A stable isotope-mass spectrometric method for measuring human spermatogenesis kinetics in vivo.

Plain-language summary

This study set out to measure how long it takes for human sperm to be produced and ejaculated, a process traditionally estimated at 60 to 70 days. Researchers used a noninvasive method where 11 men with normal sperm counts drank heavy water (a safe isotope) daily for three weeks, then provided semen samples over 90 days to track how quickly new, labeled sperm appeared. They found that new sperm took an average of 64 days to show up in ejaculate, with a range of 42 to 76 days, confirming that the process is on the shorter end of prior estimates. The study also noted that individual differences, like how quickly old sperm are cleared from the epididymis, can affect the timing.

Key findings

• New sperm took an average of 64 days (range 42 to 76) to appear in ejaculate after heavy water labeling.
• Most men showed a gradual rise in labeled sperm, but two had a sharp increase, suggesting rapid clearance of old sperm.
• The study confirms that sperm production is on the shorter side of traditional 60- to 70-day estimates.