These days it seems unquestionable that breastfeeding is superior to formula. The American Academy of Pediatrics recommends exclusive breastfeeding for six months when possible. Nursing has been reported to help babies grow up smarter (higher IQs) and provide a number of health benefits.
Breastfeeding advocate group La Leche League states on its website that "breastfed babies have a decreased likelihood for allergies and dental caries…appropriate jaw, teeth, and speech development as well as overall facial development. This means that people who were artificially fed may experience more trips to doctors and dentists". In addition, breastfeeding has been reported to decrease the prevalence of asthma, illness with diarrhea, and many more conditions.
However, when one looks closer at the evidence, it becomes significantly less clear that nursing offers any advantage over formula at all. This is not to say that nursing does not provide health benefits, but that these claims may be overblown.
Most studies looking at the potential health benefits of breastfeeding compare two groups: babies who breast feed versus babies who formula feed. Health outcomes are then observed and analyzed. Those outcomes include categories such as number of respiratory infections, the child's IQ, and how likely they are to be obese.
Here's the problem with that approach: First, wealthier parents more frequently choose nursing than their poorer counterparts. Second, education appears to play a crucial role in the decision to breastfeed. The more formal education a woman has, the more likely she is to nurse her children. It has been well documented that children of better educated, wealthier parents, generally experience more positive health outcomes. Conversely, children whose parents have attained lower degrees of education or tend to live in relative poverty, do not fare as well.
Overall, breastfeeding is a difficult topic to study. Economist Emily Oster, who has written about studies on breastfeeding says, "Maternal IQ is also very strongly related to obesity, mortality, and disease. This mechanism is probably largely through the relationship with socioeconomic status more generally. Socioeconomic status is not well measured in most data - controls for education are imperfect - which means that basically all of these relationships are confounded. All of these variables are interconnected and interdependent, which makes the specific outcomes of breast versus formula feeding hard to clarify".
Randomized trials (studies in which the people being studied are randomly assigned to a group) give the highest quality data, but are nearly impossible and likely unethical to conduct in the case of breastfeeding. To study breastfeeding benefits with a randomized trial, researchers would have to assign parents to breast or bottle feeding using a lottery. It is hard to imagine parents agreeing to "Mrs. Smith, a computer has chosen you to formula feed your baby for the next year or so." As a result, there are essentially no randomized studies, and that really hurts the quality of the information existing breastfeeding studies provide.
The data we currently rely on to understand breastfeeding and children's health come from small, non-randomized trials looking at groups who are very different from each other. These additional shortcomings create even more problems.
The AAP states in its own breastfeeding recommendations, "Major methodologic issues have been raised as to the quality of some of these studies, especially as to the size of the study populations, quality of the data set, inadequate adjustment for confounders, absence of distinguishing between "any" or "exclusive" breastfeeding, and lack of a defined causal relationship between breastfeeding and the specific outcome. In addition, there are inherent practical and ethical issues that have precluded prospective randomized interventional trials of different feeding regimens." In other words, they are admitting that the science they have used to make their recommendations is particularly flawed.
Luckily, there have been a few relatively well-done studies that provide us with good quality evidence regarding these controversial questions. Their results continue to surprise. In part two of this series, I will discuss what those studies can tell us about breast versus formula feeding. I will also address other potential benefits of breastfeeding such as its role in women's health.