"Sperm counts among Western men have halved in last 40 years," The Guardian reports. A major review of research carried out since 1973 found an estimated 50-60% drop in sperm count in developed nations.

Researchers looked for studies that reported measures of either total sperm count or sperm concentration in men not known to have fertility problems.

They analysed the findings of these studies and considered trends over time to see if there had been any changes in recent decades.

They concluded that total sperm count and sperm concentration had decreased over time in Western countries, but this trend was not as strong or didn't exist in other parts of the world, such as Africa, Asia and South America.

Both the researchers and the media have a number of theories as to why this might be the case, ranging from exposure to chemicals and pesticides to The Independent's suggestion that modern life was to blame.

It's not clear why. Both the researchers and the media offered a number of suggestions. But until further research is carried out, we just don't know whether these speculations have any merit.

Talk of human extinction in the media is premature. Although the study did report a dramatic-sounding decline in average sperm count from 92.8 million/ml to 66.4 million/ml, this is still well within the range needed to conceive.

Men can help protect their sperm by avoiding smoking and not drinking too much alcohol.

Where did the story come from?

The study was carried out by researchers from the Hebrew University Center of Excellence in Agriculture and Environmental Health and the Ben-Gurion University of the Negev, both in Israel, as well as the Icahn School of Medicine in the US, the University of Copenhagen in Denmark, the Federal University of Parana in Brazil, and the University of Murcia School of Medicine and Biomedical Research Institute of Murcia in Spain.

It was published in the peer-reviewed journal Human Reproduction Update on an open access basis, so it's free to read online.

The study was funded by the Environment and Health Fund, Israel, with additional support given to individual researchers from the American Healthcare Professionals and Friends for Medicine in Israel, the Israel Medical Association, the Research Fund of Rigshospitalet, the Brazilian National Council for Scientific and Technological Development, and the Mount Sinai Transdisciplinary Centre on Early Environmental Exposures.

While the press coverage did accurately report the trends, many headlines were misleading as they focused on the researchers' comments, rather than the study's findings. The actual research didn't look into the causes of any declines in sperm count.

What kind of research was this?

This systematic review and meta-analysis aimed to find existing research that had looked directly at human sperm counts in different populations and explore whether any changes had occurred over time.

This study design has some benefits for exploring whether sperm count is declining, as it allowed the authors to look at findings from a far greater number of people and populations than would usually be possible in a single study.

But not all of the studies included were the same quality, and the researchers weren't able to look at data from every man involved in those studies.

What did the research involve?

The researchers searched databases of medical research in a systematic way and found 185 studies that had looked directly at human sperm count in men either confirmed to be fertile or who had unknown fertility status (unselected men).

The researchers analysed data on both sperm concentration and total sperm count collected between 1973 and 2011.

The authors also analysed data on a range of confounding factors that could have influenced sperm count, such as:

age

how long it had been since a man last ejaculated before providing a sperm sample (abstinence time)

whether semen collection and counting methods were reported

number of samples provided per man

If data was missing on an important factor, the authors found ways of replacing it with an estimate.

They carried out a meta-regression analysis, where the results of the different studies were combined and the influence of other factors, such as the men's ages, was taken into account. This was an appropriate method of analysis for this type of research.

If data was missing on an important factor, researchers found ways of replacing it with an estimate.

What were the basic results?

When the researchers combined the basic results of all the studies without taking into account other influencing factors, they found that from 1973-2011 there was on average a 0.75% decrease in sperm concentration every year (95% confidence interval [CI] 0.73% to 0.77%) with an overall drop of 28.5% over the period. The average sperm count had dropped from 92.8 million/ml to 66.4 million/ml.

When they looked at total sperm count, which takes into account the volume of semen, the yearly decrease was also 0.75% (95% CI 0.72% to 0.78%) with an overall drop of 28.5%. This meant a drop from 296 million to 212 million.

When other factors were taken into account in the analysis (for example, age, region, abstinence time, sperm collection methods), the results for each group were as follows:

unselected Western men had a 1.4% decrease in sperm concentration per year, with an overall drop of 52.4% from 99 million/ml in 1973 to 47 million/ml in 2011

unselected Western men had a decrease in total sperm count of 1.6% per year and 59.3% overall, reducing from 337.5 million in 1973 to 137.5 million in 2011

fertile Western men had a 0.8% decrease in sperm concentration per year, reducing from 84 million/ml to 62 million/ml, but there was no significant difference for total sperm count

There were no significant changes in sperm concentration or total sperm count for unselected and fertile men from other regions.

How did the researchers interpret the results?

The researchers concluded there had been a "significant overall decline" in both sperm concentration and total sperm count in Western countries over the study period, particularly among men who were unselected.

They noted there was no "levelling off" of the trend, which would suggest there may be further declines in the future.

The researchers have expressed concern at their findings, making calls for research into the causes of these trends to be prioritised.

Conclusion

This research presented a useful summary of existing studies in the area of human sperm count, and presented some interesting findings relating to trends over time.

But this study does have some limitations:

The research was based on a wide range of populations who, in some cases, may only have been assessed once. Following a fixed population over time in a cohort study might have had different findings.

Research that wasn't published in English wasn't included, and there also aren't many studies published before 1985 from countries in the other category. This might have an effect on whether the estimates from this population are correct, as studies from those countries might be less likely to be published in English. Having fewer studies to draw upon may be why there are no significant trends in this group.

The study looked at sperm count and concentration, not the quality of the sperm itself, because there was limited reporting of this information in older studies. Likelihood of conception depends not only on the amount of sperm but also its quality, so it would be useful to have this information to be able to make predictions about the impact of these findings on fertility rates.

The authors didn't report any kind of formal quality assessment of the studies they included in their analysis.

Although this research suggests there may be a decline in sperm count in Western countries in recent years, it doesn't offer any explanations.

It also doesn't tell us anything about the fertility of individuals, as the research was based on averages across populations.

The researchers have called for the scientific community to investigate possible reasons for the reported drop, which would seem like a good idea.

Analysis by Bazian

Edited by NHS Website