A new meta-analysis of male fertility published yesterday shows that between 1973 and 2011 the sperm counts of men from western nations have dropped by over 50 percent, reports Nicola Davis at The Guardian.
“The results are quite shocking,” Hagai Levine of the Hebrew University of Jerusalem and lead author of the study, tells Davis. “This is a classic under the radar huge public health problem that is really neglected.”
For the paper, published this week in the journal Human Reproduction Update, researchers examined 185 studies conducted between 1973 and 2011, including data on sperm counts from nearly 43,000 men. The researchers divided the study subjects into two categories: one from “western" countries including North America, Europe, Australia and New Zealand and men from non-western regions, including South America, Asia and Africa.
After adjusting for factors like the subject's age and time without ejaculation, they found that the sperm concentration in western men has fallen from an average of 99 million per milliliter in 1973 to 47.1 million per milliliter in 2011—a 52.4 percent drop. Total sperm counts, or the number of sperm in an entire sample, fell by almost 60 percent. Yet similar drops were not found in the non-western samples.
The first study to point out the trend to lower sperm counts was published in 1992, reports Robert Gebelhoff at The Washington Post. But since then, skeptics have tried to explain away these perceived drops. As Gebelhoff reports, a co-author of the latest study Shanna H. Swan was one of those doubters. Swan, professor at Icahn School of Medicine at Mount Sinai, spent decades looking at methodological problems and confounding factors in the studies, such as smoking rates, that could explain the perceived declines.
“I tried to make [the trend] go away … After 25 years of trying to do that, I haven’t been able to,” she says.
One problem with previous studies, reports Pallab Ghosh at the BBC, is that early research tended to overestimate sperm counts. Some studies also focused on men who visit fertility clinics and already have low sperm counts. Ghosh points out that there is also a bias in journals toward studies that show a drop in sperm count; those that did not find a decline might not be published. But the authors of the new study attempted to correct for these potential biases.
“I’ve never been particularly convinced by the many studies published so far claiming that human sperm counts have declined in the recent past,” Allan Pacey professor of andrology at The University of Sheffield tells Ghosh. “However, the study today by Dr Levine and his colleagues deals head-on with many of the deficiencies of previous studies.”
So what is driving the declines? The study does not delve into causes, but many culprits have been proposed over the years, reports Susan Scutti at CNN. Among the potential culprits are obesity rates, smoking, endocrine disrupting chemicals found in pesticides and fire retardants as well as factors like stress.
At this point, the drop in sperm count is not considered a crisis. Christina Cauterucci at Slate reports that an average of 47.1 million sperm per millimeter is still within the fertile range. Sperm count is not considered low until it reaches 15 million or less, though any levels below 40 million can have an impact on fertility.
The study is also stark reminder that much still remains to be learned about sperm. "We need to really understand what makes up a sperm," Daniel Johnston, chief of the Contraception Research Branch at the National Institutes of Health, told Laura Poppick for Smithsonian.com earlier this year. Johnston is working to describe sperm's full protein contents, which is important in the future development of both contraceptives and infertility treatments.
Though there is nothing to fear just yet, hopefully scientists can soon tease out the mysteries behind the tiny swimmers so we will be ready to tackle any problems that may arise in the future.