A study released in the July 23rd edition of Human Reproduction shows that soy products can have an impact on sperm concentration in human males. For those of you who are thinking that your sperm concentrations are home free because you don't eat tofu, keep in mind that soy is a key ingredient in many food products ranging from Odwalla Protein Shakes to a number of diet bars and energy bars. Soy and soy isoflavones are used in foods you would never imagine.
But before you fall into a soy-induced sperm panic, this study did not investigate whether there was a direct connection between soy ingestion and infertility--it merely investigated whether soy ingestion has an impact on human sperm. And if you are worried that soy has made you infertile, take comfort in knowing that the population of Asia has pretty much exploded over the past century, with soy products making up a much larger part of the average diet than they do in the west.
One of the reasons why soy products might not have the same impact on the sperm of Asian men as on American men is because Asian men haven't tended to be overweight--or not before McDonalds crossed The Great Wall. This study found that soy had its greatest impact on the sperm concentrations of men who are overweight. This makes sense, given that body fat produces estrogens. It could be that the xenoestrogens in soy combine or interact with the estrogens in body fat to decrease sperm concentration. This interaction might be greatest in men who are overweight.
As for why soy might decrease sperm concentration in human males, the isoflavones found in soy products have estrogenic activity. In other words, they can act like estrogens when consumed by humans. While the estrogenic activity of soy products is thought to be much weaker than the estrogen produced by the human body, the isoflavones from soy have been found to bind strongly to certain estrogen receptors in human membranes. This would not be so good for males, unless you are a man who prizes his softer side.
As for giving up soy and soy products, I can't see where the authors of this study are suggesting this or even think it is a good idea, unless you are an overweight male who is having trouble conceiving and get the nod from your private physician. While I won't be going on any tofu-eating rampages myself, I see nothing here to rock the foundations of my own uneducated perspective, which is most things in moderation.
While I did not crunch the numbers and analyze the methodology, I found this study to be well designed and it did appear to be competent--something I can't say about the majority of research studies I read in medicine, psychology or human sexuality. To access the article yourself, click on the title: Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic.
