Sperm counts vary by city and have fallen in some populations
A 2022 critical review of studies in men from several continents finds that sperm production differs clearly between cities and regions in well-designed studies. Declines appear in specific populations where studies met every quality criterion.
Key Findings
Of 27 published studies of geographical differences, 16 met the review's minimal quality criteria, and 13 of those 16 reported a statistically significant geographical contrast in sperm production.
Among those 16 studies, the contrast appeared in 7 of 8 continental or subcontinental studies, 4 of 6 national studies and 2 of 2 regional studies.
Among retrospective single-center studies of sperm concentration (or total sperm count), 57% reported a decrease in sperm production over time, 29% reported no change and 12% an increasing trend.
Six single-center studies from specific places met the review's criteria for relatively robust trend data. Five of the six reported a decrease in sperm concentration and/or sperm count, covering 1973–2019.
Four of the five solid single-center studies that found a decline calculated an end-of-study sperm concentration above the level at which fertility is affected.
Interpretation
This critical review graded published observational studies of semen quality and added no new measurements. The grading sorted the work by quality, so the findings rest on studies that met the criteria. The authors note that research on causes is still modest. The findings support larger studies of what drives these differences.
RRM Context
Restorative reproductive medicine evaluates both partners and seeks causes before choosing a treatment. Male factor belongs in that first evaluation. The review urges study of each man's exposome, meaning all the outside exposures in his life. The authors recommend repeated, harmonized semen data collection for each man.
Our editorial summary of this paper, not the article's abstract.
Abstract
Over the past four decades, studies of various designs have reported spatial and temporal trends in human semen quality. Several standardized-methodology studies in homogeneous populations that compare specific cities within a country or a continent provide clear evidence of geographical differences in sperm production, even over short distances within the same country. Human sperm production is widely believed to be declining over time, but evidence from the scientific literature is less clear. Studies based on repeated cross-sectional data from a single centre have shown mixed results. Among the numerous retrospective studies conducted in a single centre, only some included homogeneous groups of men and appropriate methods, and most of them suggest a temporal decrease in human sperm production in the geographical areas considered. Conclusions reporting temporal trends in sperm production that came from existing retrospective multicentre studies based on individual semen data and those using means, medians or estimates of sperm production are questionable, owing to intrinsic limitations in the studies performed. Regardless of study design, studies on the percentage of motile or morphologically normal spermatozoa are still limited by the inherent variability in assessment. Overall, available data do not enable us to conclude that human semen quality is deteriorating worldwide or in the Western world, but that a trend is observed in some specific areas. To understand these trends and contrasts in sperm and semen quality, prospective studies should be encouraged and combined with assessment of the male exposome.
