Research shows a relationship between breastfeeding and weight control later in life.

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.