U.K. Government Reports on Soy Phytoestrogens

LONDON--The U.K. Food Standards Authority's (FSA) Committee on Toxicity (COT) is currently consulting on a draft report outlining soy phytoestrogens' impact on human health. The COT Working Group on Phytoestrogens was established in April 2000 to review the complex issues surrounding phytoestrogens, including their link to fertility problems in animals and their benefits for heart disease and bone health. After reviewing the available scientific evidence, COT prepared a report of its findings that outline areas needing further research.

Key points that were investigated included whether soy-based infant formula posed a risk to infants; what the health implications of soy intake were for people in general; and, ultimately, if there are any benefits derived from soy consumption. The group reviewed studies published up to April 30, 2002. Although COT acknowledged there have been many anecdotal reports and reviews expressing opinions on phytoestrogens' health benefits, the group did not consider to be objective and did not include them in the review.

First, the group agreed on the definition of phytoestrogens: any plant substance or metabolite that induces biological responses in vertebrates and can mimic or modulate the actions of endogenous estrogens, usually by binding to estrogen receptors.

COT reviewed the science behind soy's phytoestrogens, rather than the whole soy compound. The group concluded that in vitro and in vivo studies have shown that phytoestrogens' primary biological effects are due to interacting with estrogen receptors. However, much research in this field has been on isoflavones, and comparatively little is known regarding prenylated flavonoids, coumestrol or lignans--soy's other phytoestrogens.

In terms of soy-based formula, the researchers reported the concentration of phytoestrogens found in these formulas is at a higher level than that found in human breast milk. It has been estimated that infants drinking these formulas consume 4 mg/kg of birth weight per day--which may be higher than any other population consumes.

Additionally, the group reported isoflavones inhibit thyroid peroxidase by lowering levels of free thyroxine (an iodine-containing hormone), but it is unknown if an infant with hypothyroidism could compensate for these lowered thyroxine levels. The group also acknowledged that, theoretically, if the thyroid status of an expectant mother is compromised, exposure to high levels of phytoestrogens might impair normal fetal development.

The group recommended that soy-based formula should be given to infants only for clinical reasons because of potential interactions between phytoestrogens and a developing thyroid.

In other populations at risk of certain health problems, the phytoestrogens' health benefits are inconclusive. Although it has been shown that phytoestrogens lower the risk of developing breast cancer, some research has shown that these chemicals stimulate progression of the disease by proliferating the breast epithelium (a tissue layer) in premenopausal cancer patients. COT suggested that until further research has shown a more conclusive benefit linking soy with breast cancer, women with estrogen-dependent breast disease should be cautious regarding their intake of phytoestrogen-rich food or dietary supplements.

In addition, the group reported no concrete benefits have been shown between phytoestrogen intake and other cancers and menopause. However, they wrote that exposure to estrogens or phytoestrogens early in life may lower the risk of certain cancers by influencing the programming of an individual's homeostasis. COT also reported there was considerable evidence linking soy with lowering low-density lipoprotein (LDL, or "bad") and total cholesterol. In addition, short-term human studies and research using rat models have demonstrated that soy may protect the lumbar spine and prevent bone loss.

In the end, the group recommended that more long-term studies are needed to better understand phytoestrogens' role in health. There is also a need to investigate drug-phytoestrogen interactions, as well as phytoestrogens' biological effects beyond their role with regard to estrogen receptor sites. Additionally, the group suggested developing biomarkers of phytoestrogen exposure would aid epidemiological studies.

The consultation runs until Dec. 3, at which time FSA will be apprised of the science behind soy and the public's health. The comment period is also open until Dec. 3; the report is scheduled to be presented to the FSA's board in February 2003.

The entire draft report can be found here.