Did you know that scientists have been collecting data on sperm quality for about seven decades? And did you also know that, according to at least one famous analysis of these data, the overall quality of sperm has been declining steadily over the years? During a recent panel discussion, a colleague leaned over and whispered to me, “What constitutes ‘normal’ today wouldn’t even have made the cut a few decades ago!”
Most of these data have come from men donating to sperm banks. And most of the sperm banks are in urban centres in Western countries. This probably constitutes what we epidemiologists call “selection bias.” It’s when the way in which the data have been selected may be affecting the conclusions we attempt to draw from such data.
For example, men who choose to donate are inherently different from men who don’t. Populations with sperm banks may be inherently different from populations without them. And countries with well-organized data collection systems for their sperm banks are probably different from countries without such systems.
So, in our paper, “Global Decline in Semen Quality: Ignoring the Developing World Introduces Selection Bias,” which was published in the International Journal of General Medicine earlier this year, Marya Jaleel (a uOttawa Health Sciences student) and I proposed that the global sperm data is incomplete, because it under-samples potential contributions from men in rural, poor and non-Western countries.
Men in such places probably do more physical labour than the rest of us, are less likely to eat a processed, chemical diet, and are less likely to be exposed to many environmental pollutants, in particular pollutants that may mimic human hormones.
This is important for a couple of reasons. First, if it is true that there is a global decline in sperm quality, and if that decline is not seen in poor, non-industrialized populations, then it means that the cause of any observed decline is probably related to the modern, Western lifestyle. Things like our processed diets, obesity rates, aging populations, pollution and hormonal additives in our environment may be causing our sperm to be less motile, plentiful and healthy.
And second, it’s important because any environmental impact on our reproductive systems might be pointing to some less-easily seen effects on our overall health. In other words, a decline in sperm quality may be a sign of deeper biological damage.
The global industry of reproductive technologies is now worth billions of dollars, and its growth is accelerating. The issue of declining human reproductive capacity is bound to garner an increasing amount of attention and concern in coming years.
What do you think? If the decline in sperm quality is real—and it still might not be!—what do you think might be causing it?
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