As the American diet turns towards the perception of more natural and “healthy” foods, a surge in soy products has taken hold in the market. Soy is touted as a superfood full of protein, fiber, and antioxidants, and able to do everything from lower cholesterol to fight cancer. However, not all the physiological properties of soybeans are positive. Compared to the number of outlets proclaiming the benefits of soy, relatively few sources discuss the side effects of a soy-rich diet.

Unfermented soy products like soy milk and tofu form the bulk of American soy consumption. In Asian countries, which consume higher amounts of soy overall, the predominant form is fermented as found in soy sauce and miso. The fermentation process is significant because it removes much of the biologically active phylates and isoflavones from the soy.

Phylates are a form of insoluble fiber, and in low quantities are beneficial because they can absorb low density lipoprotiens and therefore lower bad cholesterol levels. They are also known to reduce risks of gastrointestinal cancers. However, in higher quantities they can also block the absorption of other nutrients, leading to vitamin and mineral deficiencies. In particular, calcium, magnesium, copper, and zinc are prone to blockage by phylates, and this can paradoxically increase the risks of osteoporosis that soy is supposed to prevent.

Soy is purported to prevent osteoporosis because of its isoflavone content. Isoflavones are a collection of complex chemical compounds which are manufactured by plants, but physiologically similar to human proteins. One type of isoflavone found in large quantities in soy is called genistein, and in humans this protein binds with some affinity to estrogen receptors. This effect is what gives soy protein the ability to fight menopausal and premenstrual symptoms, and potentially prevent osteoporosis as mentioned above. On the contrary, overconsumption of genistein can lead to the same problems associated with high estrogen levels, namely increased risks of breast, uterine, and ovarian cancer. In males, the weak estrogen activity has translated into a reported reduction in the risk of prostate cancer. High intake has been suggested to suppress testicular cell lines, although this has not been proven. Perhaps the most widely accepted consequence of a high soy diet is an impact on thyroid function. Hypothyroidism and autoimmune disorders such as Hashimoto’s thyroiditis have been linked to excess soy intake in several animal studies.

Although the benefits of soy protein are well documented, no substance can be considered a panacea. Several problems can arise when soy is ingested in excess. Most dietitians recommend 25 grams of soy protein per day, and even the American Heart Association does not recommend exceeding 50 grams per day. The public would do well to adhere to these guidelines, to avoid any potential complications of soy toxicity.

References

R DIVI, H CHANG, D DOERGE (1997). Anti-Thyroid Isoflavones from Soybean Biochemical Pharmacology, 54 (10), 1087-1096 DOI: 10.1016/S0006-2952(97)00301-8

Kumi-Diaka J, Rodriguez R, Goudaze G. Influence of genistein (4′,5,7-trihydroxyisoflavone) on the growth and proliferation of testicular cell lines. Biol Cell. 1998 Jul;90(4):349-54.

M. Lakshman, L. Xu, V. Ananthanarayanan, J. Cooper, C. H. Takimoto, I. Helenowski, J. C. Pelling, R. C. Bergan (2008). Dietary Genistein Inhibits Metastasis of Human Prostate Cancer in Mice Cancer Research, 68 (6), 2024-2032 DOI: 10.1158/0008-5472.CAN-07-1246

Vucenik I, Shamsuddin AM. Cancer inhibition by inositol hexaphosphate (IP6) and inositol: from laboratory to clinic. J Nutr. 2003 Nov;133(11 Suppl 1):3778S-3784S.

W.-F. Chen (2004). Genistein Enhances Insulin-Like Growth Factor Signaling Pathway in Human Breast Cancer (MCF-7) Cells Journal of Clinical Endocrinology & Metabolism, 89 (5), 2351-2359 DOI: 10.1210/jc.2003-032065