Carlsen, E, SH Swan, JH Petersen and NE Skakkebæk. 2005. Longitudinal changes in semen parameters in young Danish men from the Copenhagen area. Human Reproduction, in press.
|In 2000, Andersen et al. published data (graph to right) showing strikingly low sperm counts in young Danish men (military recruits). Fully 40% of men sampled had sperm counts below 40 million per ml; sperm counts beneath that are associated with increasing difficulties in impregnating a partner.
One interpretation offered for these low counts was that the men simply were not yet mature, and that as they aged their sperm counts would rise. To test that hypothesis, Carlsen et al. tracked a subset of the group studied by Andersen et al. over time. They report their findings in this paper, showing definitively that this explanation is not valid. The sperm counts of these young men remained low for as long as they tracked them over the subsequent four years.
What did they do? The initial study by Andersen et al. obtained samples from 708 men as they appeared for medical examinations for compulsory military service. In this study, Carlsen et al. invited 292 men from Copenhagen who had participated in the original study to participate in an ongoing study in which they would obtain quarterly blood and semen samples and perform a yearly physical. Of those 292, 158 agreed to participate. Subjects were asked to abstain from sex for 48 hrs prior to donating their sperm sample; the sample was accepted whether they did or not and abstinance time was recorded and used as a variable in the analysis.
Of the 158, 37 were identified with testicular abnormalities (e.g., cryptorchidism) or urogenital infections and were segregated in some of the analyses from the main group. This subset was identified as the 'genital disease group.'
For quality control and to minimize measurement error, this laboratory exchanges 5 semen samples 10 times a year with other labs, where the measurements are repeated independently and compared for consistency.
What did they find? Characteristics of men participating in the follow-up were very similar to the original sample and to the men who chose not to participate, although there were some differences. There were fewer smokers in the group and on average their sperm counts were slightly higher. There was no difference in age or BMI.
The median age of the participants was 19.1 yrs on their first visit to the lab, ranging from 18.2 to 26.1. Most had been born in 1977 and 1978. Only 8.9% were older than 20. Hence this study tracked possible changes as most of the participants aged from the late teens into their early 20s.
In the 4 years of tracking, men participating in the study gave a total of 1838 semen samples, averaging 11.7 samples per man. Most gave one sample per quarter, while a few gave fewer.
There was no significant change in sperm concentration over the 4 year duration of the study as the men aged, comparing each man's subsequent samples against his first, nor were there changes in total sperm count or the percentage of morphologically normal sperm. There were small increases in semen volume and the percentage of "rapid progressive motile sperm." The change in sperm volume was only significant for men under 23 yrs of age.
Men in the 'genital disease group' had significantly (p = 0.007) lower sperm counts (median 22 million sperm/ml) than normal men (52 million sperm/ml).
What does it mean? These data indicate that the low sperm counts reported by Andersen et al. 2000 were not a result of sexual immaturity. Instead, they indicate that a large percentage of men within this birth cohort (most born in 1977 and 1978) have low sperm counts, compared to earlier levels reported in Denmark.