Women who take estrogen replacement therapy, even for as little as a year, are less likely to die of colon cancer than those who have never used estrogen, an American Cancer Society investigation has found.

In a study of 422,373 women who were followed over 7 years, researchers found that the longer a woman took estrogen, the less likely she was to die of colon cancer, and that those who were currently using estrogen had a lower risk than those who took it for a while and then stopped. Women who used estrogen for a year or less reduced their risk by 19%, while those who took it for 11 years or more were 46% less likely to die of the disease, even if they had stopped taking the hormone. Current users who had been taking estrogen for 11 years or more reduced their risk by 55%.

It isn't clear how estrogen acts to reduce fatal colon cancer risk. One theory is that it lowers the concentration of bile adds, perhaps creating an environment that is hostile to the growth of cancer cells in the colon; another is that estrogen acts directly on the lining of the colon to suppress tumor growth.

The study looked only at the risk of dying from colon cancer; it did not examine whether taking estrogen reduced a woman's risk of developing colon cancer. According to the investigators, it's possible that the women who took estrogen had higher survival rates simply because they had healthier lifestyles than those who didn't take estrogen. Other studies have shown that, in general, estrogen users are better educated, exercise more, smoke less, and seek preventive care more often than women who don't use estrogen. Because of their estrogen use, they're also more likely to be monitored closely by their physicians. To determine whether estrogen indeed prevents colon cancer would require a controlled clinical trial in which two similar groups of women were assigned to receive estrogen or no hormone-replacement therapy, and followed for several years to determine which group developed more colon cancers.

The report was published in the April 5, 1995, issue of the Journal of the National Cancer Institute.

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