Breast milk is practically the only food eaten during the first months of life in fully breastfed infants and it is assumed to match the nutritional needs during these first months of postnatal life. Breastfeeding compared with infant formula feeding confers protection against several metabolic and physiological changes later on in life and, particularly, against obesity and related medical complications. Recent data from our laboratory, identifying leptin as the first specific compound responsible for these beneficial effects, are reviewed and discussed.