Some 30 years ago, Günter Dörner stated that the concentrations of hormones, metabolites and neurotransmitters during critical periods of early development will program disease risk in human adulthood, a concept that since has received enormous scientific support and broad attention. Evidence has also accumulated showing that early nutrition programs later obesity risk. Breastfeeding reduces the odds ratio for obesity at school age by about 20%, relative to formula feeding, adjusted for biological and sociodemographic confounding variables. We propose that the protective effect of breastfeeding is explained at least in part by the induction of lower rates of infant weight gain, which may be related to differences in substrate intakes with breast milk and standard infant formulae. Protein intake per kilogram body weight is some 55-80% higher in formula-fed than in breast-fed infants. We hypothesize that high early protein intakes in excess of metabolic requirements may enhance weight gain in infancy and later obesity risk (the 'early protein hypothesis'). The European Childhood Obesity Project is testing this hypothesis in a randomized double-blind intervention trial in more than 1,000 infants in 5 European countries. Infants that are not breast fed are randomized to formulae with higher or lower protein contents and are followed up to school age. If an effect of infant feeding habits on later obesity risk should be established, there is great potential for effective preventive intervention with a significant potential health benefit for the child and adult population.