Dubai, United Arab Emirates (CNN) — Over the past 50 years, human sperm counts appear to have declined by more than 50% worldwide, according to an updated review of the medical literature.
If the results are confirmed and the decline continues, this could have important implications for human reproduction. The researchers believed that this may also portend the deterioration of men’s health in general, as the quality of semen may be an important indicator of public health.
The review and its conclusions have sparked controversy among male fertility experts. Some say the results are real and compelling, but others say they aren’t convinced by the data because methods for counting sperm have changed so much over time that historical numbers can’t be compared with modern ones.
Most experts agree that this issue deserves further study.
“I think reproduction is one of the primary functions of any species,” said Dr. Michael Eisenberg, a urologist at Stanford Medicine who was not involved in the research. “So to my mind if there is an indication of a decline in reproduction, I think this is a very important finding.”
“There is a strong link between a man’s reproductive health and his general health. So this could indicate that we are not as healthy as we used to be,” he added.
Other experts thought that while the review was good, they were skeptical of its conclusions.
“The method of semen analysis has changed over the decades. It’s gotten better. It’s more standard, though it’s not perfect,” said Dr. Alexander Pastuzak, a surgeon and assistant professor at the University of Utah College of Medicine in Salt Lake City, who was not involved in the research. “.
“Even if you took the same semen sample and ran it and then subjected it to a semen analysis in the 60’s and 70’s compared to today, you would get two different answers,” he added.
Pastoczak said that in a number of contemporary studies of semen analysis, those that relied on samples that were analyzed in a different way, “we don’t see these trends.” In fact, some studies in northern European regions show that sperm counts are rising and not declining over time, he says.
The updated revision adds data for a number of countries
The new analysis updates a review published in 2017 and includes for the first time new data from Central and South America, Asia and Africa. It is published in the journal Human Reproduction Update.
An international team of researchers looked at nearly 3,000 studies that recorded men’s sperm count, published between 2014 and 2020, years not included in their previous analysis.
The researchers excluded studies that included only men diagnosed with infertility, those that included only men with a normal sperm count, and those that selected study participants based on genital abnormalities or diseases.
The studies, published in English only, included those involving 10 or more men and those whose sperm was collected in the standard way and counted using a device called a cytometer.
In the end, only 38 studies met the required criteria. The researchers added them to the studies included in their previous review and extracted their data, which was then entered into the forms.
In all, the researchers determined that the sperm count fell visually by more than 1% per year between 1973 and 2018. The study concluded that the average sperm count globally had declined by 52% by 2018.
When the study authors narrowed their analyzes to specific years, they found that the decline in sperm count appeared to be accelerating, from an average of 1.16% per year after 1973 to 2.64% per year after 2020.
“It’s really remarkable that the decline is actually increasing,” said study author Dr. Hagai Levin, an epidemiologist and public health researcher at the Brown School of Public Health and Community Medicine.
At the population level, the average sperm count fell from 104 million to 49 million per milliliter between 1973 and 2019. It is believed that the normal sperm count is over 40 million per milliliter.
The reasons for the decline are unknown
The study authors said that they do not have enough data from different regions to be able to know if the average sperm count in some countries is lower than in others, or if the sperm count is declining faster in certain regions. The research included data from 53 countries.
The authors did not investigate the reasons for the decline. “It has to be studied,” Levine said.
Levine noted that in other research, he and others discovered some of the factors associated with low sperm count.
Harm to reproductive health may begin in the uterus.
“We know that maternal stress, smoking during pregnancy and especially exposure to man-made chemicals found in plastics, such as phthalates, disrupt the development of the male reproductive system,” Levine said.
Lifestyle also plays an important role. He pointed out that obesity, lack of physical activity, and diets rich in ultra-processed foods may contribute to this.
“The same factors that harm health in general also harm semen quality,” he explained.
Ultimately attempting this type of study, one expert said, is fraught with problems that will complicate results.
“The paper is very robust scientifically and statistically, and does a good job of summarizing the data that is available in our field, but it is important to realize that this data is still very limited in how it is collected and how Report it.”
Lundy said that standards and methods for counting sperm have changed dramatically over time, making it difficult to compare recent censuses with historical data.
Despite this, in his opinion, historical data is all that is available.
Lundy concluded, “However, these results do not constitute a cause for panic, because the population is still quite normal, on average, but there is a risk that it will become abnormal in the future, and we have to know this and study it further.”