Global figures suggest sperm concentration has halved in 40 years – and the rate of decline is accelerating, say researchers, who warn that the human race could face a reproductive crisis if action is not taken.
A study published in the journal Human Reproduction Update, based on 153 estimates from men who were probably unaware of their fertility, suggests that the average sperm concentration fell from an estimated 101.2m per ml to 49.0m per ml between 1973 and 2018 – a drop of 51.6%. Total sperm counts fell by 62.3% during the same period.
Research by the same team, which includes scientists from Israel, Brazil, Spain and the US, reported in 2017 that sperm concentration had more than halved in the past 40 years. However, at the time a lack of data for other parts of the world meant the findings were focused on a region encompassing Europe, North America and Australia. The latest study includes more recent data from 53 countries.
The Guardian reports that declines in sperm concentration were seen not only in the region previously studied, but in Central and South America, Africa and Asia.
Moreover, the rate of decline appears to be increasing: looking at data collected in all continents since 1972, the researchers found sperm concentrations declined by 1.16% per year. However, when they looked only at data collected since the year 2000, the decline was 2.64% per year.
“I think this is another signal that something is wrong with the globe and that we need to do something about it. So yes, I think it’s a crisis, that we (had) better tackle now, before it may reach a tipping point which may not be reversible,” said Professor Hagai Levine, first author of the research from the Hebrew University of Jerusalem.
Previous studies have suggested that fertility is compromised if sperm concentration falls below about 40m per ml. While the latest estimate is above this threshold, Levine said this was a mean figure, suggesting the percentage of men below this threshold would have increased.
“Such a decline clearly represents a decline in the capacity of the population to reproduce,” he said.
While the study accounted for factors including age and how long men had gone without ejaculation, and excluded men known to suffer from infertility, it has limitations, including that it did not look at other markers of sperm quality.
Allan Pacey, professor of andrology at the University of Sheffield, who was not involved in the work, praised the analysis, but said he remained on the fence over whether there was a decline.
“Counting sperm, even with the gold standard technique of (the laboratory process) haemocytometry, is really difficult,” he said. “I believe that over time we have simply got better at it because of the development of training and quality control programmes around the world. I still think this is much of what we are seeing in the data.”
However, Levine dismissed such concerns, adding that, in any case, the decline has been more pronounced in more recent years.
While it is unclear what might be behind the apparent trend, one hypothesis is that endocrine-disrupting chemicals or other environmental factors may play a role, acting on the foetus in the womb. Experts say factors such as smoking, drinking, obesity and poor diet might also play a role, and that a healthy lifestyle may help to boost sperm counts.
Tina Kold Jensen of the University of Southern Denmark said the new study recapitulated a concerning trend. “You keep on finding the same trend, no matter how many studies you include – that is a bit scary,” she said.
Professor Richard Sharpe, an expert in male reproductive health at the University of Edinburgh, said the new data showed that the trend appeared to be a worldwide phenomenon.
He said the decline could mean it takes longer for couples to conceive and, for many, time was not on their side as some delay trying to conceive until the woman is in her 30s or 40s, when her fertility was already reduced.
“The key point that needs to be made is that this is desperately bad news for couple fertility,” he said. “These issues are not just a problem for couples trying to have kids. They are also a huge problem for society in the next 50-odd years as fewer and fewer young people will be around to work and support the increasing bulge of elderly folk.”
Study details
Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries
Hagai Levine, Niels Jørgensen, Anderson Martino-Andrade, Jaime Mendiola et al.
Published in Human Reproduction Update on 15 November 2022
Abstract
Background
Numerous studies have reported declines in semen quality and other markers of male reproductive health. Our previous meta-analysis reported a significant decrease in sperm concentration (SC) and total sperm count (TSC) among men from North America–Europe–Australia (NEA) based on studies published during 1981–2013. At that time, there were too few studies with data from South/Central America–Asia–Africa (SAA) to reliably estimate trends among men from these continents.
Objective and rationale
The aim of this study was to examine trends in sperm count among men from all continents. The broader implications of a global decline in sperm count, the knowledge gaps left unfilled by our prior analysis and the controversies surrounding this issue warranted an up-to-date meta-analysis.
Search methods
We searched PubMed/MEDLINE and EMBASE to identify studies of human SC and TSC published during 2014–2019. After review of 2936 abstracts and 868 full articles, 44 estimates of SC and TSC from 38 studies met the protocol criteria. Data were extracted on semen parameters (SC, TSC, semen volume), collection year and covariates. Combining these new data with data from our previous meta-analysis, the current meta-analysis includes results from 223 studies, yielding 288 estimates based on semen samples collected 1973–2018. Slopes of SC and TSC were estimated as functions of sample collection year using simple linear regression as well as weighted meta-regression. The latter models were adjusted for predetermined covariates and examined for modification by fertility status (unselected by fertility versus fertile), and by two groups of continents: NEA and SAA. These analyses were repeated for data collected post-2000. Multiple sensitivity analyses were conducted to examine assumptions, including linearity.
Outcomes
Overall, SC declined appreciably between 1973 and 2018 (slope in the simple linear model: –0.87 million/ml/year, 95% CI: –0.89 to –0.86; P < 0.001). In an adjusted meta-regression model, which included two interaction terms [time × fertility group (P = 0.012) and time × continents (P = 0.058)], declines were seen among unselected men from NEA (–1.27; –1.78 to –0.77; P < 0.001) and unselected men from SAA (–0.65; –1.29 to –0.01; P = 0.045) and fertile men from NEA (–0.50; –1.00 to –0.01; P = 0.046). Among unselected men from all continents, the mean SC declined by 51.6% between 1973 and 2018 (–1.17: –1.66 to –0.68; P < 0.001). The slope for SC among unselected men was steeper in a model restricted to post-2000 data (–1.73: –3.23 to –0.24; P = 0.024) and the percent decline per year doubled, increasing from 1.16% post-1972 to 2.64% post-2000. Results were similar for TSC, with a 62.3% overall decline among unselected men (–4.70 million/year; –6.56 to –2.83; P < 0.001) in the adjusted meta-regression model. All results changed only minimally in multiple sensitivity analyses.
Wider implications
This analysis is the first to report a decline in sperm count among unselected men from South/Central America–Asia–Africa, in contrast to our previous meta-analysis that was underpowered to examine those continents. Furthermore, data suggest that this world-wide decline is continuing in the 21st century at an accelerated pace. Research on the causes of this continuing decline and actions to prevent further disruption of male reproductive health are urgently needed.
Human Reproduction Update article (2017) Temporal trends in sperm count: a systematic review and meta-regression analysis (Open access)
The Guardian article – Humans could face reproductive crisis as sperm count declines, study finds (Open access)
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