Myth #4: Soyfoods contain estrogen and men who eat them may experience feminization or even impair their fertility.
|Soyfoods do not contain estrogen and the preponderance of clinical evidence indicates soyfoods do not feminize men. Further, soyfoods potentially offer men several health benefits.|
It is ironic that there are claims that soyfoods exert feminizing effects when these foods have been consumed for centuries by Asian men without any such effects being identified. Nevertheless, it is important to address all concerns as nonsensical as they appear to be, by relying upon the findings from well-designed studies.
The first misconception about soyfoods is that they contain estrogen, the female sex hormone. Estrogen is produced primarily by the ovaries in premenopausal women whereas in postmenopausal women circulating estrogen levels decrease markedly because ovarian production largely ceases. The estrogen that is produced after menopause is synthesized mostly by adipose tissue via the enzyme aromatase, which acts upon the hormone androstenedione, which is produced by the adrenals. Often not recognized however is that men also produce estrogen throughout life and that estrogen has an important physiological role; in fact, estrogen levels in older men are actually higher than they are in postmenopausal women [1, 2]. The estrogen that is produced is occurs via the aromatization of circulating androgens in tissues such as the adipose, skin and muscle .
Although soyfoods don’t contain estrogen they are a rich source of isoflavones, a group of naturally occurring plant chemicals classified as phytoestrogens (plant estrogens). There are many different types of phytoestrogens in foods (such as lignans in flaxseed) but soy is essentially unique among commonly consumed foods in providing large amounts of isoflavones . Despite being classified as phytoestrogens, isoflavones and estrogen are not equal, they have different chemical structures and although they share some properties in common they are also very different. That these two molecules are different is not at all surprising. Small differences in chemical structure can result in very different physiological effects. In fact, isoflavones are more appropriately classified as selective estrogen receptor modulators (SERMs) (mixed estrogen receptor agonist/antagonists) rather than as phytoestrogens.
SERMs have tissue selective effects; i.e., they can have estrogen-like effects in some tissues in which estrogen receptors are present, antiestrogenic effects in others and still in others, no effects at all despite the presence of estrogen receptors. There is no class effect of SERMs in that the properties of each SERM need to be determined individually .
Claims that soyfoods exert feminizing effects are partially based on the mistaken premise that these foods lower testosterone levels. However, although some studies in rodents have shown in response to large amounts of soy or isoflavones circulating testosterone levels decrease, this does not occur in men. A meta-analysis of the clinical studies recently published in the journal Fertility and Sterility showed conclusively this to be the case as there were no effects on total or free testosterone levels or levels of dihydrotestosterone – the biologically active metabolite of testosterone . Similarly, the human evidence shows that soy does not raise estrogen levels – nine studies have examined this issue .
Despite the large populations of soyfood-consuming countries concern that soyfoods might impair male fertility are occasionally raised. This concern is based primarily on the results of one small pilot (preliminary) epidemiologic study which found men who consumed soyfoods were more likely to have lower sperm concentrations although the findings have no implications for fertility because in all men sperm concentration was within the normal range . Nevertheless, because so little of this type of research has been conducted it is important to point out of some of the limitation of this study. For example, other than soy intake, no dietary information was collected. Also, much of the decreased sperm concentration noted in the soy consumers was because ejaculate volume tended to increase in these men. It seems unlikely that soy could actually increase ejaculate volume.
In any event, epidemiologic studies aren’t designed to establish cause and effect relationships; they are designed to show correlations but not causation. In contrast, clinical studies are designed specifically to show one factor causes another. Consequently, clinical studies are the gold standard for reaching conclusions. Three clinical studies have examined the effects of either soyfoods or isoflavone supplements on sperm and semen parameters and none have found any adverse effects [9-11]. Furthermore, there is actually a case report published in a medical journal of a man with low sperm count unable to father a child who after being given isoflavones for six months experienced an improvement in sperm concentration and motility that resulted in a successful pregnancy .
Finally, not only should men not fear soyfoods but they would be well-advised to make them a part of their diet as they may offer important benefits. For example, the epidemiologic data indicate soyfood consumption reduces risk of developing prostate cancer by approximately 30 percent . Furthermore, because they contain isoflavones, soyfoods may inhibit the spread of prostate tumors [14, 15]. Also, soyfoods are very heart-healthy. In addition to providing high-quality protein and being low in saturated fat, the protein itself modestly lowers blood cholesterol levels .
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