A new Canadian-led study finds that breastfeeding does not protect children against developing asthma or allergies, contradicting a long-touted benefit about nursing.

The study led by McGill University's Dr. Michael Kramer is published online by the British Medical Journal.

Kramer and his colleagues followed 13,889 children born at 31 maternity hospitals in Belarus, between December 2002 to April 2005. The hospitals in the experimental group were trained to teach better breastfeeding techniques and to encourage mothers to breastfeed as long and as exclusively as possible. A control group of maternity hospitals were told to continue their traditional practices, which did not include encouraging breastfeeding.

At the end of the trial, when the children were 6� years old, mothers were asked questions about whether their child suffered from asthma, wheezing or hay fever symptoms or had itchy rashes or eczema. The children were also given skin tests to test for common allergens, including pet dander, dust mites and tree pollen.

The researchers found that breastfeeding did not provide any protection against asthma or allergies.

"We found, not only was there no protective effect," said Kramer, "but the results even suggested an increased risk of positive allergic skin tests."

This is believed to be the first randomized study on the effects of breastfeeding. Previous studies have been observational, meaning researchers looked at outcomes of two groups of subjects without intervening. Kramer's study, on the other hand, was able to randomly assign subjects at the beginning of a study to either an experimental group or a control group.

A randomized study on breastfeeding would have been impossible to conduct in Canada, because obstetrics units here are required to actively encourage breastfeeding. But in Belarus in the mid 1990s, when this study started, there were no programs in maternity hospitals encouraging breastfeeding.

The study encountered a major problem along the way. Six of the 31 maternity hospitals were found to have "suspiciously" high rates of positive results for the allergy skin tests. That led the researchers to conclude something was wrong at the sites, so they excluded their results from the study.

When the data from all 31 sites were analyzed, there were no statistically significant differences between the two groups of children. When the six sites with the suspicious results were excluded, those children whose mothers were urged to breastfeed longer were actually 1.5 to 3.5 per cent more likely to have positive allergy skin tests.

The authors say the results confirm that prolonged or exclusive breastfeeding offers no protection against allergies and asthma in children. But given the problems they had with the six excluded sites, the authors say they "cannot be confident" that prolonged breastfeeding "actually caused the increased risks."

Despite his findings, Kramer said he remains positive about the benefits of breastfeeding.

"In the first phase of our project, we observed reductions in gastrointestinal infections and atopic eczema for the first year of life. I urge mothers to continue to breastfeed," he said in a statement.

The authors say their research underlines the importance of looking for other explanations for the recent epidemic of allergy and asthma.