Most of the evidence supporting appreciable medium or long term benefits of bf rely on observational studies which are subject to confounding. All of the counfounding factors (maternai IQ, smoking, income age) point one way (beneficial effects) and it is just not possible to be sure that all confounders have been identified or adjusted for appropriately.
If only there were a study based on randomised control testing!
Well there is. PROBIT. desigend by Prof Michael Kramer, posibly the most respected expert in this field. They got around the ethical dilemma by randomising bf promotion rather than bf itself. The children whose mothers were subjected to ( had the benefit of..) intensive bf promotion had significantly more bm than those in the control group, and significantly more exlusivly bf for longer
The results make for interesting reading.
www.ncbi.nlm.nih.gov/pmc/articles/PMC2034727/
no effect on allergy. Actually found that exclusive bf increased sensitivity to common allergens such as grass pollem
www.ncbi.nlm.nih.gov/pubmed/19106322
www.ncbi.nlm.nih.gov/pubmed/23483175
No effect on obesity or markers pointing towards diabetes
www.ncbi.nlm.nih.gov/pubmed/18065591
no effect on blood pressure height or weight
www.ncbi.nlm.nih.gov/pubmed/19710187
Summary of results at 6 years old- Non of commonly claimed health benefits observed at 6 years old
I could go and find some of the many studies which show that bf is "associated with" an INCREASE in rates of asthma and ezcema but I have to have my dinner!