"Studies have found that breast-fed children are 20 to 45 percent less likely to be obese than children who were never breast-fed, said to Kathryn G. Dewey, a professor of nutrition at the University of California, Davis. The longer a child is breast-fed, the better, according to one analysis published last year, which concluded that a child's risk of being overweight dropped by 4 percent for each month of breast-feeding. But research has not established that nursing actually causes children to be leaner. American mothers who breast-feed are different from those who don't — they tend to be older, more educated and wealthier. Obese women often have difficulty breast-feeding, so those who breast-feed are less likely to be overweight. And they probably make healthier dietary and life choices for their families, so their children are probably at lower risk for being overweight in the first place." [NY Times]
