The benefits of breast milk for babies are numerous. Lower rates of childhood obesity, decreased incidence of asthma and even better brain development are all linked with drinking more of mother's milk in infancy, and despite decades of research and promising marketing claims, the formula industry has not caught up to mother nature in the milk department.

But even if technicians could develop a better food for infants, researchers are now realizing that skipping the lactation phase would be problematic for mothers' health. In fact, not breastfeeding after giving birth seems to put women at higher risk for breast and ovarian cancer, diabetes, cardiovascular disease and many other serious health conditions.

The mechanisms behind these increased risks are still being sorted out, but researchers think that by not engaging in the process that the body prepares for during pregnancy, many crucial systems can go out of whack. And the effects can last for decades after children are weaned. 

"The normal physiology is breastfeeding after pregnancy," says Alison Stuebe, an assistant professor in the Division of Maternal Fetal Medicine at the University of North Carolina in Chapel Hill, who describes breastfeeding as the fourth trimester of pregnancy. When women cannot or choose not to breastfeed, "there are myriad consequences, and we're just figuring them out," she says.

This image, based on medical imaging and computer rendering by the Visual MD, reveals the some of the biological systems affected during lactation, many of which are now thought to have lasting effects on a woman's health.

Costs of not nursing
About 85 percent of U.S. women have at least one child, and based on information about the virtues of breast milk for all those babies, most health agencies recommend that when biologically possible and safe women breastfeed infants exclusively for the first six months with the option of introducing complementary foods in addition to breast milk through 12 months.

Almost three quarters of women in 2005 (the latest year for which data are available) started breastfeeding their infants shortly after birth. By six months, however, only 42 percent of women were still feeding their babies any breast milk at all (with 12 percent still feeding exclusively breast milk at that point). Considering the improved health outcomes for the infants alone, the U.S. could save about $13 billion each year on medical costs if 90 percent of women nursed their infants exclusively for the first six months, according to an analysis by led by Melissa Bartick of the Department of Medicine at Cambridge Health Alliance and Harvard Medical School, published March 2010 in the journal Pediatrics. And that sum says nothing of the money that might be saved on health costs for mothers if they breastfed, which Bartick estimates would be "significant."

In a study of data from 139,681 postmenopausal women in the U.S., those who breastfed for less than 12 months during their reproductive years had a higher risk for cardiovascular disease, diabetes, hyperlipidemia and hypertension than women who had lactated for more than a year in total. For example, among women who had children, those who did not breastfeed had a 42.1 percent chance of developing hypertension, where mothers who breastfed for at least 12 months had a 38.6 percent chance, according to an analysis led by Eleanor Schwarz, an assistant professor of medicine, obstetrics, gynecology and reproductive sciences at the University of Pittsburgh, and published May 2009 in Obstetrics & Gynecology. For women who never become pregnant, many of their risks seem to be closer to those who have children and breastfed.

Mobilizing mothers' fat
Those breastfeeding benefits accrue in part because nursing can start to break down some of the fat that accumulates in women's bodies during pregnancy. At first, some mothers despair for their figures because having children generally leads to thicker midsections and thighs as women's bodies change to nourish a developing fetus and boost stores for feeding the baby once it is born. Although not optimal for long-term health, this extra weight serves an important evolutionary function.

"Clearly the woman's body is positioning itself to be able to put out an extra meal for another body, and it just takes saving calories," Schwarz explains. Producing milk for a single infant requires about 480 extra calories a day. Various analyses have come back with different information of the ability of breastfeeding to help women slim down more quickly after pregnancy. New research presented in March from Schwarz and Candace McClure, a postdoctoral researcher at the University of Pittsburgh's Department of Epidemiology, found that women who had not breastfed had an average of about seven and a half additional centimeters of fat around their waists (as gleaned from CT scans).

