NEW YORK (Reuters Health) - A new long-term study is challenging the notion that breast-feeding helps protect children from developing allergies and asthma, one of the widely promoted potential benefits of breast-feeding.

In fact, researchers found that of the more than 1,000 children they followed to age 26, those who were breast-fed for at least 4 weeks were more likely to develop asthma or various types of allergies.

The findings are published in the September 21st issue of The Lancet.

Health experts including the World Health Organization and the American Academy of Pediatrics (AAP) recommend that women breast-feed their babies' during their first year for a variety of reasons. Breast milk is recognized as the best source of infant nutrition, and it is thought to cut the risk of various infant infections and have numerous other health benefits for mother and baby.

"There's 101 reasons to breast-feed," Dr. Malcolm R. Sears, the new study's lead author, told Reuters Health. However, he said, the new findings indicate that lower long-term risks of allergy and asthma are not among them.

Dr. Nancy Krebs, chair of the AAP's nutrition committee, said the findings "don't take away from the recommendation to breast-feed for one year," with introduction of baby food around 6 months of age.

She told Reuters Health that "we don't need to try to attribute every (health) effect to breast-feeding" for it to be the preferred form of infant nutrition.

Sears, a researcher at McMaster University in Hamilton, Ontario, Canada, and his colleagues based their findings on data from 1,037 children born in New Zealand between 1972 and 1973. The children, about half of whom were reportedly breast-fed for at least 4 weeks, were assessed for allergies and asthma every few years between the ages of 9 and 26.

The researchers found that breast-fed children were actually more likely to suffer from allergies to cat dander, pollen and other common triggers at the age of 13 or 21, when skin tests were performed. Asthma was also more common among breast-fed children, with a two- to three-times higher risk found among 9- to 13-year-olds who had been breast-fed.

The findings stand in sharp contrast to "common knowledge" about the effects of breast-feeding on allergy and asthma risk, Sears said. However, he and his colleagues point out, past studies have produced conflicting results on the issue.

In an interview, Sears explained that much of the uncertainty may be due to the duration of these studies. He said "a dozen or more" have shown breast-feeding to be protective against wheezing in infancy, but this effect may not hold up in the long term.

Sears pointed to a Tucson, Arizona study that linked breast-feeding to less wheezing before age 2, but to a greater longer-term risk of asthma among children whose mothers had the lung disease.

In their study, Sears and his colleagues found that, unlike in the Tucson study, family history of allergies and asthma had no bearing on the risk associated with breast-feeding. Why, he said, is unclear.

Also unclear is the mechanism by which breast-feeding could raise the odds of allergy and asthma later on. Breast-feeding might have some effects on the bacterial balance in infants' intestines or on the developing immune system that could predispose children to these conditions, but a study like the current one cannot address these questions, Sears said.

It is well accepted that genetics and environmental factors conspire to put certain people at risk of allergies and asthma, two child-health experts point out in an editorial published with the report.

But in studies to date, the effect of breast-feeding on these conditions "is not consistent between studies and changes with the age of the child," write Drs. Peter D. Sly and Patrick G. Holt of the Telethon Institute for Child Health Research and Center for Child Health Research in Perth, Australia.

"Although there are many valid reasons for encouraging breast-feeding...," they write, "based on the current evidence the prevention of asthma and allergies is not one of them."

SOURCE: The Lancet 2002;360:887-888, 901-907.

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