Soy and Breast Cancer

Maxine Walker of Snohomish, WA asks:

Are you aware of the non-profit Weston A. Price Foundation research on soy products and their effects on health? You are such an invaluable source of information for so many products I am sure you would want to address the discrepancy between your recommendations for use of soy products and the extensive investigations that question the value of soy in human nutrition. The Weston A. Price Foundation is probably the most complete reference for this.

Respectfully,

Maxine Walker

The Green Guide Replies:

Soy crops are big business in America and The Green Guide has regularly reported on concerns about genetically-engineered varieties, such as the Roundup Ready Soybean, and conventional farmers' reliance on herbicides in raising them (see "Soy: What's It All About). Yet, as noted in "Why We Need To Rethink Meat," soy can also serve as an alternative protein source to animal products, whose fat contains higher levels of dioxins and other contaminants that rise in food chain. The Green Guide has informed readers about organic soy products and to avoid certain derivatives such as soy oil, lecithin, hydrolyzed vegetable protein, and soy protein isolates.

The Weston A. Price Foundation's Soy Alert! Campaign raises a number of concerns about the health value of soybeans, claming research studies link phytoestrogens in soy with infertility, breast cancer, and thyroid toxicity. The Green Guide has reported on phytoestrogens in soy in past articles on possible environmental factors in breast cancer risk (see "Female Troubles"). However, a review of the literature on the health impact of consuming soy products indicates that the potential problems and benefits of soy are more complex than the conclusions reached by the Price Foundation.

For instance, according to some recent studies, phytoestrogens, or plant estrogens, in soy may actually lower the body's own production of the estrogen estrone, increased levels of which increase breast-cancer risk. A September 2002 report published in Cancer Epidemiology Biomarkers and Prevention of a study of 144 postmenopausal Chinese women in Singapore found that estrone levels were 15% lower among individuals in the top 25 percent of soy protein intake. In this regard, soy was the exception; No other dietary factors studied affected plasma estrogen levels. Asian women living in Asia (where soy makes up a significant part of the diet) have serum estrogen levels up to 40% lower than Caucasian US women. Furthermore, Chinese and Japanese women living in Asia have a fivefold lower breast cancer risk than do white American women. However, other lifestyle and environmental factors besides soy may play a role in these low numbers.

As noted in a 2000 study in Environmental Health Perspectives, when isolated from the soybean, the phytoestrogen genistein may have both estrogenic and anti-estrogenic properties depending on dose and time of life when received. In some studies, it has been shown to stimulate the growth of human breast cancer cells, though at even higher levels, those found through pharmacology rather than diet, genistein can inhibit growth of such cells Other elements in soy, however, such as saponin and phytic acid, may temper genistein's effects in humans when it is not taken out of the soybean. In 2002, Anna Wu and associates at the Department of Preventive Medicine at the University of Southern California published research in Carcinogenesis showing that women who consumed soy at least once a week in adolescence had a significantly reduced risk of breast cancer, while women with a high soy-consumption in both adolescence and adulthood showed the lowest risk of breast cancer. A June 2003 study in the Journal of the National Cancer Institute found that Japanese women with the highest intake of genistein, as well as those who consumed three or more bowls of miso soup a day, had half the breast cancer rate of those with the lowest intake, and lowest consumption.

As for thyroid problems, in a 2002 study published in Environmental Health Perspectives, authors Daniel Doerge and Daniel Sheehan found in rat studies that genistein consumption reduced the activity of thyroid peroxidase, an enzyme needed for thyroid hormone synthesis, by up to 80%. However, other measures of thyroid function remained normal. The authors concluded that additional factors, such as iodine deficiency among others, are necessary to cause "overt thyroid toxicity." In some cases, infants fed soy formula have suffered from goiter, but the effect is generally reversed by adding iodine supplements or switching the baby to cow's milk. Manufacturers have added iodine to formula since the 1960s to mitigate thyroid effects.

Furthermore, a seven-year study published in the British Journal of Cancer (July 2002) on soy and stomach cancer in Japanese men and women found a significant reduction in the risk of stomach cancer among men with high soy consumption.

Clearly, soy and its phytoestrogenic components can affect human health and not always necessarily for the good. By the same token, there's no need to frighten people away from a source of protein useful in lowering cholesterol that may also reduce the risk of certain cancers.