Psyche said:
The researchers noticed that children who developed allergies after prolonged exclusive breastfeeding were most likely to do so during the first years of life, suggesting that environmental factors such as pollen exposure, diet and disease are the more important factors in the onset of allergies in later childhood and early adulthood.
The subject of allergies and breastfeeding is a bit weird. There is a lot of studies going from the 80-s onward about breastfeeding being protective against allergies, leukemia, breast cancer in adulthood, but they pretty much all compare breastfed and formula-fed babies (like this one: \\\http://children.webmd.com/news/20080107/breastfeeding-may-lower-allergy-risk), and consider everything after 4 or 6 months of child's age "prolonged breastfeeding". There is really pretty much nothing in the studies at all about breastfeeding being in in any way beneficial or protective past 6 months of baby's age.
The one quoted above is talking about children breastfed exclusively, i.e., mother's milk only, for 9 months or later, which is way beyond what everyone currently recommends, and actually hard to imagine -- most normally developing 6-months old who is not confined to the crib all day will take some interest in the food the parent is eating, and it's the most natural thing to give her some here and there, so it will no longer be exclusive breastfeeding then.
If we are talking about truly prolonged breastfeeding, i.e., past 15-18 months of the child's age, the only actual studies will be from developing african and south-asian countries where this is a traditional practice (example: \\\http://www.ncbi.nlm.nih.gov/pubmed/8921445). What they see there is children who are breastfed for a few years are smaller than their early weaned peers, and there's more mortality among them from all diseases but diarrhea, from which there's less mortality.
I doubt that these data are transferable onto a western population, for the following reasons. First, the scales for children's height and weight here are based on formula-fed babies from 60-s and 70-s. Formula-fed babies are on average bigger, so even if a breast-fed child in a western country is smaller than her peers, that's rarely a cause for concern. And second, going back to the breastfeeding in developing countries, when they look at mothers' actual behavior, they see that the mothers tend to wean stronger and bigger children earlier, and keep nursing longer those who are already smaller and sickly.
So, the results they see there are likely due to subconscious selection of children and parenting strategies rather than the direct results of breastfeeding. Other than diarrhea, I would guess. Diarrhea is one of the main risks for childhood morbidity and mortality in developing countries. The child who is breastfeeding more will be eating less food, and therefore be less likely to pick up germs. Also, protection from gastrointestinal infection is the strongest effect of breastfeeding which came from the studies on western population, and that's also the the one where it has been shown that maternal antibodies directly affect the virus (\\\http://pediatrics.aappublications.org/cgi/content/full/119/5/941, there was one more recent one which showed how the antibodies line the child's intestinal wall, but I can't find it at the moment). But again, there's less of a risk of diarrhea in western world anyway, so that's not exactly transferrable as a scientific justification for the practice for older children.
I think also that WHO's recommendations to breastfeed until 24 m. are based on the situation in developing countries. It agrees with the traditional practices there, and it aims to combat diarrhea. This is a more recent policy; the previous one was to push formula for better nutrition but the results were very negative. Babies on formula had mush higher morbidity and mortality rates. Turned out, the mothers were either using contaminated water, not knowing any better, or rationing formula to save it. Basically, that western practice didn't at all fit the local conditions, and that's why the change.
From what I can see, the growing practice of prolonged nursing in western countries is not based on any studies but rather on the changes in overall parenting style among certain groups. Prolonged nursing goes well with co-sleeping, slings, attachment parenting etc. The main rationale is to listen to the needs of the child and follow them. So, people decide not to quit nursing at a certain time but rather to wait until the child is done and ready. For most kids, if the parent were to wait, that moment supposedly does come anywhere between 1 and 4 years of age. And long before it comes, nursing is already very limited and stops being the main focus of nutrition or the main activity for the child. It's more like a hug at the beginning and the end of the day and part of the closeness with the mother. Some of my friends nurse a long time, and they say that their toddlers would nurse a lot when they are sick, but can even skip a day when they are healthy.
This practice can possibly be prone to transference and projections form the parent's side, but in and of itself, I am not sure that nursing for, say, 2 years instead of 1 will make a noticeable difference either way: what exactly the parent does and how he\she does it will probably be more important.
fwiw
