The theory that asthma may be passed in breast-milk is one of two put forward by scientists to explain the unexpected finding that breast-fed babies of asthmatic mothers are up to eight times more likely to develop asthma than those that are bottle-fed.
The researchers suggest there may be chemicals in asthmatic women's breast-milk affecting their children's immune response, so increasing their later risk of developing asthma.
The other theory is that as breast-feeding is known to protect against infections in early life, and exposure to infection is important to the development of the immune system, breast-feeding may be disadvantageous for susceptible children.
The study of 1 200 babies found those who were breast-fed were significantly less likely to have wheezing problems at the age of two.
But by the age of six, children with asthmatic mothers were five times more likely to wheeze if they had been breast-fed. The risk continued to rise up to the age of 13.
However, the increased risk only applied to those children who were atopic - meaning they had a tendency to allergic reactions.
There was no increase in risk among children of asthmatic mothers who were breast-fed if they were not themselves atopic.
This limits the application of the research, published in the journal Thorax.
The scientists from the Respiratory Sciences Center in Tucson, Arizona, point out that it is not known whether a child will become atopic until well past babyhood - long after decisions about whether to breast-feed or bottle-feed have been made.
Even if a link between breast-feeding and asthma is proved in susceptible children by subsequent studies, this would have to be set against the many known advantages of breast-feeding, including better promotion of growth and development.
"There is no need to reconsider current recommendations that infants be exclusively breast-fed for six months," say the authors of the report. - The Independent