I think my previous post got lost in a flurry of posting.
Anyone saying that there's no evidence of harm in weaning before 6 months is out of date. There is now evidence.
Link is here
They studied just under 16000 babies - the biggest study of this kind to date, importantly, in a developed country (the UK).
Excerpts from the abstract:
"Data analyzed by month of age, with adjustment for confounders, show that exclusive breastfeeding, compared with not breastfeeding, protects against hospitalization for diarrhea and lower respiratory tract infection. The effect of partial breastfeeding is weaker. "
"Population-attributable fractions suggest that an estimated 53% of diarrhea hospitalizations could have been prevented each month by exclusive breastfeeding and 31% by partial breastfeeding. Similarly, 27% of lower respiratory tract infection hospitalizations could have been prevented each month by exclusive breastfeeding and 25% by partial breastfeeding. The protective effect of breastfeeding for these outcomes wears off soon after breastfeeding cessation."
They followed babies up to 8 months of age and found an effect with each month, but the effect became weaker as the babies grew older. So the effect was stronger in babies up to 4 months of age than 5-7 months, but there was still an effect in the 5-7 month old babies.
There are other studies which found the same effect, but this is the biggest and best.
So, can we stop saying that there are no risks in weaning a 4-6 month old baby - there are. Anything other than exclusive breastfeeding (ie , formula feeding, mix feeding or weaning) significantly increases the risk of being admitted to hospital with infection.
What is much more important to discuss is how you would balance that potential risk against other potential risks, eg the question of whether allergy can be worsened by holding off food, and against the fact that some babies are willing and able to eat before 6 months (and therefore presumably ready).
There are no answers in the back of the book, there needs to be much more research before it's really clear, but the trend of evidence now very strongly suggests that the WHO advice of 2001 is sound, and that the vast majority of babies are weaned too soon.