But as Schwarz points out, "not all body fat is created equal." The fat that tends to accumulate during pregnancy is in part visceral fat, which sits around organs in the midsection and can put people more at risk for heart and other types of diseases. Their CT study also found that, of the 351 women aged 45 to 58, those who had children and not breastfed had 28 percent more visceral fat than those who had consistently breastfed.

Lactating women appear to be better at mobilizing these new fat stores than new mothers who are using formula. And not shedding those extra post-pregnancy pounds may put women at risk for complications in later pregnancies as well as metabolic syndrome and related health problems, Stuebe and colleagues noted in a January 2009 review article published in the American Journal of Perinatology.

One big concern about these additional fat stores is their potential role in upping chances for diabetes later in life. Pregnancy itself can decrease glucose tolerance and raise insulin resistance, hence the prevalence of gestational diabetes. But research is accumulating to suggest that the process of lactation works to re-establish the balance of these key sensitivities. According to a cohort analysis by Stuebe et al., the longer a woman had lactated during her reproductive years, the less likely she was to get type 2 diabetes, regardless of BMI, which can be a risk factor for the disease.

Women who develop gestational diabetes are generally thought to be at higher risk for developing regular diabetes later in life. But new research has shown that this increased risk is significantly lessened for women who breastfeed for more than nine months, Schwarz points out. She adds that being able to tell women who have had gestational diabetes that breastfeeding will lower their chances of getting diabetes later has given her and many of her patients renewed hope for their future health.

Helping heart health
Breastfeeding helps mothers' cardiovascular health in very specific ways, Schwartz found in her analysis of postmenopausal women.

"Women who reported longer histories of lactation had significantly lower rates of risk factors for cardiovascular disease, even after adjusting for sociodemographic and lifestyle variables, family history and BMI category," Schwarz and her colleagues concluded in their May 2009 analysis. In fact, those who had breastfed for more than 12 months were about 10 percent less likely to develop cardiovascular disease compared with women who had not breastfed.

Recent research has shown aortic calcification, a risk factor for stroke, heart attack and other cardiovascular complications, was "significantly more likely" to be found in mothers who had not breastfed than in those who had for at least three months—even after adjusting for lifestyle, family history, socioeconomic status, BMI and other health issues, according to a study led by Schwarz that was published in January 2010 in Obstetrics & Gynecology.

Schwarz and others are still trying to figure out why not nursing might lead to hardened arteries and other cardiovascular risk factors. One possible explanation hinges on cholesterol levels, which increase during pregnancy. For mothers who do not breastfeed, levels of triglycerides seem to take longer (by about three months) to reach pre-pregnancy levels. Nursing mothers also seemed to have higher levels of high-density lipoprotein (HDL, or so-called "good cholesterol") while they were breastfeeding. But these shorter-term effects do not entirely clear up some of the questions surrounding heart disease later in life.

Better long-term heart health for breastfeeding mothers might stem in part from blood pressure, which was "significantly higher" in mothers who had not breastfed than in those who had (120 mmHg and 115 mmHg, respectively), according to the 2010 Schwarz study. Research has suggested that one in 29 cases of postmenopausal hypertension could be avoided if mothers breastfed for at least 12 months during their reproductive years. 

Risks for cardiovascular disease in lactating versus non-lactating mothers seem to be firm regardless of BMI, which is usually a factor for both conditions. This finding "indicates that lactation does more than simply reduce a woman's fat stores," Schwarz and her colleagues wrote in their May 2009 paper. They proposed that hormonal stimulation is likely playing a substantial role.

The neurotransmitter oxytocin, which is released during nursing, seems to help women get to that "blissed-out state" many women have while breastfeeding, Stuebe says. And this relaxed state of mind can help women cope with all the stresses "that go with being a new mom." Simply getting the body in the habit of releasing this hormone by frequent nursing can pay off long after weaning. "Moms who breastfeed over long times get really good at releasing oxytocin" at other times, which activates the parasympathetic nervous system and can help alleviate stress later in life, Stuebe says.

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.

Assessing limitations

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."