Breastfeeding is the biological norm, honed by evolution over hundreds of thousands of years. Human milk contains thousands of active components with nutritive and immunological properties, as well as many others such as stem cells. The composition of human milk changes as the baby gets older, as the environment changes and in response to pathogens encountered by infant and parent. Breastfeeding is associated with epigenetic changes detectable at 10 years of age. Unsurprisingly infant formula therefore increases the risk of disease, particularly those related to immune and inflammatory processes. In addition, there are infective risks related to the preparation of infant formula. Despite this, two thirds of the UK general public think that there is no biological difference between human milk and formula - probably because infant formula is so prevalent in the UK that it is seen as the de facto norm.

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Health professionals need to know the specific impacts of human milk feeding (not just 'breast is best'), in order to personalise counselling for parents deciding whether to initiate breastfeeding/chestfeeding, and whether to continue in the face of challenges. Although we cannot perform randomised controlled trials (RCTs) of human milk feeding, there is robust evidence. There are very large cohort studies where multiple confounding factors are corrected for. There are studies in high and low income settings, where the impact of socioeconomic gradients on infant feeding operate in opposite directions. There are RCTs of donor human milk for preterm babies, and studies of the dose response relationship of human milk volumes to outcomes in the neonatal unit. There is also a very large cluster RCT of lactation support which resulted in a sevenfold difference in human milk feeding rates at 3 months and still shows persistent effects at age 16.

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Important relationships are summarised below. More detail is provided in a large systematic review in the Lancet from 2016.

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• Never breastfeeding is associated with a 36% increase in Sudden Infant Death Syndrome (SIDS)​

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• Shorter length of breastfeeding is associated with a 26% increase in overweight or obesity

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• 53% of hospitalisations for diarrhoea and 27% for lower respiratory tract infections in infants in the UK could be prevented each month by exclusive breastfeeding

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• Breastfeeding for less than 6 months is associated with a 19% increase in childhood leukaemia

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• At 5 years, children who have been breastfed for a few months will be 1-6 months ahead of a typical non-breastfed child in cognitive development, after allowing for a wide range of confounders and mediators

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• There is a 4% reduction in breast cancer per year of lifetime lactation

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It is notable that breastfeeding beyond a year is still associated with continued reduction in risk of several conditions, including asthma, gastroenteritis and leukaemia.

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In prematurity and major illness the impact of human milk is particularly important:

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• Very low birthweight babies receiving less than 50% of their diet as parental human milk in the first 10 days of life have a hazard ratio of 1.6 for the combined outcome of serious infection, necrotising enterocolitis (NEC) or death

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• For each day of their first month of life that babies born under 30 weeks gestation receive more than 50% of their diet as human milk, there is a measurable change in IQ at age 7 years

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• Exclusive parental human milk gives an odds ratio of 0.11 for severe retinopathy of prematurity (ROP), compared to any formula - only 17 babies would need to be given any human milk to prevent one case of severe ROP

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• Systematic review has reported that infants with congenital heart disease receiving exclusive human milk are at a lower risk for NEC

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Unsurprisingly, these health impacts lead to economic impacts. If all families in the UK breastfeeding at 1 week could continue to 4 months, savings for childhood otitis media, respiratory infections and gastroenteritis would exceed £11 million annually. If the proportion of people breastfeeding for 7-18 months of their lifetime was doubled, breast cancer savings would be £31 million annually. Including the cost to the wider economy, the overall savings would actually be in the order of billions of pounds.

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It is also important to note the mental health impact of this steep drop off in breastfeeding/chestfeeding rates. Those who plan to breastfeed and don't succeed have a 50% higher rate of postnatal depression than those who plan to breastfeed and do so. This is a useful website for parents and health professionals to work through their feelings about feeding.

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