Update on the soy controversy
The recent increase in the popularity of soy has not been without some dissenting voices, to the point where some consumers don’t know what to think. In this article we’ll sum up the facts and set the record straight!
Soy and breast cancer
Prevention
A study of the results of a number of other studies, or meta-analysis, was published in April 2006, showing that women who consume a lot of soy have a lower risk of developing breast cancer (1). Soy products appear to be most effective against breast cancer when they are consumed before adulthood. It is therefore a good idea to serve soy products, such as ready-to-drink soy beverages, to young girls.
Treatment
On the other hand, the effects of soy on existing breast cancers are not well understood. Many breast cancers are of a type known as “hormone-dependent,” which means that tumors seems to grow faster when exposed to hormones, particularly estrogens.
Soy contains isoflavones, whose molecular structure resembles that of estrogen. Although isoflavones are considered weak estrogens, specialists are concerned that they may interfere with breast cancer recovery. However, the studies that gave rise to this concern were carried out in vitro or on animals, not on cancerous human cells. In order to clear up this controversy, future studies should determine soy’s effect on cancerous human cells (2) in the meantime, women who have or have had breast cancer are advised to consume soy products only in moderation (one serving of soy products per day) and to avoid taking soy isoflavone supplements, as these have been extracted from their natural medium and concentrated.
In brief: For breast cancer prevention, consumption of soy products appears to be beneficial. From a curative perspective, however, it is currently recommended that women who have or have had breast cancer consume soy products only in moderation.
Soy and the thyroid gland (3)
Hypothyroidism
Soy contains isoflavones. These isoflavones have a similar structure to thyroid hormones and may interfere with an enzyme involved in the production of thyroid hormones. However, studies involving post-menopausal women have shown no negative effect on the thyroid gland from consumption of soy products. The only cases of hypothyroidism caused by soy involved people with iodine deficiencies. In such cases, iodine supplementation is sufficient to resolve the problem. Within the general population, vegans are the most at risk, since they exclude primary sources of iodine such as milk products, fish, and often iodized salt from their diet. Vegans may consider taking an iodine supplement of 75 to 150 mcg per day.
Taken at the same time, soy may reduce the absorption of thyroid medication (synthyroid). If you are taking medication for hypothyroidism, do not consume soy products within three hours of taking your medication.
Thyroid cancer
Consumption of soy products has been associated with a reduction of thyroid cancer risk. However, further study is needed to confirm these findings.
In brief: For people whose iodine intake is ensured through nutritional variety (a conventional diet), the consumption of soy products is completely safe and may even provide some protection against thyroid cancer.
Soy and cardiovascular disease
Because soy contains polyunsaturated fats, replacing some meat and high-fat milk products with soy products may contribute to cardiovascular health (4). However, adding soy products without changing bad eating habits (particularly an excess of saturated fats) will not provide any cardiovascular health benefits.
Soy and dementia
A myth has been circulating about soy’s supposed adverse effects on cognitive functioning since the release of a study showing a link between consumption of tofu and a diminution of cognitive abilities (5). A number of other studies have shown the opposite, that soy is associated with improved cognitive abilitities (6). One of them, conducted among Seventh-Day Adventists, demonstrated a lower incidence of dementia than among the population in general. Soy is an important part of the diet of Seventh-Day Adventists.
Conclusion
Based on the evidence of the thousands of soy studies that have been published to date, it is absolutely safe to consume 2 to 3 servings of soy products per day. One serving of soy is equal to 1 cup of a soy beverage or about ½ cup of tofu, tempeh, soybeans, or textured soy protein. Only individuals who have or have had breast cancer should limit their consumption of soy products.
References
1. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006 Apr 5;98(7):459-71.
2. Messina M, McCaskill-Stevens W, Lampe JW. Addressing the soy and breast cancer relationship: review, commentary, and workshop proceedings. J Natl Cancer Inst. 2006 Sep 20;98(18):1275-84.
3. Duncan AM, Dillungham BL. Soy & Thyroid FuncTion: Safety issues examined. The Soy Connection. 2006 (summer);14(3):1-3.
4. Sacks FM, Lichtenstein A, Van Horn L, Harris W, Kris-Etherton P, Winston M. Soy protein, isoflavones, and cardiovascular health: a summary of a statement for professionals from the American Heart Association Nutrition Committee.
Arterioscler Thromb Vasc Biol. 2006 Aug;26(8):1689-92. Review.
5.Guo C, Wilkens LR,Maskarinec G, Murphy S. Examining associations of brain aging with midlife tofu consumption.
J Am Coll Nutr. 2000 Aug;19(4):467-8.
6. Lee YB, Lee HJ, Sohn HS. Soy isoflavones and cognitive function.
J Nutr Biochem. 2005 Nov;16(11):641-9. Epub 2005 Aug 10.
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