Air Date: Week of November 28, 1997
Five years ago a Danish research team reported in the British Medical Journal that sperm counts world-wide had fallen sharply since 1938. The team could not say what caused the change, but their study suggested that synthetic chemicals could be disrupting the human endocrine system. Skeptics of that theory declared the research flawed. Now, a new study, in the current issue of the journal Environmental Health Perspectives, takes a second look at the sperm count controversy. Doctor Shanna Swann, chief of reproductive Epidemiology at California’s Department of Health Services was skeptical of the earlier study, but was tapped to head the re-analysis. Steve Curwood reached her at her home in Berkeley, California.
CURWOOD: Five years ago a Danish research team, including Neils Skakkabaek and Elizabeth Carlsen, reported in a British medical journal that sperm counts worldwide had fallen sharply since 1938. The team could not say what caused the change, but their study suggested that synthetic chemicals could be disrupting the human endocrine system. Skeptics of that theory declared the research flawed. Now a new study in the current issue of the journal Environmental Health Perspectives takes a second look at the sperm count controversy. Dr. Shanna Swann, Chief of Reproductive Epidemiology at California's Department of Health Services, was skeptical of the earlier study, but was tapped to head the reanalysis. I reached her at her home in Berkeley, California.
SWANN: Well, we found that when we reanalyzed the data, which Carlsen had originally used, but using more sophisticated methods this time, that in fact we found a stronger decline in Europe and the US than they had reported, and little or no decline and possibly an increase in the few non-Western countries for which data were available.
CURWOOD: Why should we be concerned about decreased sperm counts?
SWANN: Well, many people ask that question and it's not for the reason that you might think. You know, it just takes one sperm to make a baby, so the correlation with fertility is not strong. But we believe that sperm decline is in fact a marker for decreased testicular function. And in countries where sperm counts have gone down, testicular cancer rates have gone up. And conversely, for example in Finland where the sperm counts are high, the testicular cancer rate is low and not increasing greatly.
CURWOOD: Your study is what sometimes is called a meta-analysis, meaning that you looked at a whole bunch of papers, I think it was what, 61 different scientific papers about sperm count, published between 1938 and 1990, right?
SWANN: That's correct.
CURWOOD: And then you extracted the data that chart how the concentration of sperm in semen has changed. I'm wondering how this differs from what Dr. Carlsen and Skakkabaek and their colleagues did.
SWANN: Well, what Carlsen and Skakkabaek did was simply to look at the year the study was published and the sperm count, and draw a straight line between the sperm counts as the publication year increased. We looked at a lot more variables. We looked at the age of the men, the mean age; the abstinence time, that is the interval before sample collection; the method of analysis, the way the sperm was counted. And we put these all in a statistical model in what's called a multiple regression model, which basically accounts for differences between populations, to make them comparable. Kind of sets them at a baseline and makes them comparable. And when we did that, we were very surprised to see that the model fit was very good, and the decline was stronger in Western countries than Carlsen had originally claimed.
CURWOOD: There was a lot of criticism of the last study, saying that there just were too many confounding variables in it. For example, while older men may have lower sperm counts, they tend to have longer periods of abstinence, so that a particular sample might show more sperm from an older man. Which really isn't true. And that overall, the study was just too crude an instrument. Are you saying that your reanalysis of the data shows that in fact Carlsen and Skakkabaek had it just about right?
SWANN: Yes. The interesting thing is that when you actually account for those variables that people were suggesting were making the study too crude, such as exactly the ones you explained, age and abstinence time and also changes in the way sperm were counted, when you actually account for those statistically, we found that the model fit very, very well. And I was surprised by this.
CURWOOD: What could be causing this drop in sperm concentration?
SWANN: Well, in my opinion that is the next important question. And I don't have the answer to that question. However, we have some clues from laboratory studies in which sperm counts can be reduced by prenatal exposure of laboratory animals to very small amounts of chemicals which have been referred to recently as endocrine disrupting chemicals.
CURWOOD: Why are you looking at chemicals rather than other factors, say, radiation or there might be some other reason that sperm counts might decline.
SWANN: Yes. That's certainly possible and radiation is, you know, certainly a possible candidate. But you would want to look for things that would differ across countries, to explain regional differences which we now see exist. And also things that have changed over time. For example, the use of DDT, PCB and other of these endocrine-disrupting chemicals, has changed quite dramatically from the beginning in the 30's to an increase up to 1970 and then a decline after that.
CURWOOD: Thanks so much for taking this time with us today.
SWANN: Thank you.
CURWOOD: Shanna Swann is the Chief of Reproductive Epidemiology at California's Department of Health Services.
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