Daidzein is a soy isoflavone with estrogenic activity present in plant-based food items and health foods and used as an alternative therapy for cancer and cardiovascular diseases. The present study was designed to investigate the effects of daidzein on the cavernosal components, including smooth muscle cells, collagen fibers, and elastic fibers, that are the key structures fundamental for erection.

A total of 30 adult male Sprague-Dawley rats were equally divided into a normal control group, three experimental groups, and one positive control group. The three experimental groups were given daidzein at a dose of 2, 20, and 100 mg/kg body weight daily, and the positive control group received 0.1 mg diethylstilbestrol per animal daily for 90 days. The collagen fibers and elastic fibers in the corpora cavernosa were measured using histochemical or immunohistochemical techniques, and their relative contents were evaluated quantitatively or semiquantitatively.

The relative content of collagen fibers in the corpus cavernosa in rats treated with low-dose daidzein (2 mg/kg) was not significantly different from that of controls, as was the case for the smooth muscle and elastic fiber content (all P >0.05). However, the relative content of the collagen fibers was significantly increased in rats treated with a medium dose (20 mg/kg) and a high dose (100 mg/kg) of daidzein. The smooth muscle cell and elastic fiber content was reduced significantly compared with that of the controls (all P <0.01). Similar alterations were observed in the diethylstilbestrol-treated rats.

These results suggest that daidzein, if ingested in a relatively large amount, could induce histopathologic alterations in the penile cavernosal structures characterized by an increase in the collagen content and a reduction in smooth muscle cell and elastic fiber content, which might be suggestive of erectile dysfunction.