Breast-feeding can lower the chance of a child getting asthma, but a new study says that eating fast food cancels out the benefit.

"Like other studies, we found that fast-food consumption was associated with asthma," said senior author Dr. Anita Kozyrskyj of the University of Alberta.

She also found that the longer a child was breast-fed exclusively, the lower the risk of asthma.

"But this beneficial effect was only seen in children who did not consume fast food, or only occasionally had fast food," she said.

More than half of the 700 Canadian children in the study studied ate fast food more than twice a week. About 250 of those in the study had asthma. A news release on the work did not give information on how much lower the risk of asthma was in those who ate fast food, nor the ages at which the children were analyzed.

The research team did not look at why fast food might cause asthma, but the authors suggested the high fat content and high salt levels may be to blame.

The article was published in the journal Clinical and Experimental Allergy.