Any breastfeeding advocate can rattle off a list of the advantages of breastfeeding to infants, their mothers and society. The list of benefits ranges from better emotional attachment to a lower risk of childhood leukaemia or dental malocclusion. However, claims of benefit have been contentious, with many studies failing to demonstrate a clear advantage and some even showing increased risk of a negative health outcome with breastfeeding.

The constituents of human milk suggest that it has evolved to promote infant health and human growth, particularly to protect children from infections and to support the rapid growth of the human brain. Despite advances in science and manufacturing, infant formula will always be a poor copy of human milk. It is improbable that a substitute will ever reproduce the full spectrum of human milk proteins, milk sugars (numbering over 100), live white cells and antibodies programmed by infections in the infant's environment, together with constant variations in milk content to meet the needs of the growing infant. But does this matter in a developed country such as Australia? Is there justification in the argument that women are being pushed too hard to breastfeed, or is there more to infant nutrition than the provision of clean water and biologically safe artificial feeds?

Many studies reporting the effects of breastfeeding, both positive and negative, have major methodological flaws. Indeed, robust studies are difficult to carry out. Random allocation of study participants to either a "breastfeeding" or an "artificial feeding" group is unethical, especially if we accept the belief that "the epidemiologic evidence is now overwhelming that, even in developed countries, breastfeeding protects against gastrointestinal and (to a lesser extent) respiratory infection, and that the protective effect is enhanced with greater duration and exclusivity of breastfeeding".1 So evidence for the effects of breastfeeding must rely largely on observational cohort and case–control studies. Blinding to the intervention group is difficult, except at the analysis level. Moreover, mothers who elect to breastfeed differ from those who don't — the former group are generally better-educated women of higher socioeconomic status, who are more likely to have partners and less likely to smoke. All of these factors are likely to be independently associated with positive health outcomes, so failure to adequately adjust for these confounders generally favours positive breastfeeding outcomes. On the other hand, apparently poorer outcomes may result if mothers preferentially breastfeed more vulnerable infants. Difficulties with defining terms such as "breastfeeding" and "exclusively breastfed", and with defining endpoints such as "atopic disease", also make comparisons between studies difficult.

So, have the roughly 2000 papers on breastfeeding and human milk listed in Medline since the year 2000 helped answer the question of whether or not it matters if a term infant born in a country like Australia is breastfed? I will confine my comments to the three most common health problems in Australian children: infection, obesity, and asthma.

In Australia, there are almost 20 000 hospital admissions a year for acute gastroenteritis in children under five years old. Rotavirus accounts for about half of these episodes and is also one of the most important nosocomial infections in paediatrics. A Belarussian study2 (involving 17 000 healthy mother–infant pairs intending to breastfeed) showed that, in centres randomly assigned to deliver support for breastfeeding (as outlined by the Baby Friendly Hospital Initiative3), there were increases in exclusive and continued breastfeeding and reductions in episodes of gastrointestinal infection. In an Italian study4 of infants aged 1–18 months admitted to an infant ward, fewer breastfed infants contracted rotavirus infection (10.6% v 32.4%), and none of these became symptomatic. It has been shown that human milk lactadherin prevents symptoms in breastfed infants infected with rotavirus by binding to the virus and inhibiting its replication.5

Obesity is the commonest chronic health problem in Australian children. This has health implications both in and beyond childhood, as about half of obese children become obese adults. The effect of breastfeeding on later overweight/obesity remains contentious. A 2001 review of 18 studies6 concluded that most studies examining the effects of breastfeeding on later obesity had found an insignificant effect, although two of the studies actually found a positive association between breastfeeding and later body fatness. Since that review, four large studies7-10 (involving between 2000 and 14 000 children) have shown a lower prevalence of overweight in previously breastfed children, with several showing decreasing risk with longer duration of breastfeeding. The three studies of older children (aged 5–14 years)8-10 also found a negative association between breastfeeding and obesity (with adjusted odds ratios [ORs] ranging from 0.66 to 0.8). Similar results in these three studies, despite differences in method, suggest that the finding may be robust.

The second most common chronic health problem in Australian children is asthma. A large Western Australian study showed a substantial reduction in risk of childhood asthma, as assessed at six years of age, if exclusive breastfeeding is continued for at least the first four months of life.11 This was included in a systematic review with meta-analysis of 12 prospective studies,12 which gave a summary OR of 0.70 (95% CI, 0.60–0.81) for asthma among children who had been breastfed, with a greater effect in children from families with a history of atopy (OR, 0.52; 95% CI, 0.35–0.79). The studies were mainly of younger children but included children up to 8.4 years. Another prospective study of more than 330 000 children followed to age 24 months supported these findings: breastfeeding of less than nine months' duration was associated with an increased risk of asthma or wheezing, and a dose–response effect was observed with breastfeeding duration.13 However, a recent report has suggested that atopic children with asthmatic mothers are more likely to develop asthma in later childhood if they have been exclusively breastfed.14 There may be interesting possibilities for intervening to further reduce atopy in at-risk families. Mothers of infants who develop atopic sensitisation have a higher intake of saturated fat,15 and their breast milk is much lower in some long-chain unsaturated fatty acids (which may have antiallergenic properties16). Perhaps improving maternal diets and the use of probiotics17 may reduce the prevalence of atopic disease in their children.

I have selected three childhood conditions for which small changes in prevalence or disease severity would have a major impact on the health of the nation. The US Department of Agriculture, Food and Nutrition chose otitis media, gastroenteritis and necrotising enterocolitis, and estimated that the United States would save a minimum of US$3.6 billion dollars a year if breastfeeding rates increased by 10% at initiation and by 20% at the age of six months.18

While we should be careful not to convey to mothers that breastfeeding will make their child a trim, non-atopic, infection-free genius, the balance of the evidence is that breastfeeding is of benefit in many ways. Every healthcare professional in contact with young children and parents should be familiar with their responsibilities under the World Health Organization Code19 to encourage and promote breastfeeding — for the health of the nation.