Global sperm counts not falling after all?

A 1992 paper said that human sperm counts were falling around the world. A new study say that might no be not so.

The story started in 1992, when Danish researchers led by Neils Skakkebaek of the University of Copenhagen published a paper in the British Journal of Medicine reporting falling sperm counts. Their findings were based on their analysis of dozens of published papers describing semen quality. According to their results, global sperm counts had dropped by 50% in the five decades before the report.

Sperm production doesn’t happen without hormones. Endocrine-disrupting compounds are chemicals that may act like hormones and disrupt the work hormones do. Although some experts expressed concerns about the study design and analysis, the publication set off a blizzard of interest in the so-called endocrine-disrupting compounds as a potential culprit in, as one documentary put it, this “Assault on the Male.” Reams of research data later, many chemicals have been identified that have hormonal effects either in lab dishes or in life. But effects on sperm counts? Where are we now?

That screeeeeeching sound you hear may be some scientists throwing on the brakes, at least when it comes to trying to link falling sperm counts to anything. Drawing such links might be difficult if sperm counts aren’t really falling, as a recent commentary in the journal Epidemiology suggests.

In their commentary (available as a free PDF download), authors Jens Peter Bonde, Cecilia Host Ramlau-Hansen, and Jorn Olsen evaluated the results of sperm counts on about 5000 young Danish men reporting for compulsory fitness exams for the military. While providing a sperm count was not compulsory, it was “encouraged,” giving new meaning to the idea of “national service.”

According to the authors, these data and more from a Swedish study—are the Scandinavians the only ones who give it up for science?—show no recent drop whatsoever in sperm counts. Indeed, a graph in their commentary covering each year from 1996 to 2010 shows very little change in median counts over time. These data have the merit, according to the authors, of being “the best longitudinal semen data yet available.” Loosely translated, that means the researchers analyzed a lot of semen over a lot of years and no one else has done that.

That said, the Bonde and co-authors note another study that found a “surprisingly large” proportion of young men who turned up with low sperm counts. In other words, sperm counts may not be falling, but for many men, the norm may be “low.” According to the commentary writers, while environmental hormones may be one issue, they should not be the sole concern. Other factors influence sperm counts, such as maternal alcohol consumption or smoking during pregnancy or a man’s obesity.

Taking on this finding of many men with low-ish sperm counts, the authors shift gears in their paper from the upbeat “sperm counts are not falling” to mentioning the post-repro-apocalypto book and film Children of Men, in which humans can no longer reproduce. Bonde and colleagues warn that, “It may be too early for society to dismiss the concerns” that the book and film depict. In other words, based on the Epidemiology commentary from Bonde, Ramlau-Hanse, and Olsen regarding Danish sperm counts, no worries about falling counts, but don’t take your reproductive capacity for granted. Those factors that negatively influence reproductive ability…whatever they are…may still be out there with effects waiting in the wings…or in the testes.

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Emily Willingham