This study aimed to analyze the associations between urinary genistein, daidzein, and equol concentrations and semen parameters in Japanese men exposed to high levels of isoflavone in their diet. Between September 2020 and March 2021, men seeking fertility treatment (n = 157) at Sanno Hospital (Tokyo, Japan) and Tsukuba Gakuen Hospital (Ibaraki, Japan) provided spot urine and semen specimens on the same day. Sperm concentration, sperm count, and total sperm motility were quantified. Urinary isoflavones were measured using liquid chromatography-tandem mass spectrometry and adjusted for specific gravity. The median (interquartile range [IQR]) total sperm count and concentration were 112 × 106 (50 × 106, 221 × 106) and 39 × 106 (17 × 106, 72 × 106) ml−1, respectively. Men in the second, third, and fourth quartile of urinary daidzein concentration had −40% (95% confidence interval [CI]: −59%, −13%), −37% (95% CI: −56%, −9%), and −32% (95% CI: −53%, −3%) low sperm count, respectively, than those in the lowest quartile. Men in the second, third, and fourth quartile of urinary genistein concentration had −23% (95% CI: −47%, 12%), −50% (95% CI: −66%, −26%), and −29% (95% CI: −51%, 3%) low sperm count than those in the lowest quartile, respectively. Sperm count showed no association with urinary equol concentration (P > 0.05). No associations were observed between urinary isoflavones and total sperm motility. A higher isoflavone intake may be associated with reduced sperm concentration and count. The effect of these alterations in semen parameters on the fecundity of couples trying to conceive remains unknown.

Keywords: Asia; daidzein; equol; genistein; semen