Early nutrition in infancy may influence later child health outcomes including overweight through 'programming'. Systematic reviews suggest that breastfeeding is associated with a modest reduction in the risk of later overweight and obesity. This commentary explores some of these mechanisms behind this association. Generally breastfed infants are leaner than artificially (formula)-fed infants and behavioural and hormonal mechanisms may explain this difference. The theory is that a high nutrient diet in infancy adversely programs the principal components of the metabolic syndrome in the child (body mass index, blood pressure and blood lipids) by promoting growth acceleration, whereas slower growth benefits later cardiovascular disease and its risk factors. Artificial-feeding stimulates a higher postnatal growth velocity with the adiposity rebound occurring earlier in those children who have greater fatness later, whereas breastfeeding has been shown to promote slower growth. The adverse long-term effects of early growth acceleration emerge as fundamental in later overweight and obesity. The higher protein content of artificial baby milk compared to the lower protein content in breastmilk is responsible for the increased growth rate and adiposity during the influential period of infancy of formula-fed infants. Breastfeeding, on the other hand, has a protective effect on child overweight and obesity by inducing lower plasma insulin levels, thereby decreasing fat storage and preventing excessive early adipocyte development. Plausible biological mechanisms underlying the protective effect of breastfeeding against obesity are based on the unique composition of human milk and the metabolic and physiological responses to human milk.