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This is a very practical question that every new mother faces – how much effort should they make to breastfeed their children, should they breastfeed exclusively, and for how long? The consensus of official recommendations is that mothers should exclusively breastfeed for the first six months, and then breastfeed along with other food for up to two years.

These recommendations are based on a large literature that shows that breastfeeding correlates with fewer infections, higher IQ, and overall better health. These claims are plausible. Breast milk contains passive antibodies from the mother, which is something you cannot get in formula. Breast milk has also evolved over millions of years to provide optimal nutrition. The best we can hope to do with formula is match it, but it’s easy to miss micronutrients.

Many women, however, have difficulty breastfeeding for various possible reasons. They may be on medication that will pass through the breast milk. They may struggle with borderline nutrition themselves, and need to take advantage of the availability of formula. Or they may simply have difficulty producing sufficient milk and breastfeeding. So the real question is – how strong is the recommendation that “breast is best?” Also, is there any downside to putting excessive pressure on mothers who may be having difficulty?

The answer to these important questions depends partly on how strong the evidence is for the superiority of breastfeeding, and the magnitude of any apparent benefit. Relevant to this has been a looming question in the literature – is the measured beneficial effect of breastfeeding due to the breastfeeding itself, or to confounding factors?

The reason this is a burning question is that the vast majority of the breastfeeding literature is observational, not experimental. This means that mothers are typically questioned about their intention to breastfeed, and their breastfeeding activity, but they are not themselves randomized to either breastfeed or formula. Therefore – breastfeeding mothers are a self-selective group, which introduces a host of potential confounding factors. Therefore, while there is a solid correlation between breastfeeding and positive outcomes, it is much more difficult to draw any causal conclusions.

A 2007 systematic review made this issue clear:

A history of breastfeeding is associated with a reduced risk of many diseases in infants and mothers from developed countries. Because almost all the data in this review were gathered from observational studies, one should not infer causality based on these findings. Also, there is a wide range of quality of the body of evidence across different health outcomes.

A new study throws a little gas on this burning controversy. The study authors took a clever approach to this question – they asked pregnant mothers enrolled in the study if they intended to breastfeed, and then followed their actual breastfeeding behavior, and various health outcomes for their children. The data is based on 1,000 participants in the Infant Feeding Practices Study II:

The researchers found that mothers who while pregnant said they intended to exclusively breastfeed, but then used formula once the baby was born, had children with health outcomes similar to exclusively breastfed infants. Only about half of mothers who intend to breastfeed are able to do so, and often their ability to breastfeed is beyond their control and not known until their baby is actually born.

This was a good way to get a peek at possible confounding factors. The intention to exclusively breastfeed, even if the mothers did not breastfeed, was very similar in outcomes to mothers who actually breastfed. This supports the hypothesis that the improved outcomes from breastfeeding are at least partly explained by the characteristics of the self-selective population who breastfeed. To further support this:

What we found is that intending mothers had more information about nutrition and diet; they more frequently consulted their physicians; and had better access to information related to infant health than those moms who did not intend to breastfeed.

So breastfeeding mothers simply were better informed and took better care of themselves and their children. This is exactly the confounding factors that were suspected. Similarly, the data on breastfeeding and IQ is hugely confounded by the fact that breastfeeding mothers have a higher baseline IQ, and when you control for this factor the benefits of breastmilk are greatly reduced.

None of this means that there is no benefit from breastfeeding. Especially within the first six months, there is enough data and plausibility to suggest that breastfed babies have fewer infections and improved outcomes. However, a simplistic interpretation of the evidence may overstate the benefits. It should also be mentioned that the long term benefits, beyond two years, are not well established. By five years of age any differences disappear.

What all of this suggests is that, while the basic recommendation that breast is best is valid and a reasonable default, we should not overstate the benefits to the public. Further, new mothers should not be overly pressured or shamed into breastfeeding. Formula feeding is a perfectly acceptable option for those mothers who have difficulty breastfeeding, cannot breastfeed, or choose not to for various reasons. Most of the apparent benefits of breastfeeding can be had through better educating new parents, supporting them, and making sure they have access to health care.

The more general lesson from this new study, relevant to science-based medicine broadly, is the power of confounding factors. Simply being better educated and taking better care of oneself or one’s children can have a powerful effect on outcomes, and if such factors are not carefully considered they can make any intervention seem to be effective. This is essentially part of what we consider “placebo” or non-specific effects. When assessing any study, review, or question in the literature, such factors should be carefully considered.