Just skimmed through this thread but I noticed that someone mentioned intelligence and breast feeding. There have been many studies done on this which show higher points on intelligence tests for breastfed babies. It is however difficult to show directly what factors contribute to intelligence because of the many environmental factors involved. However in a study quoted in the paper below results were selected from 11 studies in order to perform a meta-analysis. Using this approach, they quantitated a 5.3-point intelligence quotient (IQ) difference in cognitive development favouring breast-fed children; after adjustment for covariates the difference dropped to 3.2 points. So there is a difference although it is small. The difference is due to the composition of breast milk and also could be due to the act of breastfeeding itself promoting positive social behaviour thus contributing to a better cognitive development. This is explained below.
?Recent information on breast-milk composition revealed that despite the efforts of formula manufacturers, artificial formulas remain significantly different from breast milk. Several nutrients and other components present in breast milk could contribute to the enhancement of mental development. Long-chain polyunsaturated fatty acids (LCPUFAs) in human milk have an effect on the chemical composition of the brain and enhance retinal and cortical function? The presence of growth factors and hormones in breast milk are also of potential interest. These agents can act directly, influencing brain biochemistry and functional development, or indirectly, modifying sensory systems that affect brain development?
?The act of breast-feeding itself, as a mode of mother-infant interaction, may favor cognitive development. Taking milk from the breast has profound effects on both mother and infant. Hormonal responses in the dyad triggered by breast-feeding, prolonged skin contact, the reduction in maternal stress with feeding, and the improved mother-infant interaction all contribute to bonding and may enhance cognitive development.?
Breast is best: human milk is the optimal food for brain development, Ricardo Uauy and Patricio Peirano, American Journal of Clinical Nutrition, Vol. 70, No. 4, 433-434, October 1999
The difference is higher in tests done on children who were premature because of their relatively immature brains at the time of birth. The difference narrows with time to reach a plateau of breastfeeding at around 6 months of age. Therefore this may not affect the discussion here about follow on milk. There are of course other benefits to longer breastfeeding such as the continued immunological protection for example.