Breast-feeding is a major factor that helps to reduce a woman's lifetime risk of developing breast cancer, according to a new analysis of research data from 30 countries.
The relatively high breast cancer rates found in developed countries are largely explained by the fact that women in those countries have chosen to have few children and to breast-feed them briefly or not at all, according to the detailed analysis of 47 studies by a British research group.
"It's really the number of children and the duration of breast-feeding that is the key to the differences between developed and developing countries" in breast cancer rates, said Valerie Beral, an Oxford University epidemiologist who led the project. "It really changes the way one looks at the cause of breast cancer."
Researchers have long known that having a full-term pregnancy before the age of 30 lowers breast cancer risk and that having additional pregnancies further reduce the chances of developing the disease. For the first time, the analysis confirms that breast-feeding itself can protect against breast cancer.
"This is a spectacular piece of work," said Miriam Labbok, a senior adviser on infant and young child feeding at UNICEF in New York. "The conclusion that if women breast-fed, they would bring their cancer rate down is highly substantiated."
20,000 fewer cases
The analysis found that a woman's relative risk of breast cancer declines by 7 percent with each birth and by an additional 4.5 percent for each year that she breast-feeds. Although those figures translate into modest risk reductions for an individual, the researchers concluded that if all women in developed countries breast-fed their infants for an extra six months, about 25,000 cases of breast cancer -- or about 5 percent of cases -- would be prevented each year. Researchers speculate that breast-feeding may provoke hormonal or tissue changes that protect against cancer.
The new findings help explain why breast cancer rates had been rising in developed countries while rates in developing countries remained relatively low -- a trend that has baffled scientists. Researchers have tended to focus on other factors, such as hormone use, diet and obesity, that women can more easily modify.
The findings, published in Saturday's issue of the journal the Lancet, add to the many recognized benefits of breast-feeding for children, such as better nutrition, a reduction in allergies and better immunity to infections.
"If you look back to the 17th century or even earlier, people knew ... that nuns had high rates of breast cancer," said Beral, the epidemiologist. "People have thought for centuries that childbearing and breast-feeding were important."