Isoflavones (daidzein, genistein and glycitein), found mainly in soy beans and soy-derived products, are plant-derived compounds with estrogenic effects. While animal studies have linked the high consumption of isoflavones with infertility, there had been little evidence of this effect in humans.
Now research published in the journal Human Reproduction states that men who eat an average of half a serving of soy foods a day have lower concentrations of sperm than men who do not eat soy foods. The association was particularly marked in men who were overweight or obese, the study found.
In the largest study in humans to examine the relationship between semen quality and phytoestrogens (plant compounds that can behave like the hormone, oestrogen), Dr Jorge Chavarro, a research fellow in the department of nutrition at Harvard School of Public Health, Boston, USA, and his colleagues found that men who ate the most soy food had 41 million sperm per millilitre less than men who did not consume soy products. (The "normal" sperm concentration for men ranges between 80-120 million/ml).