Pediatricians have been scrutinizing breast milk since the late 1800s. But the public didn’t pay much attention until an international scandal in the ’70s over “killer baby bottles.” Studies in South America and Africa showed that babies who were fed formula instead of breast milk were more likely to die. The mothers, it turned out, were using contaminated water or rationing formula because it was so expensive. Still, in the U.S., the whole episode turned breast-feeding advocates and formula makers into Crips and Bloods, and introduced the take-no-prisoners turf war between them that continues to this day.
Some of the magical thinking about breast-feeding stems from a common misconception. Even many doctors believe that breast milk is full of maternal antibodies that get absorbed into the baby’s bloodstream, says Sydney Spiesel, a clinical professor of pediatrics at Yale University’s School of Medicine. That is how it works for most mammals. But in humans, the process is more pedestrian, and less powerful. A human baby is born with antibodies already in place, having absorbed them from the placenta. Breast milk dumps another layer of antibodies, primarily secretory IgA, directly into the baby’s gastrointestinal tract. As the baby is nursing, these extra antibodies provide some added protection against infection, but they never get into the blood.
Since the identification of sIgA, in 1961, labs have hunted for other marvels. Could the oligosaccharides in milk prevent diarrhea? Do the fatty acids boost brain development? The past few decades have turned up many promising leads, hypotheses, and theories, all suggestive and nifty but never confirmed in the lab. Instead, most of the claims about breast-feeding’s benefits lean on research conducted outside the lab: comparing one group of infants being breast-fed against another being breast-fed less, or not at all. Thousands of such studies have been published, linking breast-feeding with healthier, happier, smarter children. But they all share one glaring flaw.
An ideal study would randomly divide a group of mothers, tell one half to breast-feed and the other not to, and then measure the outcomes. But researchers cannot ethically tell mothers what to feed their babies. Instead they have to settle for “observational” studies. These simply look for differences in two populations, one breast-fed and one not. The problem is, breast-fed infants are typically brought up in very different families from those raised on the bottle. In the U.S., breast-feeding is on the rise—69 percent of mothers initiate the practice at the hospital, and 17 percent nurse exclusively for at least six months. But the numbers are much higher among women who are white, older, and educated; a woman who attended college, for instance, is roughly twice as likely to nurse for six months. Researchers try to factor out all these “confounding variables” that might affect the babies’ health and development. But they still can’t know if they’ve missed some critical factor. “Studies about the benefits of breast-feeding are extremely difficult and complex because of who breast-feeds and who doesn’t,” says Michael Kramer, a highly respected researcher at McGill University. “There have been claims that it prevents everything—cancer, diabetes. A reasonable person would be cautious about every new amazing discovery.”
The study about obesity I saw in my pediatrician’s office that morning is a good example of the complexity of breast-feeding research—and of the pitfalls it contains. Some studies have found a link between nursing and slimmer kids, but they haven’t proved that one causes the other. This study surveyed 2,685 children between the ages of 3 and 5. After adjusting for race, parental education, maternal smoking, and other factors—all of which are thought to affect a child’s risk of obesity—the study found little correlation between breast-feeding and weight. Instead, the strongest predictor of the child’s weight was the mother’s. Whether obese mothers nursed or used formula, their children were more likely to be heavy. The breast-feeding advocates’ dream—that something in the milk somehow reprograms appetite—is still a long shot.
In the past decade, researchers have come up with ever more elaborate ways to tease out the truth. One 2005 paper focused on 523 sibling pairs who were fed differently, and its results put a big question mark over all the previous research. The economists Eirik Evenhouse and Siobhan Reilly compared rates of diabetes, asthma, and allergies; childhood weight; various measures of mother-child bonding; and levels of intelligence. Almost all the differences turned out to be statistically insignificant. For the most part, the “long-term effects of breast feeding have been overstated,” they wrote.
Nearly all the researchers I talked to pointed me to a series of studies designed by Kramer, published starting in 2001. Kramer followed 17,000 infants born in Belarus throughout their childhoods. He came up with a clever way to randomize his study, at least somewhat, without doing anything unethical. He took mothers who had already started nursing, and then subjected half of them to an intervention strongly encouraging them to nurse exclusively for several months. The intervention worked: many women nursed longer as a result. And extended breast-feeding did reduce the risk of a gastrointestinal infection by 40 percent. This result seems to be consistent with the protection that sIgA provides; in real life, it adds up to about four out of 100 babies having one less incident of diarrhea or vomiting. Kramer also noted some reduction in infant rashes. Otherwise, his studies found very few significant differences: none, for instance, in weight, blood pressure, ear infections, or allergies—some of the most commonly cited benefits in the breast-feeding literature.
Both the Kramer study and the sibling study did turn up one interesting finding: a bump in “cognitive ability” among breast-fed children. But intelligence is tricky to measure, because it’s subjective and affected by so many factors. Other recent studies, particularly those that have factored out the mother’s IQ, have found no difference at all between breast-fed and formula-fed babies. In Kramer’s study, the mean scores varied widely and mysteriously from clinic to clinic. What’s more, the connection he found “could be banal,” he told me—simply the result of “breast-feeding mothers’ interacting more with their babies, rather than of anything in the milk.”
The IQ studies run into the central problem of breast-feeding research: it is impossible to separate a mother’s decision to breast-feed—and everything that goes along with it—from the breast-feeding itself. Even sibling studies can’t get around this problem. With her first child, for instance, a mother may be extra cautious, keeping the neighbor’s germy brats away and slapping the nurse who gives out the free formula sample. By her third child, she may no longer breast-feed—giving researchers the sibling comparison that they crave—but many other things may have changed as well. Maybe she is now using day care, exposing the baby to more illnesses. Surely she is not noticing that kid No.2 has the baby’s pacifier in his mouth, or that the cat is sleeping in the crib (trust me on this one). She is also not staring lovingly into the baby’s eyes all day, singing songs, reading book after infant book, because she has to make sure that the other two kids are not drowning each other in the tub. On paper, the three siblings are equivalent, but their experiences are not.
What does all the evidence add up to? We have clear indications that breast-feeding helps prevent an extra incident of gastrointestinal illness in some kids—an unpleasant few days of diarrhea or vomiting, but rarely life-threatening in developed countries. We have murky correlations with a whole bunch of long-term conditions. The evidence on IQs is intriguing but not all that compelling, and at best suggests a small advantage, perhaps five points; an individual kid’s IQ score can vary that much from test to test or day to day. If a child is disadvantaged in other ways, this bump might make a difference. But for the kids in my playground set, the ones whose mothers obsess about breast-feeding, it gets lost in a wash of Baby Einstein videos, piano lessons, and the rest. And in any case, if a breast-feeding mother is miserable, or stressed out, or alienated by nursing, as many women are, if her marriage is under stress and breast-feeding is making things worse, surely that can have a greater effect on a kid’s future success than a few IQ points.
So overall, yes, breast is probably best. But not so much better that formula deserves the label of “public health menace,” alongside smoking. Given what we know so far, it seems reasonable to put breast-feeding’s health benefits on the plus side of the ledger and other things—modesty, independence, career, sanity—on the minus side, and then tally them up and make a decision. But in this risk-averse age of parenting, that’s not how it’s done.