Men’s spunk may be getting noticeably less spunky in some high-income countries, according to a meta-analysis of international swimmers.

Skimming and re-examining sperm data from 185 past independent studies, researchers estimated that sperm counts of men from select high-income regions—North America, Australia, New Zealand, and Europe—dropped about 52 percent between 1973 to 2011, from 99 million sperm per milliliter to about 47 million per milliliter. Likewise, estimates of total sperm count per batch dropped 59 percent, from 337.5 million in 1973 to 137.5 million in 2011.

The researchers, led by Hagai Levine of Hebrew University, also looked at data from what they referred to as “other” countries, including some in South America, Asia, and Africa. They saw no trends in these places, but they also had relatively little data from them.

Publishing the results this week in the journal Human Reproduction Update, Levine and colleagues say the decline “implies that an increasing proportion of men have sperm counts below any given threshold for sub-fertility or infertility.” They conclude that the modeled findings have “significant public health implications” that require “urgent” research.

That was enough for several media outlets to speculate that the end of human reproduction, and therefore existence, was in sight.

Outside experts agreed that the analysis should prompt further study, but they were far more reserved in their reactions. For one thing, the study is a meta-analysis, a type of study that is fraught with caveats, relying solely on re-purposed data from disparate studies conducted by different researchers, in different places, at different times, with different methods and standards, on different cohorts of study participants. Levine and co-authors tried to address some of these issues, excluding from their analysis thousands of studies that had poor methods and data quality or those focused on men with known or suspected fertility problems. In the end, they included data from 42,935 men and focused on the data from those that were not selected based on any fertility status. In that way, it is the best quality meta-analysis on the subject so far.

More than meets the eye

But a gold-standard study would be one conducted in one place, taking random sampling of a large group of men, around the same age, year-over-year for a long period. Such a study was in fact conducted—a 2012 study in BMJ Open recorded young Danish men’s sperm counts from 1996 to 2010. It found no decline.

The fact that the new meta-analysis and the 2012 study contradict is a problem, Martin Blomberg-Jensen, a senior researcher at Righospitalet, in Denmark, said in a media statement. “For more than 20 years there has been an ongoing discussion about a decline in sperm counts,” he notes. “The implication of this work is that stronger evidence exist for the reported decrease in sperm,” he concluded. But researchers need far more data to address whether this is real and, if so, what’s causing it—and if it even matters.

Allan Pacey, an andrologist at University of Sheffield, notes, “Whilst an apparent 52.4% decline in sperm counts may sound a lot… it represents an average change from ‘normal’ (99 million sperm per milliliter) to ‘normal’ (47 million sperm per milliliter).”

The World Health Organization considers any sperm concentration above 15 million and up to 200 million per milliliter to be normal. Men are only considered to have low sperm counts if they fall below 15 million per milliliter or if they have less than 39 million per ejaculate. (The average total sperm counts in the study only fell to 137.5 million.)

“As such, I would urge journalists and editors to treat this study with caution as the debate has not yet been resolved and there is clearly much work still to be done,” Pacey adds.

And last is the issue of what might be causing any such decline in sperm counts. The authors speculate that exposure to pesticides and ubiquitous but extremely low-level hormone-disrupting chemicals may be the issue. But Pacey notes that there’s little epidemiological data to support that hypothesis. Instead, health issues that are of large concern in high-income countries, such as obesity, smoking, stress, or other lifestyle factors may play a role.

"The paper does represent a step forward in the clarity of the data, which might ultimately allow us to define better studies to examine this issue," Pacey said. "Ideally, we would have funded large prospective epidemiological studies of healthy males 25 years ago and this would by now have given us a clear answer one way or the other. Unfortunately, it seems as though we might have to wait another 25 years before we might get to know the real answer.”

Human Reproduction Update, 2017. DOI: 10.1093/humupd/dmx022 (About DOIs).