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Soy: Good or Bad?

Does the soybean help the body, or create more problems?

soy bean
A question many of my patients ask is: is soy good for me, or not? This is an especially important question for the woman with a history of breast or uterine cancer.

The fact is soy is thought to contain ‘phytoestrogens’. The term phytoestrogen refers to the idea that plant chemicals can act in similar ways as an estrogen molecule. Interestingly, the fellow who coined this phrase is Dr. Herman Aldercreutz of the University of Helsinki. Dr. Aldercreutz himself admits that coining the term phytoestrogen was one of his greatest regrets over his career, for this term has lead the medical community to ascribe estrogenic activity to soy that it really does not possess.

It is true that soy isoflavones can act like weak estrogens. Interestingly, this is how tamoxifen, a breast cancer drug that serves to protect estrogen sensitive cells, works as well by decreasing the effect of aggressive estrogens made in the body. The weak ‘estrogens’ in soy actually block the more aggressive, cancer-causing estrogens – as such, they lower the effect of cancer-causing estrogens. Additionally, soy isoflavones also help shut off genes that can cause cancer by acting as methylators. Methylators are molecules which can lock up cancer genes so they are not expressed.

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One case-controlled study from 2008 looked at over 24,000 Japanese women and found that those with the highest soy isoflavone level had the lowest cancer rates. Breast cancer in Western countries is much higher than Asia – where isoflavone levels is about 0.5% of that in the Asian countries.

When consumed in excessively large amounts, soy may inhibit thyroid function, lower mineral absorption, or even create allergic response in an individual.  Eaten in reasonable amounts, this is not typical.  

In our clinic, we highly recommend natural and fermented forms of soy, such as edamame, natto, miso, and tempeh. While some soy milk and tofu may not be harmful, these sources of soy are more processed forms that may increase sensitivity in some individuals. Persons who are blood type A seem to do fairly well with soy in the diet, while blood type O individuals seem to be more reactive.


Iwasaki M, Inoue M, Otani T, Sasazuki S, Kurahashi N, Miura T, Yamamoto S, Tsugane S; Plasma isoflavone level and subsequent risk of breast cancer among Japanese women: a nested case-control study from the Japan Public Health Center-based prospective study group. J Clin Oncol. 2008 Apr 1;26(10):1677-83. Epub 2008 Mar 3.

Peter Bongiorno is a naturopathic doctor and author of Healing Depression: Integrated Naturopathic and Conventional Treatments.


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