British researchers have cast doubt on the safety of the exclusive breastfeeding of babies until they are six months old.
Exclusive breastfeeding - giving babies only breast milk and no solids, infant formula or other fluids - for their first six months is recommended by the World Health Organisation.
Governments including New Zealand's and Britain's have adopted the advice.
Fourteen per cent of New Zealand babies were exclusively breastfed until six months old in 2007 - up from 9 per cent in 2002.
But paediatrician Dr Mary Fewtrell, of University College London, and her colleagues say in the latest British Medical Journal that exclusive breastfeeding may not be the best idea.
"Exclusive breastfeeding for six months is readily defensible in resource-poor countries with high morbidity [sickness] and mortality from infections".
"In the West, exclusive breastfeeding for six months is linked to reduced risk of infection.
"Nevertheless, the studies are observational and some evidence suggests that introducing solids - rather than formula - before six months may not significantly affect risk of infection.
"By contrast, exclusive breastfeeding to six months raises concerns [such as] ... higher risk of iron deficiency anaemia ... concerns over a higher incidence of food allergies [and] higher risk of coeliac disease."
They say iron deficiency anaemia is linked to irreversible adverse mental, motor and psychosocial effects.
They also add the "relatively unexplored concern" that by failing to expose babies to new tastes early in life, exclusive breastfeeding to six months may lead to children never developing a liking for bitter foods.
This could put them off green leafy vegetables for life and "may influence health outcomes such as obesity".
The "proposed beneficial effects" of exclusive breastfeeding to six months must be weighed against the "plausible or at least suggestive" evidence of adverse effects.
New Zealand's Ministry of Health is standing by its recommendation of exclusive breastfeeding for the first six months.
"The Ministry of Health's advice remains unchanged until we have looked at the study and see what implications there may be for New Zealand women and their babies," senior adviser on maternity services Bronwen Pelvin said yesterday.
Britain's College of Midwives challenged the suggestion the country should reconsider its official advice on breastfeeding.
"I believe this is a retrograde step and plays into the hands of the baby food industry," said the college's professional policy adviser, Janet Fyle.
She said that if earlier introduction of solids was encouraged, many parents would offer sugar-based foods to their babies, potentially nurturing a sweet tooth.
- additional reporting, Telegraph Group Ltd