Apart from the beneficial health effects of soy isoflavones related to various hormone-related health conditions in both men and women, there are also some concerns regarding the safety of soyfoods and soy isoflavones. There has been some specific concern whether consumption of isoflavones might result in adverse effects on men’s fertility, such as modified testosterone levels and reduced semen quality. This document has been made to address this topic by presenting an overview of published literature.
Habitual diets rich in phytoestrogens like isoflavones have been linked to a decreased incidence of hormone-dependent conditions such as breast cancer and menopausal complaints [1-5]. Despite the impressive scientific body of evidence that has been established over the past decades, some concerns have been raised over the estrogen-like effects of isoflavones that may exert a feminizing effect in men including reduced fertility. These concerns are based on published reports linking soy consumption in men to feminization  and reduced fertility [7-9] and are fuelled by studies indicating that isoflavones may affect sperm quality in vitro [10-14] and might impair reproductive health in animal studies [15-20]. However, animal studies mainly involved high soy diets leading to high serum levels of isoflavones and equol that far exceed those reached in humans consuming a typical Asian diet [20-21]. It is further well known that the negative effects of isoflavones on the reproductive system such as erectile dysfunction is due to a much higher rate of isoflavone metabolism in rodents compared to the human metabolism as well as the excessively high amount of isoflavones to which the animals were exposed .
In 2002, a report was published and presented at the Fourth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease . In three intervention studies that were reviewed it was shown that in men consuming soyfoods or supplements containing 40–70 mg/d of soy isoflavones, minor effects on plasma hormones or semen quality were observed. The author concluded that these data do not support concerns about effects on reproductive hormones and semen quality. The intake levels from these studies are similar compared with those found in typical Asian diets .
An early review on the safety aspects concerning soy supplementation and men, published in 2003, presented an overview of clinical studies on soy isoflavones . In this review, several short and long term studies on the effects on men were evaluated. In one study, six healthy male subjects were given an unrestricted diet supplemented with an intake of approximately 137 mg total isoflavones (aglycone equivalents). No significant changes in the metabolic pathways of soy isoflavones and no adverse effects were observed. In another longer-term study, no effects on endocrine function, including plasma estradiol concentrations, testicular volume and semen quality were reported in 14 healthy male volunteers consuming 40 mg aglycone isoflavones/d for 2 months. In a recent study, the effect of SoyLife 40 supplementation (116 mg aglycones/d) on the cognitive function of healthy men was investigated . In this 12-week clinical study, the isoflavone supplements were well tolerated by the subjects and there were no adverse effects were reported.
The concerns regarding possible feminizing effects of soy consumption in men are mainly based on three recent human studies [6-7, 26]. Two studies have described the possible feminizing effects of soy in two male individuals consuming excessive amounts of soy (as many as 14-20 servings per day) [6, 26]. Soyfoods consumption of this degree is considered abnormal and can, like nearly any other food, be expected to have negative effects on health. A more recent epidemiologic study investigated the relationship between soy consumption with fertility in sub-fertile men . Although the soy consumption was associated with a reduced sperm concentration, there was no effect of soy intake on sperm morphology and motility. It should be noted that these results are in contrast with another similar study that reported that higher isoflavone intake was associated with levels of good sperm DNA integrity, sperm count and sperm motility . Furthermore, one of the main weaknesses of the epidemiological study is that the researchers only looked at soy intake and no other dietary information was collected. It has been reported that different dietary factors such as supplementation with zinc and folic acid increase sperm count in fertile and infertile men, whereas a diet high in omega-6 fatty acids can have the opposite effect [28-29].
Furthermore, the concern regarding male fertility is based on the hypothesis that environmental estrogens might play a role in the observed decline of male fertility over the past decades [30-33]. The negative association between soy consumption and sperm concentration reported in the epidemiological study by Chavarro et al. (2008) , however, appear to follow the general trend of declining male fertility and this trend was not taken into account.
In contrast to the three human studies investigating association between soy isoflavone consumption and male fertility , a recently published (2010) review and meta-analysis discussed the clinical evidence related to the effect of soy isoflavones on the reproductive system in men, including the above mentioned reports. It was concluded that neither soyfoods nor isoflavones affect semen or sperm, or the levels of the male sex hormone, testosterone. It was generally concluded that soyfoods do not exert feminizing effects and that there was no clinical evidence that men’s fertility is negatively affected by soyfoods or isoflavones. It is generally advised that men can feel confident in making soy a part of their diet and that this will not compromise their virility or reproductive health.
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