There are three possible explanations for an association between breastfeeding and reduced risk of obesity at an older age. Breastfed infants may self-regulate their intake; breastfed infants have lower plasma insulin levels, which stimulates greater adipose tissue deposition; and breastfed infants have an increased level of leptin, a protein hormone in human milk. Leptin is thought to be a key regulator of appetite and body fatness. In reviewing 11 studies, 8 showed a lower risk of obesity in children who had been breastfed.
Dewey, K.G. Is breastfeeding protective against child obesity? J Human Lact 2003; 19 (1) 9-18.