The evidence is not strong enough to recommend that men should shun tofu.
Ample evidence in animals demonstrates that isoflavones, with their weak estrogenic activity, can negatively affect the male reproductive system, but evidence in humans is scarce. Boston investigators assessed whether intake of isoflavone-rich soy foods affected sperm quality in 99 men who presented with their partners for evaluation of infertility.
The men completed questionnaires that addressed their intake of 15 soy-based foods during the 3 months before semen analysis. There was an inverse association between soy food intake and sperm concentration that remained significant after adjusting for potentially confounding variables. Results for individual soy isoflavones were similar but did not achieve statistical significance. This inverse association was more pronounced among overweight and obese men and was stronger at the higher end of the sperm concentration distribution. However, intake of soy food and isoflavones was not associated with total sperm count, ejaculate volume, sperm motility, or sperm morphology.
Comment: These results, which have been reported widely by the lay press, reflect — at best — a weak association between soy intake and semen parameters; indeed, the preservation of total sperm count per ejaculate casts doubt on the clinical importance of a modest decrease in sperm concentration. Although we can construct hypotheses about why soy products might affect male reproductive function, this study merely highlights the need for randomized controlled trials. Based on the findings, recommending a change in diet to men who consume large quantities of isoflavones would be premature.
— Robert W. Rebar, MD
Published in Journal Watch General Medicine August 7, 2008
Chavarro JE et al. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod 2008 Jul 23; [e-pub ahead of print].