Will Formula Feeding Harm My Baby?

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Because breastfeeding is assumed to be medically superior to formula-feeding, scientists consider it unethical to construct randomized controlled trials (RCT) in which babies are randomly assigned to either breast or bottle.  Instead, scientists systematically observe people who have chosen, for their own reasons, to breast or bottle-feed.  The results of observational research are often inconsistent with one another, and the ability to replicate such studies is much lower than that for RCTs.  One main reason for this is that crucial confounding variables are often not considered in observational studies. Confounding variables are unexamined factors in a study that plausibly could explain the outcome. Potential confounding makes it difficult to isolate the protective powers of breast milk itself, or to rule out the possibility that something associated with breastfeeding is responsible for the benefits attributed to milk.  For example, if research were to find that most mothers who breastfeed also eat low-carbohydrate diets or exercise five times a week, it would be difficult to determine whether distinct health outcomes were attributable to breastfeeding or to behavior – eating or exercise habits – associated with breastfeeding.

Selection bias, or the possibility that different types of people “self-select” into different behavioral groups and that their pre-existing dissimilarities are responsible for distinct outcomes, is a serious problem in infant-feeding research. It might be that mothers who breastfeed tend to behave differently in a variety of health-promoting ways and that it is this behavior, not breastfeeding per se, that is responsible for better health. In survey data on the health behavior of middle and high school students, University of Chicago political scientist Eric Oliver found tooth-brushing to be the best predictor of obesity. Dismissing the notion that tooth-brushing plays a direct role in children’s weight, Oliver argues that it is nonetheless a good indicator of homes where health and hygiene are a priority, households where children eat healthier foods and engage in fewer sedentary activities. People who brush their teeth regularly, in other words, undertake multiple health-promoting behaviors. Women who breastfeed might distinguish themselves in similar ways.

Confounding and selection bias are of particular concern in studies finding only small differences between groups, as is the case in research finding a benefit to breastfeeding, or a risk from formula-feeding. Consistency of result does not solve this problem: one study with critical methodological flaws is not convincing; one hundred or one thousand studies with the same weaknesses are no more compelling. Meta-analyses, literature reviews, and commentaries that do not seriously take into account confounding and bias are also unreliable. As the number of years between breastfeeding and the measured health outcome grows, so, too, does the list of possibly influential factors, which means that the challenge of isolating cause and effect is magnified when trying to evaluate long-term benefits. Claims that bottle-fed babies are more likely to be obese or to suffer from cardiovascular disease and diabetes as adults face the nearly insurmountable task of establishing a link between infant feeding and the onset of disease several years or decades later.

In short: when studies find a correlation between breastfeeding and desirable outcomes, this does not mean that breastfeeding causes better health. The process of breastfeeding cannot be distinguished from the decision to initiate or continue breastfeeding, which, irrespective of socioeconomic status or education, could represent an orientation toward parenting that is itself likely to have a positive impact on children’s health.  It might be, in other words, that breastfeeding is part of a package of ostensibly health promoting behaviors, any one of which is insignificant on its own.  Hence the assertion that formula-feeding itself is dangerous has not been demonstrated.  Nor have the biological mechanisms by which breastfeeding might be beneficial, or how breastfeeding actually works in the body to produce better health, been demonstrated.  A great deal of evidence, in fact, suggests that the choice between breast and bottle-feeding has little impact on most babies in the United States.