There’s some question as to whether soy consumption affects male reproductive parameters such as sperm count and testosterone levels, some studies answering in the positive, some not. But there are enough studies showing an effect that it’s cause for concern. For instance: Inverse Association of Soy Product Intake With Serum Androgen and Estrogen Concentrations in Japanese Men
Serum estradiol concentration was significantly inversely correlated with soy product intake (r = -0.32, p = 0.009), and serum estrone concentration was nonsignificantly inversely correlated with soy product intake (r = -0.24, p = 0.05) after controlling for age, body mass index, smoking status, and ethanol intake.
However, the study found only borderline effects on T levels. Still, borderline effects are enough to make me wary.
Hormonal Effects of Soy in Premenopausal Women and Men. This study found little effect on hormones or sperm counts.
But this one found a large effect on sperm count: Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic
RESULTS There was an inverse association between soy food intake and sperm concentration that remained significant after accounting for age, abstinence time, body mass index, caffeine and alcohol intake and smoking. In the multivariate-adjusted analyses, men in the highest category of soy food intake had 41 million sperm/ml less than men who did not consume soy foods (95% confidence interval = –74, –8; P, trend = 0.02). Results for individual soy isoflavones were similar to the results for soy foods and were strongest for glycitein, but did not reach statistical significance. The inverse relation between soy food intake and sperm concentration was more pronounced in the high end of the distribution (90th and 75th percentile) and among overweight or obese men. Soy food and soy isoflavone intake were unrelated to sperm motility, sperm morphology or ejaculate volume.
CONCLUSIONS These data suggest that higher intake of soy foods and soy isoflavones is associated with lower sperm concentration.
An animal study found large effects on T:
Plasma testosterone and androstenedione levels were significantly lower in the animals fed the phytoestrogen-rich diet compared with animals fed the phytoestrogen-free diet. However, there were no significant differences in plasma LH or estradiol levels between the diet groups. Testicular StAR levels were not significantly different between the phytoestrogen-rich vs the phytoestrogen-free fed animals. These results indicated that consumption of dietary phytoestrogens resulting in very high plasma isoflavone levels over a relatively short period can significantly alter body and prostate weight and plasma androgen hormone levels without affecting gonadotropin or testicular StAR levels.
All in all, I would (and do) avoid soy consumption.