I've seen many articles cautioning men about consuming too much soy. It seems soy can have a feminizing effect.
Here's one. T-Nation.com | Soy is Still Bad Protein
Science has shown soy, more importantly its phyto-estrogen components, namely genistein, has the ability to attach to estrogen receptor sites and through transcription, act as female hormones such as estradiol. This, in some cases, can have benefits so it's not strange that soy would receive some well-deserved attention. The problem with this attention is that individuals who have no need of soy, and even some to which soy could be hazardous, have started using it. Science is now beginning to see what this "benign" protein can do, though.There are many other articles with similar warnings. So no soy for me!The next major concern is genistein's estrogenic and anti-androgenic effects on adult male animals and humans. This effect was shown clearly in a study on adult male reproductive tracts.(8) In intact adult male mice, genistein (2.5 mg per kg of body weight per day for only 9 days) reduced testicular and serum Testosterone concentrations and pituitary LH-content. These results suggest that genistein — in doses comparble to those that would exist in a soy-based diet — induced typical estrogenic effects.
A second study showed plasma Testosterone and androstenedione levels were significantly lower in the animals fed a phytoestrogen-rich diet compared with animals fed a phytoestrogen-free diet.(9) These results indicated that consumption of dietary phytoestrogens over a relatively short period can significantly alter plasma androgen hormone levels.
In a study of Japanese men, total and free Testosterone concentrations were inversely correlated with soy product intake.