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(fwd)--new info on sperm count metastudy
saw this posed to helath-e list, pretty sure it hasn't made it to dioxin-l
yet. it's IMPORTANT to note this section of NIH's press release:
'However, these studies did not take into account such factors as the
age of the subjects, the length of abstinence prior to sample
collection, and method of sample collection, each of which can
influence the observed trend. Swan said, "Most of the critics have
suggested ways in which the data analysis might be skewed, but no
one has ever looked at the data from these earlier studies to see
whether these hypothetical biases are actually present."
Using a statistical model that corrects for individual
differences in
these key variables as well as geographic area, Swan and her
colleagues reanalyzed the data from 56 of the studies cited in the
1992 paper. The investigators excluded three non-English
language..'
---
NATIONAL INSTITUTES OF HEALTH
National Institute of Environmental Health Sciences
EMBARGOED FOR RELEASE
Monday, November 24, 1997
7:00 AM Eastern Time
John Peterson
(
919) 541-7860
Reanalysis of International Data Finds Sharp Decline in Sperm
Density
After an extensive review of data from 61 published studies, three
California researchers have concluded that a decline in average
sperm density reported in the U.S. and other Western countries may
be even greater than previously estimated.
Their analysis of data collected from 1938 to 1990 indicates that
sperm densities in the United States have exhibited an average
annual decrease of 1.5 million sperm per milliliter of collected
sample, or about 1.5 percent per year, while those in European
countries have declined at about twice that rate (3.1 percent per
year).
The study was conducted by epidemiologists Shanna Swan, Eric Elkin
and Laura Fenster of the California Department of Health
Services. It
appears in the November issue of Environmental Health
Perspectives,
the monthly scientific journal published by the National
Institute of
Environmental Health Sciences.
Since the early 1930's, there has been considerable interest in
declining semen quality as a key predictor of male reproductive
dysfunction. The vast majority of studies designed to answer this
question have focused on sperm density - the number of sperm
contained in one milliliter of sample. (One milliliter is
approximately 1/30th of an ounce.)
Despite enormous differences in data collection methods, study
population and time period, most studies have come to the same
conclusion - that sperm density has declined. In fact, a 1992
review
of 61 such studies (Evidence for decreasing quality of semen
during
past 50 years, E. Carlsen, A. Giwercman, and N. Skakkebaek,
British
Medical Journal, vol. 305, page 609) revealed a steady
decline, from
113 million sperm per milliliter in 1938 to 66 million in 1990, or
about 1 million sperm per milliliter per year.
However, these studies did not take into account such factors
as the
age of the subjects, the length of abstinence prior to sample
collection, and method of sample collection, each of which can
influence the observed trend. Swan said, "Most of the critics have
suggested ways in which the data analysis might be skewed, but no
one has ever looked at the data from these earlier studies to see
whether these hypothetical biases are actually present."
Using a statistical model that corrects for individual
differences in
these key variables as well as geographic area, Swan and her
colleagues reanalyzed the data from 56 of the studies cited in the
1992 paper. The investigators excluded three non-English language
studies and two others that included men who had conceived only
after an infertility workup.
While the results of their analyses also showed a significant
decline
in sperm density, it was the rate of the decline, particularly in
Western countries, that was most surprising. "We observed a
decrease of about 1.5 million sperm per milliliter per year in the
United States, and a corresponding decrease of about 3 million
sperm per year in Europe," reports Swan.
For non-Western countries, a group that included Brazil, India,
Israel, Hong Kong, Kuwait, Nigeria and Thailand, the trend was
slightly positive. However, because these data were taken
from only
13 studies, all of which were published after 1978, this trend was
not statistically significant.
Since no mathematical model can ever fit the data perfectly, there
is always a certain discrepancy between what the model predicts
and what actually happens - this is often referred to as
variability.
"A perfect model would explain 100 percent of this
variability," says
Swan. "Our model accounts for 80 percent - that is the best fit of
any model that has been proposed."
Although the authors do not address the specific causes of this
phenomenon, some recent studies have focused on the relationship
between environmental exposures and declining sperm quality.
In one
such study, researchers reported a significant correlation between
lowered sperm densities and increased levels of organochlorine
compounds in the subjects' seminal fluid. In another,
investigators
found that a general decline in sperm concentration during
the years
1949 to 1981 was statistically linked to an overall increase in
several environmental exposures.
While there is no evidence that this apparent decline in sperm
density has led to reduced fertility, the authors say sperm count
may be a surrogate indicator of effects on the male reproductive
system. They say, for example, that in countries such as Denmark,
England and the United States, where sperm counts have fallen, the
incidence of testicular cancer has increased dramatically over the
last 25 years, while in Finland, where sperm counts are still
relatively high, testicular cancer rates have remained low.
Researcher Shanna H. Swan, Ph.D., can be reached at (510)
450-3818.