Researchers are raising the alarm that declining sperm counts "threaten mankind's survival" — yet experts remain cautious about the results of a new study.
A controversial paper recently published in the journal Human Reproduction Update noted that sperm counts declined globally by about half since the 1970s — and the trend has been accelerating since the year 2000.
"It is the first study to examine global trends in semen quality in recent years and the first to demonstrate declining sperm counts among men from South and Central America, Asia and Africa," said lead author Hagai Levine, professor at Hebrew University of Jerusalem’s Hadassah Braun School of Public Health.
But many experts in the scientific community remain skeptical of the findings.
"The conclusions of the Levine group — that sperm concentrations are declining globally and that the decline has accelerated — are not shared by many in the andrology community," said Dr. John K. Amory, professor of medicine at the University of Washington School of Medicine in Seattle.
"Moreover, the mean sperm concentrations observed in these studies remain well within the range of values thought to be consistent with normal fertility in men," Amory said.
He also said, "More data will be needed over time to fully understand these phenomena."
Levine’s international team from Denmark, Brazil, Spain, Israel and the United States performed a meta-analysis, which combined findings from more than 250 previous studies from 53 countries, including the United States, between 1973 and 2018.
"This meta-analysis looks at worldwide trends in sperm concentration (SC) and total sperm concentration (TCS) between 1973 and 2018," Levine told Fox News Digital.
The present study builds on previously published data in 2017 on sperm counts in North America, Europe and Australia.
It examined seven more years of data from 2011 to 2018 to focus on regions of the world that were not reviewed in their first study — South and Central America, Asia and Africa.
Levine noted that the study found sperm concentration declined globally by more than half, with a 62% decline in total sperm concentration between 1973 and 2018.
"We found that the pace of decline increased from 1.2% each year since 1972, to 2.6% each year since 2000," Levine noted.
"The increased data and statistical power allowed us, for the first time, to assess trends in the 21st century, [up to] 2018."
The study concluded there is global decline in sperm concentration not only in North America, Europe and Australia, but also in South and Central America, Africa and Asia.
"Sperm count is an indicator of men's overall health, with low levels being associated with increased risk of chronic disease, testicular cancer and a decreased lifespan," Levine told Fox News Digital.
"On a larger scope, the decline signifies a global problem that is related to modern environment and lifestyle, reflecting a disrupted sick world, at least for human reproduction."
Sperm counts alone, however, are not a great indicator for infertility because they need to be considered in the context of a couple — namely, how they interact with the egg and female reproductive tract, according to The New York Times.
Sperm also grows from stem cells in the testes, but the development can take around two months, so one sperm count is only a "snapshot" in time, The Times also noted.
The authors noted the study’s limitations because they only analyzed the sperm count and concentration of participants — but not how the sperm moved or their shape.
These are qualities infertility specialists use to assess reproductive potential.
"The authors of this paper have conducted a very elegant meta analysis and I have no criticism at all about the way they have done this," said Allan Pacey, professor of andrology at the University of Sheffield in the United Kingdom.
But Pacey told Fox News Digital that he is "concerned" about the quality of the data on which the study’s analysis was based.
"The quality of the meta-analysis is as good as the original data we have, as noted by Prof. Pacey," Levine told Fox News Digital.
"Luckily for us, the methods for counting sperm are rather simple and haven't really changed in the last 50 years."
The paper notes that "counting by hemocytometer is the classical way to assess [sperm count] and has been recommended by the World Health Organization in all versions of organizations semen analysis manuals."
But Pacey said that counting sperm, even with the "gold standard" technique of haemocytometry, "is really difficult."
"I believe that over time we have simply gotten better at it because of the development of training and quality control programs around the world," Pacey added.
"I still think this is much of what we are seeing in the data."
Levine told Fox News Digital, "Nevertheless, as [with] any study, we are limited by the fact that we see what we look at."
"We had 41 estimates [data from studies] from the USA but only one from Israel or one from Cuba — and some countries are not represented at all," he added.
"So we can summarize that there is strong evidence for global decline, including in Latin America, Asia and Africa, but we can't be certain for a specific population or country."
He stated more studies are needed to monitor semen quality and to better understand the causes of the decline.
The study comes after a recent United Nations report noted "the world’s population continues to grow, but the pace of growth is slowing down."
As of Nov. 15, 2022, the world’s population is estimated at 8 billion.
"In 2020, the global population growth rate fell under 1 percent per year for the first time since 1950."
Although the study did not examine the cause of sperm count declines, Levine suggested our modern environment and lifestyle are playing a role.
"We have previously shown that disturbances in [the] male reproductive system are determined by prenatal exposure to environmental chemicals as well poor health behaviors in adulthood," Levine said.
"The study should serve as a wake-up call for clinicians, researchers, governments and the public, to address the reduced sperm crisis by investing in research for unknown causes and mitigating the known causes."
Nevertheless, Pacey remains "on the fence" about the findings.
"The problem is that the notion of a decline in sperm counts has [gotten] into popular culture and so it’s very difficult to have an even-handed debate about the issue — even among scientists."