“Soy it isn’t so:” Why you needn’t fear this healthy protein source

Few, if any, foods have been linked to more health benefits than soy. Yet they have often been a source of concern for couples that are trying to get pregnant or even some women that are currently pregnant. Ironically, the reasons for concern were based upon theories that have been essentially disproven. Here’s what we know.

Soybeans are essentially the only food that contains significant amounts of estrogen-like compounds called isoflavones.  One gram of soy protein contains about 3.5 mg of isoflavones or about 4% by weight. The average serving size among Japanese adults is 10 g per day which is about a ½ cup. The isoflavones—named genistein and daidzein—are referred to “estrogen-like” because they can act as estrogens at some biological sites and have little or no activity at others. Although it is these estrogen-like properties that are believed to convey heart healthy and cancer fighting properties to them; it is these same effects have fueled the fear that soy can reduce male fertility or trigger a hormone imbalance in pregnant women.

The myth that isoflavones can cause infertility can be traced back to the 1950’s and 1960’s when Australian farmers noted that sheep that ate a diet rich in red clover had developed fertility problems. Subsequent studies on rodents only created more confusing data in that it was inconsistent. Although extreme case reports from individual patients—like the 60 year old man [G1] that drank 3 quarts of soy milk per day for 3 months—have fueled concerns; the actual studies on people have been very reassuring.

Men are considered more susceptible to estrogen imbalance so much of the fertility related research has focused on them. Recently, a large review [G2] was published as a summary and conclusion of the 47 clinical studies that have been performed. They found that neither soy foods nor soy containing supplements had any influence on male hormones or reproductive function. This was after another large clinical review [G3] also had reassuring findings. A more recent study  [G4] deliberately compared the semen analysis for men at baseline and then on either a soy protein supplement or a milk protein supplement. Again there no apparent negative effect. So we can now conclusively say that soy does not contribute to male infertility.

When it comes to women’s fertility and health, the data is even more reassuring. One study compared a protein boost of either a soy supplement or a meat supplement in 20 to 30 year old women. They found that the soy supplement actually improved the balance between the key reproductive hormones FSH and LH suggesting that it may actually improve fertility. By contrast, the meat supplement found a worsening of this key ratio tilting the ratio closer to that associated with PCOS.

In summary, soy beans are a very healthy source of dietary protein that may also have other benefits beyond the nutrients that they provide. At the very least, it is time that we put to rest the unwarranted concerns raised by outdated ideas.

8 thoughts on ““Soy it isn’t so:” Why you needn’t fear this healthy protein source

  1. Personally, I am a little concerned about the inhibition of thyroid peroxidase-catalyzed reactions which are essential to thyroid hormone synthesis and caused by genistein and daidzein, the isoflavone components in soy protein.

    Obviously moderation in consumption is key, as is finding a source of soy which is not genetically modified and hasn’t been overly contaminated with organophosphates.

    Divi RL, Chang HC, Doerge DR, Anti-thyroid isoflavones from soybean: isolation, characterization, mechanisms of action. Biochem Pharmacol 1997 Nov 15;54(10):1087-96 http://www.ncbi.nlm.nih.gov/pubmed/9464451

    1. Dear Anne,
      The concerns that you pose are reasonable. In fact, there is a study just published demonstrating that soy can tip someone with borderline thyroid function into the “hypothyroid zone.” In the larger scheme of things however, this is a relatively small risk. In fact, on the flip side; MANY studies have shown that replacing at least some of your dietary animal protein with a soy products can profoundly reduce your health risks from a variety of sources. So please continue the dialogue and continue to follow the research.
      Best thoughts,

  2. Hi Dr. Greene. I just love your book the Perfect Hormone Balance for Pregnancy. I’ve read with each of my pregnancies. I am currently pregnant with my third and am wondering which Soy supplements you might recommend. I am not a fan of soy beans, but would like to use soy as an alternate protein during pregnancy.
    Thank you!

    1. Hello Jackie,
      Sorry that I did not respond to this sooner…had a bit of chaos as the fertility clinic where I work was recently sold to a new owner. Hope that your pregnancy is going well. Regarding your question about soy supplements–I’m not a fan. They are an attempt to replicate the ingredients of soy in order to create the benefits but the research typically fails to support their use. There are many soy products available today which can provide you with dietary soy that you should find palatable. I would encourage you to experiment with those instead. My concern with soy supplements is that their absorption and contents may not convey the benefits of dietary soy; especially during pregnancy. Some studies suggest that soy supplements provide spikes in blood levels of phytoestrogens that are hundreds of times higher then soy is consumed as food. This may not be a problem but we just don’t know. Hope this helps.
      Best thoughts,

  3. Anne,
    Not sure what reference you were searching for and I fear that it might be a bit more taxing for me to find it right now if I did. The IVF center that I work for was recently purchased by California IVF Fertility Center Inc. Unfortunately, my computers and much of my data stayed with the old owner. Please forgive this inconvenience.
    Best thoughts,

  4. I know this is not the purview of your blog, but as a mother of vegan teen-age boys I have read that too much soy can interfere with proper growth and development of boys because of its estrogen-like properties. Has your research on soy ever addressed the development of post-gestation individuals (adolescents)?
    Also, as a former high school classmate, I have to comment on how well you write!!! You must have gotten a good education at BHS.

    1. Hey Becky,
      You are far too kind. Thank you for your reading the blog and even more for providing feedback. Rest assured, your sons are safe. I have thoroughly reviewed this subject for my own health as well. I have also been vegetarian since 1994. Veganism is a bit more rigorous than my lacto-ovo choices but it can still be an excellent and healthy lifestyle. Always great to hear from a former classmate. Please feel free to email directly if you’d like to discuss this further.
      Very best thoughts,

      Robert Greene, MD, FACOG
      CNY Fertility Center

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