Reports on breastfeeding sibling study are vastly overstated

A recent analysis of breastfeeding’s effects on child health is making headlines that some of the benefits of breastfeeding have been overstated.

The authors examined behavioral assessments of children born between 1978 and 2006. When they compared breastfed children with formula-fed children, they found that the breastfed kids were healthier and smarter, as many other studies have previously reported. However, they then looked at families in which only some of the children had been breastfed, and they found that whether or not siblings were breastfed did not significantly affect their health outcomes. The authors argue that this proves that a child’s family – not infant feeding – is what really determines long-term child health, and breastfeeding doesn’t really matter.

The biggest problem with this conclusion is that the study ignored anything that happened in these families before their children reached the age of 4, disregarding well-established links between ear infections, pneumonia, vomiting and diarrhea and the amount of human milk a baby receives. There’s strong biological evidence for these relationships, because formula lacks the antibodies and other immune factors in breast milk that block bacteria from binding to the infant gut and airway. For preterm infants, formula exposure raises rates of necrotizing enterocolitis, a devastating and often deadline complication of prematurity. And evidence continues to mount that formula feeding increases risk of Sudden Infant Death Syndrome. Furthermore, mothers who don’t breastfeed face higher rates of breast cancer, ovarian cancer, diabetes, high blood pressure and heart attacks. None of these outcomes were addressed by the recent sibling study. The paper’s authors note they were interested in longer-term outcomes in childhood, but that’s been lost in the news coverage, which has effectively thrown out the breastfeeding mom and baby with the bath water.

There are also other, more subtle issues. The study’s within-family design is a relatively novel attempt to solve an age-old problem. What we really want to know is this: If the same child lived exactly the same life, but in one universe he was formula fed, and in the other, he was breastfed, would it make a difference? In the absence of access to parallel universes, the “gold standard” way to ask this question is to randomly assign study participants to an intervention and see what happens.

The problem is most families would not agree to have a researcher tell them how to feed their infants. Furthermore, it’s unethical to randomly assign people to an exposure if there’s evidence that it is harmful. Researchers have tried to solve this problem by randomizing families to breastfeeding support, but these studies are limited by “crossover” – many families randomized to the control group breastfeed anyway, and many families getting extra support formula-feed. Despite these challenges, the largest trial of breastfeeding support, the PROBIT study, found significant differences in child IQ favoring breastfeeding.

In observational studies – where researchers simply collect data on feeding and outcomes over time – the problem becomes accounting for the differences between families who both decide to breastfeed and succeed vs. those who don’t. In the US, income and education are strongly related to breastfeeding. That’s why the authors of this paper looked within families, where things like income, education, and access to healthy foods were similar. The only thing that differed among siblings was whether they were breastfed or not. Essentially, these siblings were supposed to occupy their own, private, parallel universe—except that they didn’t.

And the assumptions these authors made about these different siblings’ lives are important. For example, in the paper, the authors argue that any differences among siblings would favor the healthier child being breastfed. But, in fact, a preterm sibling of a bottle-fed child might be much more likely to be breastfed, if mom was counseled on the importance of mother’s milk for a NICU baby, than the term sibling of a bottle-fed child. Other issues, such as birth order, age difference between siblings, and changes in parental employment, education and marital status might also affect why a mother breastfed one child and formula fed another. These issues are not explored in any way by this paper, and cannot be “adjusted away,” any more than the differences between families could be adjusted away by prior observational studies. Something was different at the time of that child’s birth that affected the way he or she was fed, destroying the possibility of parallel universes.

But let’s assume, as a thought experiment, that it is the conditions that make breastfeeding possible, and not breastfeeding itself, that reduce child obesity, raise IQs and improve school performance. What does that mean for health policy? If the secret ingredient is “being born in a family where breastfeeding is possible,” then creating the conditions that enable families to breastfeed must be our highest priority. The take-away is that we need to fight for paid parental leave, high-quality childcare and a living wage for every family, regardless of how they decide to feed their infants.

The study’s authors say as much in the conclusion of their paper:

Efforts to increase breastfeeding that solely focus on individually based behavior change without addressing the economic and social realities women face and the difficult tradeoffs they are forced to make in the months following the birth of their child risk alienating and stigmatizing the very women they hope to help. Instead, they need to be considered in conjunction with social policies that also influence a mother’s ability to breastfeed, especially when current recommendations are that women exclusively do so for at least 6 months of age… A truly comprehensive approach to increasing breastfeeding in the U.S., with a particular focus on reducing racial and SES disparities, will need to work toward increasing and improving parental leave policies, flexible work schedules and health benefits even for low-wage workers, and access to high quality child care that can ease the transition back to work for both mother and child.

Feminist scholars have articulating this point eloquently, arguing that breastfeeding is not a “choice” but a reproductive right. If the conditions that allow breastfeeding make our children smarter and healthier, then we’d best stop fighting about how much breastfeeding matters and focus on fighting for the policies and programs that enable all families to optimize the health of their children.

Alison Stuebe, MD, MSc, is a maternal-fetal medicine physician, breastfeeding researcher, and assistant professor of Obstetrics and Gynecology at the University of North Carolina School of Medicine. She is a member of the board of the Academy of Breastfeeding Medicine. You can follow her on Twitter at @astuebe.

Eleanor Bimla Schwarz, MD, MS is a clinician and researcher at the University of Pittsburgh, Departments of Medicine, Epidemiology, and Obstetrics, Gynecology and Reproductive sciences.

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