LONDON - Breastfeeding is the best nutrition for babies and helps to reduce the risk of childhood infections, but scientists said on Friday it might actually increase the risk of asthma and allergies.

Canadian and New Zealand researchers found that infants fed naturally for more than four weeks were about twice as likely as bottle-fed babies to develop asthma or allergies.

"Breastfeeding does not protect children against atopy (allergy) and asthma and may even increase the risk," Malcolm Sears, of McMaster University in Canada, said.

The findings, reported in The Lancet medical journal, contradict earlier studies that showed beneficial effects against allergies from breastfeeding for four months or longer.

Sears said the conflicting results could be explained by the different duration of the follow-up periods in the various studies and the ages at which the children were assessed for allergies and asthma.

Sears and colleagues from the University of Otago in New Zealand studied 1,000 children from Dunedin, New Zealand, who were born between 1972-1973.

Each child was tested for allergies and breathing function every two to five years up to the age of 26.

More breastfed children were allergic to cats, grass pollen and house-dust mites between the ages of three to 21 than babies who had been fed formula. Asthma was also more prevalent between the ages of nine and 26 among naturally fed babies.

"Breastfeeding for four weeks or longer increased the likelihood of skin test responses to common allergens at age 13, and more than doubled the risk of diagnosed asthma in mid-childhood, with effects persisting into adulthood," said Sears.

In a commentary in the journal, Peter Sly, of the Telethon Institute for Child Health Research, and Patrick Holt, of the Center for Child Health Research in Perth, Australia, agreed that the results of studies into the effects of breastfeeding are conflicting and change with the age of the child.

"Although there are many valid reasons for encouraging breastfeeding during the first four to six months of life, based on the current evidence the prevention of asthma and allergies is not one of them," they said.

Breastfeeding has been shown to affect intelligence and behavior and to decrease the incidence of ear infections, diarrhoea, eczema, influenza and respiratory illnesses.