)

Abstract

Phytoestrogens have become one of the more topical areas of interest in clinical nutrition. These non-nutrient bioactive compounds are ubiquitous to the plant kingdom and possess a wide range of biological properties that contribute to the many different health-related benefits reported for soy foods and flaxseeds—two of the most abundant dietary sources of phytoestrogens. Reviewed is the recent knowledge related to their pharmacokinetics and clinical effects, focusing mainly on isoflavones that are found in high concentrations in soy foods. Arguments are made for considering soy isoflavones as natural selective estrogen receptor modulators (SERMs) based upon recent data of their conformational binding to estrogen receptors. Rebuttal is made to several key and important issues related to the recent concerns about the safety of soy and its constituent isoflavones. This article is not intended to be a comprehensive review of the literature but merely highlight recent research with key historical perspectives.

• soy isoflavones
• phytoestrogens
• pharmacokinetics
• phytoprotectants

Key teaching points:

• Soy is the richest dietary source of bioactive phytoestrogens called isoflavones, and their bioavailability is highly dependent on intestinal bacterial metabolism.

• Plasma urinary concentrations of isoflavones exceed by several orders of magnitude the levels of endogenous estrogens after consuming relatively modest amounts of soy foods and biological effects can be expected.

• The pharmacokinetic behavior of isoflavones indicates that the maximal health benefits are most likely to be derived by consuming small amounts of isoflavone-rich foods throughout the day.

• Maximal health benefits from phytoestrogen-rich foods are more likely to occur from regular and lifelong consumption.

• Isoflavones have characteristics that are consistent with selective estrogen receptor modulators and not estrogens. As such, when consumed at usual dietary intakes consistent with intakes by Asians, isoflavones are unlikely to have the negative effects associated with estrogens.

Key teaching points:

• Soy is the richest dietary source of bioactive phytoestrogens called isoflavones, and their bioavailability is highly dependent on intestinal bacterial metabolism.

• Plasma urinary concentrations of isoflavones exceed by several orders of magnitude the levels of endogenous estrogens after consuming relatively modest amounts of soy foods and biological effects can be expected.

• The pharmacokinetic behavior of isoflavones indicates that the maximal health benefits are most likely to be derived by consuming small amounts of isoflavone-rich foods throughout the day.

• Maximal health benefits from phytoestrogen-rich foods are more likely to occur from regular and lifelong consumption.

• Isoflavones have characteristics that are consistent with selective estrogen receptor modulators and not estrogens. As such, when consumed at usual dietary intakes consistent with intakes by Asians, isoflavones are unlikely to have the negative effects associated with estrogens.

Notes

Presented in part at Ross Products Research Conference on Medical Issues, “Synergy in Medical and Nutritional Therapy,” November 6–8, 2000. Key Largo, Florida.