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updated 11/14/2011 9:44:29 PM ET 2011-11-15T02:44:29

Countries where the highest numbers of women use oral contraceptives have the highest rates of death from prostate cancer, a new study finds.

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Birth control pills contain the female hormone estrogen, and in recent years, some experts have raised concerns about the presence of estrogen and similar compounds in foods and the water supply. The new study may raise those concerns further, though the authors said the findings are preliminary.

"Several studies now have found an association between estrogen exposure and prostate cancer," said study researcher Dr. David Margel, a uro-oncology fellow at the University of Toronto. In this case, he said, "We think this is environmental —[estrogen] goes into the water, into our food chain."

At the same time, Margel said, "We can't establish a cause-and-effect relationship. We definitely don't think the take-home message is women should stop taking the pill."

Birth control pills often contain a type of estrogen called ethinyloestradiol, which women taking the pills excrete in their urine. The hormone ends up in the water supply, or is taken up by plants or animals that use the water, and then passed up the food chain, according to the study.

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Margel and his colleague, Dr. Neil Fleshner, looked at prostate cancer mortality and contraceptive use in 88 countries for their analysis.

In addition to looking at oral contraceptive use, they examined use of intrauterine devices, condoms and other vaginal barriers, but found no association between those and prostate cancer death rates.

"Although the amount [of estrogen] one woman would secrete is minimal, when millions of women take it for a long period of time, it may have an environmental effect," said Margel.

Previous studies have linked estrogen and similar compounds to prostate cancer and other health issues, because of the way these compounds signal cells in the body. Perhaps the most prominent example is bisphenol A (BPA), a compound that mimics estrogen, which is proving to be harmful to health.

But in past studies examining the link between estrogen and prostate cancer, the results have been mixed. One analysis of 18 previous studies, which was cited in the new study, found no connection between blood levels of a number of sex hormones and rates of prostate cancer.

But the authors said that this may be because the hormone causes problems when it occurs in tissue, rather than blood. In follow-ups to this study, Margel said he and his colleagues are planning to examine tissue and fat samples, as well as the water supplies, to see if their hypothesis holds up.

The authors are not alone in saying proving the link between birth control and prostate cancer has a long way to go.

"Concerns have previously been raised about the environmental and health effects of endocrine disruptive compounds, including estrogens from oral contraceptives," said Eric Jacobs, strategic director of pharmacoepidemiology at the American Cancer Society.

"However, results of 'ecological' analyses like this one, that compare countries, rather than individual people, must be interpreted cautiously. Many lifestyle and medical care factors vary between countries, therefore differences in cancer rates between countries can be difficult to ascribe to particular factors," Jacobs said.

The study will be published online tomorrow (Nov. 15) in the journal BMJ Open, published by the British Medical Journal.

Pass it on: Estrogen from birth control pills that seeps into waterways and the food chain may be linked to an increased risk of prostate cancer, though further research is needed to firm up the link.

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