Auger, J., Eustache, F., Chevrier, C., Jégou, B., 2022. Spatiotemporal trends in human semen quality. Nature Reviews Urology 19, 597–626.. doi:10.1038/s41585-022-00626-w
Healthy Male is proud to be a recognised source of evidence-based information and education to improve the lives of Australian men and boys. Occasionally, our critical interpretation of research seemingly puts us at odds with others on our own side, who are also working to improve male health. So, we appreciate it when people take a rigorous and detailed look at research evidence and provide sensible – rather than sensationalised – interpretations.
In their recent review article, Auger et al. examine the methodological quality of the individual studies “which might have led to distorted conclusions and undue attention” behind the belief that the quality of human semen is declining.
People have suspected for about 50 years that semen quality might be falling, after noticing an increase in the number of men seeking help for infertility. The reasoning behind this hypothesis is straightforward but perhaps simplistic, given the growing interest in donor insemination and the commercialisation of sperm banking that was occurring at the time. Perhaps it was an increase in awareness, rather than a decrease in sperm counts, behind the observations that started this whole line of enquiry.
Auger et al. outline the evidence from “numerous appropriately designed studies” that provide “unambiguous” evidence of geographical variations in semen quality, “even over short distances within the same country”. The cause of such variations is beyond the scope of their review, but they suggest environmental and genetic contributions as possibilities.
The geographical variation in semen quality is important when it comes to the examination of studies claiming a temporal decline, which have been used to support overblown claims of an apocalyptic failure of human reproduction that is “imperilling the future of the human race”.
Auger et al. describe the various study designs and results from five single-centre studies with repeated cross-sectional data, 68 retrospective single-centre studies with individual data, five retrospective multi-centre studies with individual data, and nine retrospective multi-centre studies based on means, medians or estimated values.
In general, multi-centre studies fail to consider or control for the heterogeneity of subjects, the methods used for semen assessment or the influence of many confounding variables. For example, established seasonal effects on sperm count (counts are highest in winter and lowest in summer) are rarely mentioned.
Only six single-centre studies were judged as being of sufficient quality. Five of these reported declining semen quality over time, but “these centrespecific findings cannot be generalised as a worldwide trend or even a trend across the Western world, even though this is where these few studies were conducted”.
The potentially inaccurate but dogmatic view that human semen quality is declining is often linked to concerns about environmental pollution, and as evidence to support human effects of endocrine-disrupting chemicals that affect male sexual development and function in animal experiments. But we don’t need spurious claims to motivate interventions aimed at improving male fertility; there’s enough evidence already.
Auger et al. mention a number of factors associated with reduced semen quality and male infertility, including poor mental health, obesity and other chronic diseases. Promoting factors beyond individuals’ control, like pollution in the environment, might discourage men from taking responsibility for things they can control to improve their health and wellbeing and, in turn, their fertility