Researchers disagree on whether the trend has implications for human fertility
In the last 50 years, average human sperm concentrations dropped by 51.6 percent, and total sperm counts dropped by 62.3 percent, according to a study published last week in the journal Human Reproduction Update.
The researchers conducted a meta-analysis of 223 papers published between 1973 and 2018. The studies analyzed sperm samples of a combined 57,000 men across 53 countries, writes Euronews Next’s Natalie Huet.
The study expands on a 2017 paper by several of the same authors that found a similar decline. While the former study focused mostly on people in Europe, North America and Australia, the new analysis includes data from South America, Asia and Africa, per USA Today’s Karen Weintraub.
But fears of a sperm decline, which largely began when the 2017 paper came out, might be overstated, Rachel E. Gross wrote in the New York Times last year. It’s unclear what lower sperm counts would really mean for human fertility: As long as people have a concentration of at least 40 million sperm per milliliter of semen, having additional sperm doesn’t increase fertility, per the Times. The new paper found average sperm concentrations have dropped from 104 to 49 million per milliliter.
“We don’t see [sperm count] predicting much of anything,” Germaine M. Buck Louis, a reproductive epidemiologist at George Mason University, told the Times.
While the researchers found average sperm concentrations did not drop below the 40 million threshhold, the decline suggests that the number of individuals with sperm concentrations below that level is increasing, Hagai Levine, a co-author of both studies and an epidemiologist at the Hebrew University of Jerusalem in Israel, tells the Guardian’s Nicola Davis. “I think it’s a crisis that we [had] better tackle now,” he says to the publication.
Other researchers caution that finding definitive proof of a decline is difficult, since sperm levels are hard to capture.
“Counting sperm, even with the gold standard technique of [the laboratory process] haemocytometry, is really difficult,” Allan Pacey, who studies sperm and male reproductive health at the University of Sheffield in the U.K. and was not involved in the study, tells the Guardian. “I believe that over time we have simply got better at it because of the development of training and quality control programs around the world. I still think this is much of what we are seeing in the data.”
Studying sperm counts is challenging, but “it’s probably something we have to continue to take seriously and look at,” Bruce Redmon, who studies male reproductive disorders at the University of Minnesota Medical School and did not contribute to the research, tells USA Today.
Beyond the question of fertility, sperm count is thought to be an indicator of other health issues. Poor sperm counts are correlated with increased risk of cardiovascular disease and certain cancers, Michael Eisenberg, a urologist at Stanford University who was not involved in the study, tells Environmental Health News’ Grace van Deelen.
The study did not investigate potential causes of sperm declines. Previous research has tied pesticide exposure to sperm quality, according to Environmental Health News. Other environmental factors, like exposure to pollutants from fossil fuels, may be linked to lower fertility rates, per the Guardian. Levine tells USA Today that a lack of physical activity, a poor diet and smoking can all contribute to lower sperm counts.
“We don’t understand why we’re seeing this pattern, so I think it’s hard to be alarmist for an individual,” Eisenberg tells USA Today. “But at a policy level, this should be a wake-up call to try and understand.”
Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.