Cutting chances of cancer
Perhaps the most well-known maternal benefit of breastfeeding is reduced risks for breast and ovarian cancers. New research is underscoring that link and shedding new light on some of the ways in which not nursing puts some women at higher risk.
For every 12 months a woman breastfed, her risk of breast cancer dropped 4.3 percent, Stuebe and Schwarz noted in a 2010 Journal of Perinatology paper. (Scientific American is part of Nature Publishing Group.) And for women who have a family history of breast cancer, breastfeeding seemed to decrease the risk of getting it, according to an analysis of data from more than 60,000 women led by Stuebe, published August 2009 in the Archives of Internal Medicine. Whereas the potent drug Tamoxifen can reduce risk for those with a mother or sister who had breast cancer by about half, "moms who had breastfed at all had about 60 percent less risk," Stuebe says. "That's a pretty significant statistic," she adds.
The mechanisms behind these statistics remain unclear. Women who had taken medication to suppress lactation also seemed to have a lower risk of developing breast cancer compared with women who gave birth but did not breastfeed, according to recent research by Stuebe and colleagues (though these drugs have questionable safety records). These findings hint that the changes in breasts that become engorged with milk that is not expressed could up the chances for breast cancer down the road.
Ovarian cancer risk also appears to be partially tied in with breastfeeding. When compared with women who had breastfed for at least 18 months, mothers who never breastfed had a 1.5-fold increased risk of developing ovarian cancer, according to one analysis. Another study, published November 2009 in Cancer Causes & Control, found that the protective effect of lactating on ovarian cancer was strongest if women had breastfed their last child.
One hypothesis for the ovarian cancer connection is that small infections that often occur during breastfeeding (known as mastitis) might serve to protect the body against tumors in the future. Antibodies that develop to fight the mastitis would persist in the body, and corollary evidence has shown that women who breastfed but did not have these antibodies were more likely to develop ovarian cancer than those who breastfed and did have the antibodies.
Although knowledge on the health rewards of breastfeeding only seems to be growing, many of these apparently protective effects do seem to diminish with time. In Schwarz's study of postmenopausal women, those aged 60 to 69 were only significantly less likely to get cardiovascular disease than those who had formula-fed if they had lactated for a total of 13 to 23 months, according to the analysis published last May. And women who were 70 to 79 years old appeared to be at about the same risk for cardiovascular disease as their formula-feeding compatriots—regardless of how much they had breastfed during their younger years.
Despite the multitude of studies on maternal health outcomes and breastfeeding, many are small or based on methodologies that can fall prey to unintentional biases. "Findings must be interpreted with caution," Stuebe and her colleagues noted in their 2009 American Journal of Perinatology paper. Although many studies try to control for lifestyle, they asserted, "women who breastfeed are more likely to engage in other healthy behaviors" and these are challenging to fully take into account. There also might be a sort of long-term feedback loop, in which women who were breastfed as infants would be more likely to breastfeed their own children, thus benefiting from both their own early breast milk diet and the effects of lactating themselves, as Schwarz has pointed out.
No matter how many factors researchers have controlled for, confounding variables continue to abound. Big questions about the complex relationship between breastfeeding and obesity remain. Although some studies have tied breastfeeding to faster weight loss, the dynamic is much more complicated. Being obese at delivery has also been linked to difficulty breastfeeding and decreased release of the hormone prolactin, which stimulates lactation, as Stuebe pointed out in her 2009 paper. So teasing out finer elements of causation—and possibilities for mitigation—continues to challenge researchers looking for effective ways to study breastfeeding.
The best way to study the effects of breast and formula feeding on mothers and children would be to design a long-term, randomized controlled trial, which is the research gold standard in many medical fields. But given the accumulated literature on the benefits of breastfeeding, such a trial would be "ethically problematic," depriving half of the mother-baby dyads, Stuebe and her colleague noted in their 2009 paper.
One subtle but central adjustment in both the research and popular opinion has been to shift the act of breastfeeding from the experimental group to the baseline assessment. "Breastfeeding is the standard to which all other feeding methods should be compared," Bartick says. "It's not best, it's not optimal—it's just normal." Although many of the results from studies that use formula feeding as a baseline remain valid, the mindset behind it is overdue for change, she says. When assessed as the norm, breastfeeding should not bestow benefits as much as formula feeding increases risks.
The research is still evolving, however, and Stuebe is not sure we have found all of the reasons breastfeeding should be a no-brainer health choice when it is an option. "I think there are going to be many answers," she says. "It's like saying, 'How does exercising improve health?' It's the physiological norm."