Men don’t produce as much sperm as they did decades ago. It is a trend observed all over the world. The rate of decline is accelerating.
Those are the key findings of a new analysis of sperm count studies published Tuesday in the journal Human Reproduction Update. It is the largest report to investigate the issue.
In a press release, the researchers behind the analysis framed their findings as a “looming crisis” and a “canary in a coal mine” that could “threaten the survival of humanity.”
Shanna Swan, an author of the new analysis, said in an interview that the new research should raise alarms about men’s overall health and reproductive fitness.
“There’s a decline in reproductive function,” said Swan, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai in New York City. “This is not an isolated phenomenon.”
Sperm count is an imperfect measure of fertility, and some outside researchers said they have reservations about the new analysis. But even those critics say the research raises crucial questions about men’s reproductive health — a topic some say has been neglected by science and is ripe for more thorough investigation.
“We still don’t know much about normal sperm concentrations in men around the world after decades, and to date this is the best attempt to collect and assemble all the available data,” said Dr. Bradley Anawalt, a reproductive endocrinologist and chief of medicine at the UW Medical Center in Seattle.
But Anawalt said the inherent limitations of this type of analysis — which combined the results of more than 200 sperm count studies — could lead to misleading conclusions. More research is needed to better understand whether sperm concentrations are dropping so dramatically and what could be behind the problem.
“I wouldn’t want people to think we’re in grave danger of imploding as a species,” Anawalt said. “One still has to ask the question: Is this possibly a smoke signal?”
The researchers behind the study first caused a stir in 2017, when they published a paper showing declining sperm counts in North America, Europe and Australia – places where data was readily available.
That paper received significant media attention and sparked scholarly debate, including criticism from a Harvard research group about the narrow geographic focus and language used to describe the areas studied. According to the Harvard researchers, the paper has been used as fodder for wild speculation about men’s health, as well as baseless and racist theories by some white supremacist and alt-right groups.
“We were challenged by some critics who felt we were only talking about white males, and that was not our intention,” Swan said. “In areas where laboratories were less available and resources were scarcer, there were fewer studies.”
Swan said more high-quality sperm count studies have been produced since the 2017 paper, and the research group was now able to fill in the geographic gaps.
To assess sperm counts worldwide, researchers reviewed hundreds of scientific papers, eventually combining the data and findings from 223 previous papers on sperm concentration. The researchers evaluated the estimates, which included data on semen samples from 1973 to 2018. The authors tried to control for factors such as age and abstinence time.
The new data, which includes studies from around the world, “followed the same trend” as the 2017 study, Swan said. “To our surprise, the pace had accelerated. The decline had become stronger.”
Despite the negative trend, the average sperm count for men in 2018 remained above the levels considered normal by the World Health Organization.
Swan said previous research of small groups of men has linked reductions in sperm count to pesticides and chemicals that disrupt the endocrine system.
“I don’t think there is any doubt that these chemicals affect sperm quality. Where there is doubt is how you parse how much change is due to lifestyle factors and how much is due to chemicals,” Swan said.
External researchers said the new analysis was thorough and careful, but that filtering and combining so many different studies by separate research groups could inevitably create bias.
“You combine all kinds of methodologies. You’re going to introduce bias,” Anawalt said. “We have to take this with a grain of salt.”
Trends in how previous research has been conducted or promoted over decades may skew the overall picture of the problem, researchers said.
It’s possible that medical and scientific journals are more likely to publish results that show declines, said Dr. John Amory, a professor of medicine at the University of Washington who was not involved in the analysis. It is also possible that the type of men involved in sperm count studies are different from those who were not.
Sperm can be difficult to count and accurately characterize, meaning numbers can vary from study to study and over time depending on how sperm are counted.
“You’re talking about millions of cells and they move,” Amory said. “There are other things in ejaculate that aren’t sperm — round cells and debris.”
In addition, sperm count is only one factor in determining fertility. Motility — how effectively sperm can swim — and morphology — size and shape of sperm — are also important indicators of male fertility, Amory said.
The authors of the new analysis acknowledged those limitations and did their best to limit their effects on outcome. They just used studies that counted sperm according to World Health Organization guidelines or used the same techniques, Swan said.
Other studies have shown drops in other sperm parameters and an increase in men seeking treatment for fertility problems, said Dr. Ryan Smith, an associate professor of urology at the University of Virginia, who was not involved in the new analysis.
“I think the consistency of what’s found in the research today is absolutely concerning,” Smith said. “We can’t say anything conclusive at this point, but I think as clinicians and researchers we need to focus research support and advocacy on this.”
Male fertility can be a strong indicator of general health.
Men with infertility problems are at increased risk for other diseases, said Amory, who believes these problems could be a precursor to health problems or alert patients to diseases they didn’t know were affecting their lives.
Obesity, opioid use, and other health factors can cause infertility. Some prescription medications can negatively impact fertility.
It is possible that environmental factors and pollution can broadly affect sperm count.
“Identifying individual perpetrators is challenging,” Smith said.
This article was originally published on NBCNews.com