במאמר משנת 2008 מצביעים חוקרים על הבדלים בכמות הזרע בין גברים שצרכו כמויות גבוהות של סויה, לבין גברים אחרים. ירידה בכמות הזרע (אבל לא בפרמטרים אחרים) הייתה ניכרת במיוחד אצל צרכני סויה כבדים, כאשר משקל עודף החמיר את המצב.

Hum Reprod. 2008 Nov;23(11):2584-90. doi: 10.1093/humrep/den243. Epub 2008 Jul 23.
Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic.
Chavarro JE1, Toth TL, Sadio SM, Hauser R.

Abstract
High isoflavone intake has been related to decreased fertility in animal studies, but data in humans are scarce. Thus, we examined the association of soy foods and isoflavones intake with semen quality parameters.
The intake of 15 soy-based foods in the previous 3 months was assessed for 99 male partners of subfertile couples who presented for semen analyses to the Massachusetts General Hospital Fertility Center. Linear and quantile regression were used to determine the association of soy foods and isoflavones intake with semen quality parameters while adjusting for personal characteristics.
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.

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