Cancer-Preventive Soy Isoflavones Appear Harmless to Male Reproduction

Dietary phytoestrogens, such as genistein, the principal soy isoflavone, are believed to play an important role in cancer chemoprevention, leading to increasing use as part of the Western diet.

The estrogenic potency of phytoestrogen may trigger many of the biological responses that are evoked by the physiological estrogens. Exposure to estrogen during early, critical periods of development has been associated with alterations to the endocrine system and reproductive tract.

Effects Evaluated in Rodent Study

To identify possible effects of soy isoflavones on male fertility, IITRI's Dr. Ali Faqi evaluated reproductive parameters in Wistar-Unilever rats receiving dietary exposure to a characterized mixture of isoflavones containing 45% genistein, 23% daidzein and 4% glycitein. He presented his findings in a poster display at the American College of Toxicology meeting November 4-7, 2002, in Washington, D.C.

Beginning at 10 weeks of age, rats received chronic dietary exposure to isoflavones (2000 or 200 mg/kg diet) for 13 months. Control rats received unsupplemented basal diet (Teklad 4% Fat Rat/Mouse Diet) only.

No Toxicity Observed

Dietary exposure to isoflavones induced no gross toxicity or alterations in body weight gain. Absolute and relative weights of the testis and epididymis in both groups receiving isoflavones were comparable to those of controls.

Similarly, no significant differences between groups were identified in evaluations of spermatid count, sperm production or sperm morphology. Testicular morphology was comparable in all study groups.

Chronic dietary administration of a characterized soy isoflavone mixture conferred modest, but statistically significant protection against the induction of prostate cancer in male Wistar- Unilever rats.

These data demonstrate that dose levels of soy isoflavones that confer protection against prostate carcinogenesis in rats have no adverse effects on sperm quality or on the morphology of male reproductive organs.

Assuming that the results in this rodent model are an adequate predictor of human responses, it appears unlikely that male reproductive function will be adversely impacted by long-term administration of these agents for cancer prevention or other purposes.