Sperm counts among men worldwide are declining rapidly after halving in the past 40 years, according to a large new study released on Tuesday.
The study, led by Israeli epidemiologist Hagai Levine, updates a 2017 study criticized for including only North America, Europe, Australia and New Zealand. The new study includes data from more than 57,000 men pooled from 223 studies in 53 countries, making it the largest meta-analysis ever conducted.
It confirmed 2017 findings that the sperm count in men has halved over the past 4 decades as new countries added. Sperm concentration in non-sterile men decreased by more than 51 percent between 1973 and 2018, from 101.2 million sperm cells to 49 million sperm cells per millimeter of semen, according to the new study.
"Furthermore, the data suggest that this global decline continues at an accelerating rate into the 21st century," according to the study announced in the journal Human Reproduction Update. Sperm count is declining at a rate of about 1.1 percent per year, according to the study.
More action and research is urgently needed "to prevent further disruption to male reproductive health," according to the report. "Honestly, we don't know why."
Sperm count isn't the only factor that affects fertility; the speed of movement of male sperm, which was not measured in the study, is also important.
And the lowest male sperm concentration of 49 million is still above the World Health Organization's "normal" range of 15 million to 200 million sperm per milliliter. According to Sarah Martins Da Silva, a reproductive medicine expert at the University of Dundee in Scotland who was not involved in the study, the rate of decline in sperm counts has more than doubled since 2000.
“Men's exposure to pollutants, plastics, smoking, pharmaceutical and medicinal drugs, as well as lifestyle factors such as obesity and poor diet, have been proposed as contributing factors, although the effects are not known or they define themselves well.
Other experts said the new sperm study didn't dispel their doubts about the 2017 study.
"I remain skeptical of the quality of data in published papers, particularly in the distant past," Allan Pacey of the University of Sheffield in the UK told USA Today. Pacey praised the "very nifty meta-analysis" but added that he thinks "we just got better" in sperm counts, which could explain the drop in rates.
On the other hand, Martins Da Silva dismissed critics of the sperm study findings, saying "the sperm count and consistent results are hard to ignore."