Exposure to the Phytoestrogen Daidzein Attenuates Apomorphine-Induced Penile Erection Concomitant with Plasma Testosterone Level Reduction in Dose and Time-Related Manner in Adult Rats

Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence

“…As already discussed, no such decrease in T occurs in men (108). Thus, it would appear that the rodent findings do not provide a reasonable basis for raising a concern that soy food consumption increases the risk of developing ED (97,121,122). Of potential relevance, and in support of this conclusion, is the recent finding in prostate cancer patients undergoing androgen deprivation therapy that exposure to high dose isoflavones (160 mg/d) was neither beneficial nor harmful in terms of libido or erectile function (132).…”

“…Consequently, concern has been raised that isoflavones have feminizing effects in men and adversely affect male reproductive health. Findings that support this concern include those from a pilot epidemiologic study that linked soy intake with lower sperm concentration among infertile men (10), a case report linking excessive isoflavone intake with raised estrogen levels and gynecomastia (8), limited clinical research showing soy intake lowers blood T levels (88,107), and rodent research suggesting isoflavone (daidzein) exposure can lead to ED (97,121,123,133). However, the clinical evidence overwhelmingly indicates that there is essentially no basis for concern.…”

“…Finally, it should be noted that in New Zealand rabbits, a 1000-fold lower daidzein dose (100 mg/d or approximately 30 mg/kg bw) than was used in the previously discussed rat studies (97,121,122) significantly potentiated norepinephrine-induced antierectile contraction of the corpora cavernosa. In addition, in organ bath experiments, relaxant responses to acetylcholine, nitroglycerin, and nitrergic transmission were significantly attenuated compared with the control response (133).…”

“…Claims that isoflavones can lead to ED are based primarily on research in male Sprague-Dawley rats conducted by Chinese investigators (97,121,122). In their most relevant study, when 9-10-week-old rats were orally gavaged daily with daidzein at a dose of 20 or 100 mg/kg body weight (bw) for 90 d, the relative content of the collagen fibers in the corpora cavernosa (corpus cavernosum penis is one of a pair of sponge-like regions of erectile tissue that contain most of the blood in the male penis during erection) was significantly increased, and the smooth muscle cell and elastic fiber content significantly reduced compared with that of the controls (122).…”

“…Thus, there is little justification for extrapolating results from rodents given daidzein to humans given soy foods or mixed isoflavones. Furthermore, and of critical importance, is the finding that in Sprague-Dawley rats the two doses (20 and 100 mg/kg bw) of daidzein that affected the corpora cavernosa also decreased circulating T levels by more than 50% (97). It is this decrease that is thought to account for effects on the corpora cavernosa (122).…”