Multi-generational dietary exposure to the phytoestrogen genistein does not alter mating behavior in young adult male rats

Flynn, K. M.; Delclos, K. B.; Newbold, R. R.; Ferguson, S. A.

Genistein is a hormonally active isofalvone found in soy products. Estimates of human isoflavone exposure range from 0.1 mg/kg/day for adults on a Western diet to 7.5 mg/kg/day for infants consuming soy formula. Genistein has estrogenic activity; adverse effects on animal reproductive systems have been reported, though there are few data on behavioral effects. Here, male and female rats were fed soy-free diets containing 0, 5, 100, or 500 ppm genistein (approximately 0, 0.4, 8, and 40 mg/kg/day for an adult rat) beginning on postnatal (PND) 42 and continuing through mating on PND 70. Two generations of offspring continued on the same diets throughout (F1 and F2), F3 changed to the 0 ppm diet at weaning, and F4 only consumed the 0 ppm diet. From PND 40-90, male offspring (n=4-5/group) were assessed weekly in the presence of a hormonally-primed female for frequency and latency of mounts, intromissions, ejaculations, and post ejaculatory intervals. Weekly data were analyzed by 3 way ANOVA with treatment, week, and generation as factors; mean values per animal over the 6 week period and after maturity (weeks 4-6) were analyzed by 1 way ANOVA. There were no treatment effects on any measure in any generation; however, there were week effects, indicating improved sexual performance with maturity. These more extensive analyses of data from our preliminary report (Flynn, et al., SFNAbs., 1999) with data from subsequent generations indicate that there are no adverse effects on mating behaviors in young adult male rats following long term multi-generational consumption of genistein at doses approximating human exposure. These data do not eliminate the possibility of effects in older animals nor of effects on reproductive system physiology. Additional studies are underway with other estrogen mimics which might alter mating behaviors in rodents.