Breast-feeding cuts cancer risk
Risk of breast cancer halved for some women, study says
July 20, 2004 - It’s a small study dealing with only a small percentage of women. But its message is anything but trivial. The study, published today in the Journal of the National Cancer Institute, says breast-feeding reduces a woman’s risk of getting breast cancer by as much as 50% if she has a genetic predisposition to getting the disease.
“This little study highlights in a larger way the importance of breast-feeding,” said Shail Verma, a medical oncologist with the University of Ottawa’s integrated cancer program. “The bigger picture is that breast-feeding is good for all women.”
The study found breast-feeding for one year or longer cuts the risk even further.
“It’s been known for some time that breast-feeding protects against breast cancer,” said Dr. Steven Narod at the Centre for Research in Women’s Health at the University of Toronto, who led the study.
“Generally the magnitude of the risk reduction is not that great. Breast-feeding for one year might reduce the average person’s chance from 8% to 6%. So if the average woman decides to have a baby just to protect against cancer, that suggests an exaggerated concern for her mortality.”
The key is a genetic mutation known as BRCA1. Women who carry it “have an unusually high risk of getting the disease,” Dr. Narod said. “If they breast-feed, that risk goes down by half. That’s a real slice in risk reduction. Pregnancy becomes a genuine motive for longevity.”
Over the course of a lifetime, 8% of all women will get breast cancer. Of these, only 5% to 10% will have a hereditary form of the disease. The new findings are important because among those with the BRCA1 mutation, 80% will develop breast cancer.
There is no definitive answer as to why breast-feeding offers this protection. Cancer researchers have a few different theories. Perhaps fat-soluble carcinogens and pollutants are not stored as efficiently in lactating breasts. There is also evidence to suggest breast-feeding causes physical changes in breast cells that may make them more resistant to the mutations that can lead to cancer.
But the most compelling theory has to do with how pregnancy and breast-feeding reduces exposure to the female hormone estrogen.
“There are lots of risks for getting breast cancer,” Dr. Verma said. “One is to carry the genetic mutation. The other is related to endocrine, or hormonal, causes. A major concern is for women who have overly long exposure to estrogen: women who menstruate early and reach menopause late. Getting pregnant and breast-feeding when a woman doesn't menstruate, gives the breasts a break from that estrogen exposure.”
Then there are the BRCA1 mutation carriers. Dr. Narod says those who carry the mutation are inclined to a more aggressive cancer that is not as estrogen sensitive. The question is, how do you know whether you carry the gene?
Family history, Dr. Narod said, will give some pretty good clues. “Is there a history? If so, then the doctor should order testing. There are many women in the population who don't know they carry the gene. They don't have a dramatic family history so they haven't been tested.”
While this is a cause for concern, Dr. Narod is not yet advocating blanket testing. “The decision of whether or not to get tested is a complicated one. I would say that every woman should have access to the test if she’s concerned,” he said. “Once we’ve established that someone is a carrier, that makes it much easier to counsel them ... so that they're limiting their risk.”
The dramatic decline in cancer incidence among breast-feeding women with the BRCA1 gene does not appear to apply to carriers of the other breast cancer gene known as BRCA2. Studies have shown that alterations in this gene are associated with a higher risk of ovarian cancer in women and colon cancer in men.
“Breast-feeding seemed to have little effect on the long-term health of BRCA2 carriers,” Dr. Narod said. “Why that is, I’m not sure. It may be because we had such a small sampling of them.”
Dr. Verma says this study -- as well as other research looking at the relationship between breast cancer and breast-feeding -- make sense from an epidemiological standpoint. “In countries where we see high birthrates ... in places like the Philippines and Southeast Asia, we see low numbers of breast cancer,” Dr. Verma said. “Nature seems to be telling women that getting pregnant is healthy.”
China produces further evidence of the strong link between pregnancy and protection from breast cancer.
“We would expect to the same low rates of the disease here as we’ve seen in places like the Philippines,” Dr. Verma said. “But ever since they started restricting the number of children allowed, the numbers have gone up.”
It might turn out that childbearing and breast feeding have a greater bearing on whether a woman develops breast cancer than environmental factors, such as pollution, diet, smoking and alcohol.
According to the U. S. National Institute of Environmental Health Sciences , pesticides and other chemicals called environmental estrogens for the way they imitate the female sex hormone, have been under suspicion for years. While one preliminary finding out of Mount Sinai School of Medicine in New York found breast cancer patients had significantly higher levels of DDE, the major break-down product of DDT, no significant association was found with PCBs -- the acronym most bandied about in news reports about toxins and disease.
Additional studies are underway to explore links between pesticides and breast cancer, as well as possible links between the disease and air pollutants, electric and magnetic fields and municipal waste. But the latest study is putting breast feeding back in the spotlight.
Dr. Verma hopes to see a decline in breast cancer numbers now that breast-feeding has become fashionable again for European and North American women.