Although falling male sperm counts have been reported in the developed world since 1992, the first systematic review and meta-analysis of sperm count trends was only published recently by Hagai Levine and others in Human Reproduction Update.
This latest research analysed 185 studies involving nearly 43,000 men who provided semen samples from 1973 to 2011. The researchers found a 54.2 per cent drop in sperm concentration (number of sperm per millilitre) and a 59.3 per cent drop in total sperm count (total number of sperm in ejaculate) over the past 40 years among men from North America, Europe, Australia and New Zealand. No significant decline was seen in South America, Asia and Africa, but far fewer studies have been conducted in these geographical areas. The study also shows the rate of sperm count decline is not slowing down among Western men.
Further work is necessary to fully confirm this halving of male sperm counts over the past 40 years. However, if true and left unchecked, this trend could have the most serious consequences. The lead author of the study, Dr Hagai Levine of Hebrew University in Jerusalem, commented: "If we will not change the way that we are living, and the environment and the chemicals we are exposed to, I am very worried about what will happen in the future. Eventually we may have a problem, and with reproduction in general, and it may be the extinction of the human species".
Many experts doubted the reliability of previous studies reporting falling sperm counts. For example, some of these studies used semen samples donated to fertility clinics, samples that could be self-selecting for low sperm counts. However, the latest study controlled for many confounding factors that might explain previous findings of falling counts, such as selection of the study population, age, abstinence time, and more. We can therefore now have more confidence that sperm counts are indeed falling, though this new study is not the last word on this matter.
For example, one expert points out that semen analysis is a very poorly performed laboratory test with margins of error as high as 50 per cent and early methods of counting sperm may have overestimated the true count. Also, studies showing a drop in sperm counts are more likely to get published in scientific journals. All in all then, these factors and others may have created an exaggerated impression of declining sperm counts. Much further work remains to be done to finally clarify the picture.
Demographic records show that developed countries have had falling fertility rates for about 20 years now. Undoubtedly, many factors contribute to this falling fertility, including the large numbers of women now entering the workplace. However, we can now add a biological reason – falling sperm counts. Taken at face value, the declining sperm counts imply that a growing population of men have sperm counts below the reduced-fertility threshold.
The fall-off in sperm counts also has implications beyond fertility. Recent studies have correlated lower sperm count with increased risk of diabetes, heart disease and premature death in men. Of course we must remember that correlation is not causation – someone once correlated rising installed telephone statistics in Ireland with admissions to mental asylums!
However, there is no clear evidence to pinpoint the precise cause(s) of the decrease in sperm counts. The decrease has been linked to exposure to chemicals in pesticides and plastics, obesity, smoking, stress, tight underpants or even too much TV-watching.
Some people speculate that declining semen quality is a signal that something ominous is happening at a very basic level in male development. This could include exposure to hormone-disrupting chemicals called phthalates. They are added to plastics to make them more flexible, are found in a large range of products including food packaging and we are easily exposed to them. Phthalates have been reported to decrease male sex hormone levels and to diminish masculinisation in the early fetus at a time when the genitals are forming.
In the absence of a clear set of risk factors Dr Hagai Levine advises men to maintain a healthy lifestyle, eating fresh food, cutting back on processed food, and avoiding junk food, smoking and sedentary behaviour. He also calls on governments to better regulate the manufacture and use of chemicals.
William Reville is an emeritus professor of Biochemistry at UCC