science on sperm count declines
SH, RL Kruse, L Fan, DB Barr, EZ Drobnis, JB Redmon, C Wang, C Brazil
and JW Overstreet and the Study for the Future of Families Research
Group. 2003. Semen quality in relation to biomarkers of
pesticide exposure. Environmental Health Perspectives.
in the US mid-West has discovered that men with elevated exposures
to alachlor, diazinon and atrazine are dramatically more likely to
have reduced sperm quality. The study is the first to show
such a link for common, current-use pesticides, and its findings are
particularly troubling because the most likely route of exposure is
through drinking water. The three pesticides implicated by th research
are widespread contaminants in mid-West water systems. More...
SH, C Brazil, EZ Brobnis, F Liu, RL Kruse, M Hatch, JB Redmon, C
Wang, JW Overstreet, and the Study for Future Families Research
Group. 2003. Geographic differences in semen quality of
fertile US males. Environmental
Health Perspectives 111. doi:10.1289/ehp.5927
the most sophisticated study of geographic variation in US sperm
count yet conducted, scientists from four different geographic regions
across America report they find important differences in sperm density
and motility. Men in Missouri have the lowest sperm count
compared to New York, Minneapolis and Los Angeles. The
cause of these differences are not yet known. The scientists conducting
the study hypothesize it
may be related to the intensity of pesticide use in industrial agriculture
in Missouri compared to the other, more urban areas. More...
YL, PC Hsu, CC Hsu and GH Lambert. 2000. Semen quality after
prenatal exposure to polychlorinated biphenyls and dibenzofurans.
The Lancet 356:
Guo et al. revisit the children exposed in the womb to PCBs
and dibenzofurans because their mothers had ingested contaminated
rice wine in Taiwan in 1979, examining aspects of the sperm of boys
16+ yrs old and comparing these with unexposed counterparts. They
find significant impacts on sperm quality but not sperm quantity,
with increases in the percentage of abnormally-formed sperm, decreases
in motility and decreases in sperm strength in the exposed cohorts
compared to normal boys.
SH, EP Elkin and L Fenster. 2000. The Question of Declining Sperm
Density Revisited: An Analysis of 101 Studies Published 1934-1996.
Health Perspectives 108:961-966.
et al. revisit the controversy launched by the 1992 paper
et al. that reported a large average decline in sperm count
from studies around the world. In this paper, they add 47 studies
to the data set and repeat the multivariate statistical analysis
reported by Swan et al. in 1997. Their
results are remarkably consistent with both the original Carlsen
et al. paper and the subsequent Swan et al. reanalysis: on average,
human sperm density has declined significantly over the past 5 decades.
The congruence of the results suggests that the overall results
do not rest upon any particular data set within the study, supporting
the general robustness of the original conclusions. More...
AG, TK Jensen, E Carlsen, N Jørgensen, AM Andersson, T Krarup,
N Keiding and NE Skakkebæk. 2000. High frequency of sub-optimal
semen quality in an unselected population of young men. Human Reproduction
study found a median sperm concentration of 41 million sperm per
ml (mean =57.4 m/ml) among young men in Denmark participating in
a compulsory medical examination for military service. Sperm count
of normal, healthy young men is often above 100 million/ml.
is important for two reasons:
all other studies are of men participating because of aspects
of their reproductive status. Hence they are not a representative
sample of the population at large and may contain significant
biases (for example, Acacio et al.,
or Fisch et al., below).
sperm counts revealed are extraordinarily low for healthy young
men. "Among men with no history of reproductive diseases
and a period of abstinence above 48 h, as many as 18% and 40%
respectively had concentrations below 20 and 40 million/ml."
Recent data suggest that reproductive impairment begins to appear
when sperm counts drop beneath 40 million per ml.
BD, T Gottfried, R Israel and RZ Sokol. 2000. Evaluation of a large
cohort of men presenting for a screening semen analysis. Fertility
and Sterility 73:595-597.
paper purports to demonstrate that there has been no change in sperm
count the Los Angeles area over the past several decades. The authors
measured sperm count in male patients at an infertility clinic over
a three year time period (1994-97) and compared their results with
data from 1951.
methodology, however, makes it impossible to support their conclusion
with the data presented. Unfortunately, several newspapers carried
stories about this finding, presenting it as additional evidence
that sperm count is not declining. They used a measurement technique
known to produce high counts, and their sample of men was biased.
