A long-term study of breast-feeding has linked the practice to a susceptibility to asthma and allergies.

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Breast-feeding children for more than a month can double their risk of developing asthma or allergies, a 26-year study by researchers in Canada and New Zealand suggests.

This contradicts the assumption breast-feeding protects infants against both conditions and is expected to reignite the debate over whether "breast is best."

Dr. Malcolm Sears, the lead researcher and a professor of medicine at McMaster University in Hamilton, expects an onslaught of criticism after the study is published in The Lancet, a British medical journal, tomorrow.

"There are many, many reasons why breast-feeding is good," he said in an interview.

"All we're saying is that it doesn't protect against asthma and allergies long-term, and in fact seems to increase the risk. If there are 100 reasons why it [breast-feeding] is good and one reason why it's bad, then you go for the 100 reasons why it's good."

Dr. Sears and colleagues at the University of Otago in Dunedin, New Zealand, tracked 1,037 children born in that city between April 1972 and March 1973. Five hundred and four of the children (49 per cent) were breast-fed for four weeks or longer, while 533 (51 per cent) were not. The children were routinely assessed with respiratory questionnaires, measures of breathing function and skin prick allergy tests.

The researchers found children who had been breast-fed were significantly more likely to be allergic to cats, house dust mites and grass pollen by age 21 than children who had not been breast-fed. The breast-fed children were also more likely to report asthma at every assessment. The researchers say their subjects' family history of hay fever or asthma did not affect the results.

Dr. Sears was shocked by the results because they countered the research team's expectations. He initially assumed someone had erred while crunching the numbers.

What sets this study apart is its length: Most people's assumptions about the link between asthma and breast milk are based on the results of short-term studies, Dr. Sears said. Breast-feeding may indeed protect against asthma and allergies, but that trend reverses itself over the long term, he explained.

Breast milk contains antibodies that help infants fight bacteria, viruses and parasites. A plethora of studies have linked breast-feeding with higher child IQs and a reduced risk of ear infections, childhood cancers and obesity, to name a few.

The World Health Organization last increased its recommended time for exclusive breast-feeding from four months to six [with supplementary breast-feeding continuing until at least the age of two].

But a growing body of evidence suggests breast-feeding can increase the risk of certain conditions. British researchers recently linked breast-feeding to early signs of coronary artery disease: They found adults between 20 and 28 who had been breast-fed for more than four months had stiffer arteries than peers who had been bottle-fed or nursed for a shorter amount of time. A recent German study also suggested mother's milk can contain concentrations of polychlorinated biphenyls, or PCBs, that are so high they can hinder a baby's development.

Previous Canadian research has also suggested nursing mothers should avoid peanuts if their family history suggests the child might develop an allergy to them, because significant amounts of peanut protein can be passed through breast milk.

Dr. Jack Newman, a leading Canadian breast-feeding advocate who established the first hospital-based breast-feeding clinic in the country, is skeptical of the new findings. Almost all newborn babies are fed at least one or two bottles of supplementary formula before they leave the hospital, which can skew the results of any study comparing breast-feeding and formula-feeding, he said.

"Why does this study invalidate other studies that came before it? I don't think it does," he said.

The researchers acknowledge most of the children probably received some formula feeding in their first days of life, but stress that fact does not affect their results.