The recommendations from the World Health Organisation (who review their recommendations every 2 years) remains that breastmilk provides all the nutrients a baby needs for the first 6 mnths and that solids should be introduced at around 6mnths of age.
Two very good articles I have head on this lately are
http://www.analyticalarmadillo.co.uk...nd-todays.html

and
http://comms.unicef.org.uk/t/339972/5039655/2647186/0/