SH, EP Elkin and L Fenster. 1997. Have sperm densities declined? A
reanalysis of global trend data. Environmental
Health Perspectives 105(11):1228-1232.
Swan has emerged as an important analytical voice in the scientific
debate over trends in semen quality. Trained as a statistician and
a reproductive epidemiologist, she was brought into this issue at
the request of the National Research Council as an independent and
widely respected analyst who could evaluate the many competing viewpoints
on whether sperm quality or density had declined. As a newcomer
to the sperm count debate, she had no stake in its outcome. She
expected, at the outset, that a rigorous, independent review of
the data would reveal that were no trends.
|As reported in
this paper, however, Swan found just the opposite: that a more sophisticated
statistical analysis of available data in fact not only corroborated
et al.'s original result, but reinforced it.
further analysis of these studies supports a significant decline
in sperm density in the United States and Europe. However, some
intraregional differences were as large as mean decline in sperm
density between 1938 and 1990, and recent reports from Europe and
the United States further support large interarea differences in
sperm density. Identifying the cause(s) of these regional and temporal
differences, whether environmental or other, is clearly warranted."
Shanna H, and Eric P Elkin. 1999. Declining semen quality: can
the past inform the present? BioEssays
paper analyzes several of the assumptions made in uses of existing
sperm count data in studies of global trends.
[of Carlsen et al.] suggested that historical methods (of counting
sperm or conducting studies) were variable and unreliable, differing
from modern methods both qualitatively and quantitatively. To address
this issue we analyzed these studies for trends in counting methods
or their variability. We found neither. Alternative analyses produced
some differences in trend estimates, but statistical factors alone
could not count for the total decline in sperm density. We reviewed
the study populations to identify trends in population characteristics,
such as abstinence time, that might explain the decline. However,
controlling analytically for such factors only increased the rate
of decline. We conclude that historical data on sperm density, despite
large random error, are surprisingly reliable. Nonetheless, understanding
causes of temporal and geographic differences in sperm density must
await contemporary data.
then can we conclude from these analyses? Do they suggest an average
decline in sperm density between 1938-1990? The above discussion
supports this conclusion. Does this imply that uniform decline in
sperm density has occurred worldwide? The analyses of these studies
by several authors and more recent data argue against this conclusion
....Therefore we believe that the trend summarized by Carlsen et
al. reflects a significant decline in the USA (between 1938 and
1990) and northern Europe (at least post-1970), but the scarcity
of data elsewhere prohibits drawing any conclusion about the rest
of the world.
must be remembered that none of these analyses have attempted to
identify the causes of the trends in semen quality, although the
identification of such causes is of considerable interest.
we cannot hope to obtain convincing evidence of exposures that might
have produced the temporal trends described here, we have an indirect
means of addressing this question. If indeed, environmental factors
influence semen quality, these factors, unless uniformly distributed
geographically, would be expected to produce not only temporal variation
in sperm density, but geographic variation as well. A carefully
controlled, cross-sectional study of semen quality conducted in
several cities, with differing types and levels of environmental
exposures, would be invaluable for assessing such geographic variation
and its causes. Suppose such a study identified significant differences
in sperm density between geographic areas that cannot be explained
by host factors (such as age and ethnicity) and personal behaviors
(such as drinking and smoking)? It might then be possible, using
biomarkers of environmental exposures, for example, to point to
environmental factors that have contributed to these differences.
One class of chemicals that will be particularly relevant to examine
are the "endocrine disrupters," those chemicals that act
to alter hormone activity, particularly in offspring, when administered
exposure occurs during the relevant developmental period. Several
of these chemicals (methoxychlor, bisphenol A, and dioxin) are plausible
candidates because very low doses prenatally has been shown to depress
sperm counts in laboratory animals. A unique global study is now
underway (The International Study of Semen Quality in Partners of
J, P Laippala, A Penttila and PJ Karhunen. 1997. Incidence of disorders
of spermatogenesis in middle-aged Finnish men, 1981-1991: two necropsy
series British Medical Journal 314.
et al. compare the quality of sperm production of Finnish
men in 1991 vs. 1981 by conducting post-mortems of men who had died
suddenly from factors unrelated to reproductive health. This sampling
allowed them to circumvent the biases that plague most studies dependent
upon sperm donors. The Finnish team found a drop in the percentage
of men with normal, healthy sperm production from 56.4% in 1981
to 26.9% in 1991. This change was accompanied by a decrease in the
average weight of the men's testes, a decrease in the size of seminiferous
tubules, and an increase in the proportion of fibrous tissue. These
changes over time could not be explained by changes in body mass
index, smoking, alcohol drinking or exposure to drugs. "Deteriorating
spermatogenesis may thus be one important factor in the explanation
of declining sperm counts observed worldwide."
