WEDNESDAY, April 22 (HealthDay News) -- Older women using hormone replacement therapy appear to have a 28 percent reduction in their risk of developing colorectal cancer, a new study found.

But researchers don't yet know why the link seems to exist. And the main message to women remains the same: Take hormone replacement therapy at the lowest dose possible for the shortest period of time possible and only for menopausal symptoms such as hot flashes.

"This is not a primary treatment indication for hormone therapy," said Dr. James Liu, chairman of the department of obstetrics and gynecology at University Hospitals MacDonald Women's Hospital in Cleveland. "If a woman happens to be on the therapy [for menopausal symptoms] this could be a benefit but it's not a reason to stay or start the therapy."

The study, by researchers at the Mayo Clinic in Rochester, Minn., was to be presented Wednesday at the annual meeting of the American Association for Cancer Research, in Denver.

This is not the first time researchers have seen an association between hormone replacement therapy (HRT) and a lower risk of colon cancer.

"This just confirms what we knew before," said Dr. Yiwu Huang, associate program director of hematology and medical oncology at Maimonides Cancer Center in New York City.

The link was first picked up in 2002 when the results of the landmark Women's Health Initiative were first announced. That study found a 40 percent reduced colon cancer risk in women taking combined estrogen and progestin. The risk reduction disappeared, however, when women stopped taking their therapy, Liu said.

The authors of the new study looked at more than 37,000 Iowa women, who ranged in age from 55 to 69 at the start of the study in 1986. Participants had completed questionnaires on oral contraceptive and hormone therapy use. The researchers were able to analyze colorectal cancer tissues in 40 percent of the women.

There was no relationship between use of oral contraceptives and colorectal cancer.

Dr. David Limsui, lead author of the study and a fellow in the department of gastroenterology at the Mayo Clinic, said that "the ideal situation would be that we find the pathway by which estrogen may protect against colorectal cancer risk and find a drug or a therapy that can provide protection through that pathway without the adverse effects of estrogen. We need to figure out how and why."

Hormone replacement therapy is still taken by large numbers of women to relieve the symptoms of menopause -- despite evidence of various health risks, including heightened odds for breast cancer and stroke.

More information

The U.S. National Cancer Institute has more on colorectal cancer.

SOURCES: David Limsui, M.D., fellow, department of gastroenterology, Mayo Clinic, Rochester, Minn.; Yiwu Huang, M.D., associate program director of hematology and medical oncology, Maimonides Cancer Center, New York City; James Liu, M.D., chairman of the department of obstetrics and gynecology, University Hospitals MacDonald Women's Hospital, Cleveland; April 22, 2009, presentation, American Association for Cancer Research annual meeting, Denver

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