Time for researchers to use breastfeeding as the baseline norm (NOT artificial formula).

Most studies comparing breastfeeding and infant formula use a simple statistical convention: formula-fed infants are assigned a relative risk of 1.0, and researchers then calculate the ‘benefits’ of breastfeeding relative to that baseline.

A new paper argues that this seemingly harmless statistical choice fundamentally changes how we interpret infant feeding research. Rather than recognising breastfeeding as the biological standard, it unintentionally portrays human milk as providing additional benefits above a formula ‘norm’.

The authors suggest this approach introduces a second, less recognised problem: sociodemographic selection bias.

In many developed countries, mothers who exclusively breastfeed are also more likely to have higher education, higher household incomes and greater access to healthcare. These factors independently improve health outcomes, making it difficult for traditional observational studies to determine exactly how much benefit comes from breastfeeding itself versus the broader social environment.

To address this, researchers increasingly use sibling-pair fixed-effects models, comparing siblings within the same family where one was breastfed and another formula-fed. Because siblings share genetics, parents, household income and many environmental influences, these studies remove much of the socioeconomic confounding that affects conventional research. Interestingly, they often find that some previously reported long-term effects of breastfeeding-  such as reductions in adolescent obesity or behavioural disorders - become smaller than earlier estimates.

However, the paper argues that this should not be interpreted as evidence against breastfeeding. Instead, it exposes a deeper methodological problem. If formula is assumed to be the ‘normal’ reference point, epidemiology is forced to compare a highly complex biological system against an artificial substitute using a statistical framework that may not adequately reflect the underlying biology.

From an evolutionary perspective, the authors argue that exclusive breastfeeding - not formula feeding - is the appropriate biological reference standard. Mammalian lactation evolved over more than 300 million years as a living biological system that supports immune development, neurological maturation and maternal physiology. Formula should therefore be considered an alternative environmental exposure rather than the default against which breastfeeding is measured.

This proposed shift has important implications for future research. Rather than simply asking, ‘What are the benefits of breastfeeding compared with formula?’, researchers could ask a fundamentally different question: ‘What are the physiological consequences of replacing the biological norm with formula?’ The authors argue that reframing the question in this way could help separate the biological effects of human milk from the social and environmental factors that have complicated breastfeeding research for decades.