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.