A study following 1,000 New Zealanders from infancy to adulthood found that those who were breastfed had roughly twice the incidence of asthma and allergic diseases as those who were not. The findings were reported in this week's issue of The Lancet.
"This does not mean that new mothers should not breastfeed," says lead researcher Malcolm R. Sears, MD. "Breastfeeding could be promoted for many reasons, including optimum nutrition and reduction of risk of infant infections. However, the role of breastfeeding in protection of children against (allergies) and asthma cannot be supported on the bases of the present balance of evidence."
Sears and colleagues from Ontario's McMaster University assessed the long-term incidence of allergies and asthma in children from Dunedin, New Zealand, followed from birth to age 26. Roughly half of the children had been breastfed.
The researchers found that more of the children who were breastfed were allergic to cats, dust mites, and grass pollen between the ages of 13 and 21. And periodic assessment showed that breastfed infants were far more likely than bottle-fed babies to develop asthma between the ages of 9 and 26.
"There was approximately a doubling of the risk of allergies and asthma from mid childhood to adulthood," Sears tells WebMD. "These findings are not inconsistent with the few studies that have looked at long-term outcome."
One such study, reported last year by researchers from the University of Arizona at Tucson, found that children with allergies who were breastfed by mothers with asthma were at increased risk for developing asthma themselves. But the New Zealand study found no link between a parent's history of allergies and asthma and the development of disease in children.
Breastfeeding expert Richard Schanler, MD, tells WebMD it is difficult to prove breastfeeding is linked to health outcomes later in life because so many other variables come into play as children age. Though there is some evidence that breastfeeding promotes intelligence in children and protects against brittle bones in adulthood, Schanler says the findings are far from definitive.
The evidence is overwhelming, however, that breastfeeding has a positive impact on health in the short term. Studies show that breastfed babies have less diarrhea, respiratory disease, and infections than bottle-fed infants.
"There may be long-term consequences to breastfeeding, but we don't know that definitively," he says. "We can't say that breastfeeding is good because it will make your child healthier when he is 10 or 15. But there is good data that it makes healthier babies."