H, ET Goluboff, AH Olson, J Feldshuh, SJ Broder and DH Barad. 1996.
Semen analyses in 1,283 men from the United States over a 25-year
period: no decline in quality. Fertility and Sterility 65(5):
et al. examined sperm counts of men in three US metropolitan
areas (New York NY, Roseville MN and Los Angeles CA) and found no
decline in any of the cities over the 25-year data span. Mean counts
varied highly among locations, with New York, Roseville and Los Angeles
averaging 131, 101 and 73 million sperm per milliliter, respectively.
of these studies was based upon men volunteering sperm before vasectomy.
This introduces biases into the samples, as men volunteering for
vasectomy are known to have sperm counts higher than the population
average. The New York analysis, moreover, is built on small number
of samples per year. Further, it is not stated in the methodology
whether or not donors are asked to return for subsequent donations
if their first donations are determined to have low counts. This
procedure, while appropriate for the medical goals of a sperm donation
or storage clinic, would invalidate the use of these data in trend
Fisch, H and ET Goluboff. 1996. Geographic variations in sperm
counts: a potential cause of bias in studies of semen quality
Fertility and Sterility 65(5): 1044-1046.
and Goluboff challenge the statistical calculations used by Carlsen
et al. in their meta-analysis of sperm count changes in men
from samples around the world (mostly from developed countries). Their
reanalysis suggests that geographic biases misled Carlsen et al. to
reaching a conclusion unsupported by the data.
and Golubuff's reanalysis has also been questioned (e.g., in a paper
delivered by N. Skakkebaek at the 1996 Annual Meeting of the Society
of Endocrinology). The New York data used by in this analysis involve
a small number of samples taken over a long time span, with the
data entered in the analysis as if they had been accumulated in
a single year at the end of the study. This invalidates the analysis.
the meantime, it is clear that sperm count has dropped significantly
in some areas (for example, see Irvine et
al., Auger et al., Toppari
et al., Pajarinan et al.) whereas
it appears not to have changed in other areas (e.g., Fisch
et al., Paulsen et al.). This geographic
variation may provide epidemiological clues as to what is causing
sperm count declines in some areas and not in others.
CA, NG Berman and C Wang. 1996. Data from men in greater Seattle
reveals no downward trend in semen quality: further evidence that
deterioration of semen quality is not geographically uniform.
Fertility and Sterility 65(5):1015-1020.
et al. conducted a retrospective analysis of the relationship
between year of sample donation and semen quality of healthy men in
the Seattle WA area over the time span 1972-1993. They found a "very
weak" but statistically significant increase in sperm concentration
over time. A multiple regression analysis suggested that time of sample
accounted for only approximately 1% of the variation in sperm concentration.
They also examined small but statistically significant increases in
semen volume, sperm ejaculate and percentage of normal sperm in the
et al. note that this finding differs from that reported
in the meta-analysis of Carlsen
et al overall, but that the Seattle data examine only
the latter part of the period studied by Carlsen et al. When
Paulsen et al. reexamine Carlsen et al.'s data only
for the time span studied in Seattle (1972-1993), there is no significant
change. Further analysis leads Paulsen et al. to observe:
"This suggests that there may have been a true decrease in sperm
concentration earlier in the century that did not continue. Alternatively,
this could be a 'stairstep' effect, due to some changes that occurred
S, E Cawood, D Richardson, E MacDonald and J Aitken. 1996. Evidence
of deteriorating semen quality in the United Kingdom: birth cohort
study in 577 men in Scotland over 11 years. British Medical Journal
et al. report a significant decline in sperm counts of Scottish
men over a 20-year period. Sperm concentrations dropped at a rate
of 2.1 percent a year with year of birth.
study is also notable in the sperm trend literature because of the
lengths to which Irvine et al. went to avoid sampling biases. The
Irvine team recruited the men broadly through several different
methods and did not do any sort of screening that would select only
for men of proven fertility or exclude those in particular occupations.
findings support previous reports that the quality of human semen
seems to be falling. In particular, we have observed a decline in
sperm concentration and the total number of sperm and of motile
sperm in the ejaculate in association with a later year of birth,
such that men born in the 1970s are producing some 24% fewer motile
sperm in their ejaculate than are men born in the 1950s."