The first study followed 2,250 male Brazilians for 18 years, for whom detailed breast feeding information was collected in early childhood.
The results were mostly negative. Duration of breast feeding showed no association with several measures including weight and body composition. After adjusting for factors such as physical activity, diet, and smoking, two significant associations remained, but these should be interpreted with caution, say the authors.
Regardless of the role of breast feeding may have in preventing obesity, the continued protection, promotion, and support of breast feeding remains a major public health priority, they conclude.
The second study involved 2,631 British children, for whom data on duration of breast feeding, body mass index, and confounding factors (such as birth weight, mother's smoking during pregnancy, parental body mass index, and social class) were available.
The researchers found no evidence that breast feeding influenced body mass index or obesity, and adjustment for confounding factors did not alter these findings.
Promoting breast feeding is important, but evidence for an important beneficial effect on obesity is still equivocal, they conclude.