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Smoking canceled the benefits of breast-feeding, but there was a decade of difference in diagnosis among nonsmoking breast cancer patients, depending on how long they breast-fed, the researchers reported.
Nonsmokers who didn't breast-feed or did so for less than three months were diagnosed at an average age of 58, while women who didn't smoke and breast-fed longer than six months were diagnosed at an average age of 68. Those who breast-fed longer than six months but also smoked were diagnosed at an average age of 47.
"Those women diagnosed with breast cancer who did not smoke and breast-fed for longer than six months were diagnosed much later -- an average of 10 years later," said study author Emilio Gonzalez-Jimenez, a researcher at the University of Granada in Spain.
His team looked at the medical records of more than 500 women, aged 19 to 91, who had been diagnosed and treated for breast cancer from 2004 through 2009 at a university hospital in Granada.
The researchers found an association between breast-feeding and age at cancer diagnosis, but they did not prove a cause-and-effect link. The association held, however, even when they factored in a family history of cancer. Other research has linked smoking with breast cancer.
The new study is published online Aug. 15 in the Journal of Clinical Nursing.
The study, however, is relatively small, said Debbie Saslow, director of breast and gynecologic cancer for the American Cancer Society. For example, only 26 nonsmoking women in the study breast-fed for more than six months.
Previous research, including a review of dozens of studies, has produced very mixed findings, she added, and this latest finding is not definitive.
"The jury is still out," Saslow said. "There have been many studies showing breast-feeding is slightly protective [against breast cancer], but that the effect is small and it's more protective the longer you breast-feed."
In one review, the risk of breast cancer declined by about 4 percent for each 12 months of breast-feeding, whether it was done with one child or several, she said.
The American Cancer Society expects more than 234,000 cases of breast cancer to be diagnosed in 2013. About 40,000 deaths are expected this year.
According to the U.S. Centers for Disease Control and Prevention, as many as 77 percent of U.S. infants are breast-fed as newborns. Of infants born in 2010, however, just 49 percent were still breast-feeding at six months and only 27 percent were doing so at 12 months.
"There are various explanations why breast-feeding seems to prevent breast cancer," Gonzalez-Jimenez said. "The most probable of these are the hormonal changes that take place during pregnancy and lactation."
Among those changes are reduced levels of estrogen, which fuel many breast cancers, and physical changes in breast cells.
"I recommend breast-feeding for longer than six months," Gonzalez-Jimenez said. Not smoking is his other piece of advice.
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