Children who are breastfed for fewer than three months and who are overweight have an increased risk of developing asthma, new research suggests.

In a study of more than seven hundred 8- to 10-year-olds, researchers found that the combination of being breastfed for fewer than 12 weeks and excess childhood weight gain raised a child’s asthma risk by 80 percent.

Short breastfeeding duration by itself was not associated with later asthma risk. It’s not clear, though, whether breastfeeding is linked to asthma only through effects on children’s weight, according to the study authors.

“Our study suggests that short duration of breastfeeding increases the risk for overweight and asthma concurrently,” lead study author Dr. Xiao-Mei Mai told Reuters Health. “We were not able to evaluate if overweight is a mediator in the relationship.”

Regardless, the findings support experts’ opinion that “breast is best” for infants, according to Mai and colleagues at the University of Manitoba in Canada.

In general, it’s recommended that infants have breast milk only for the first six months of life, then continue breastfeeding after starting solid foods, up until at least the age of 12 months.

But Mai noted that when it comes to asthma risk, some studies suggest that exclusive breastfeeding for three months or more may be protective.

The current study included 246 children with asthma and 477 without the lung disease, all between the ages of 8 and 10. Parents answered questionnaires on breastfeeding and other factors in the child’s environment, such as whether anyone in the house smoked.

Mai’s team found that a short duration of exclusive breastfeeding - fewer than 12 weeks - was only weakly tied to the risk of developing asthma. However, short-duration breastfeeding was more strongly linked to the risk of becoming overweight, and these children were at elevated risk of asthma.

This relationship was particularly strong among children whose mothers had asthma, the researchers found.

One theory on why breastfeeding, weight and asthma are related has to do with a hormone called leptin, which helps regulate food intake by dampening feelings of hunger. According to Mai’s team, leptin is present in breast milk, and studies have found that breastfed infants tend to have higher blood levels of leptin than bottle-fed babies do. These higher leptin levels may help control weight gain early in life.

There is also evidence that leptin may affect immune system function - possibly inhibiting the abnormal immune response that triggers allergies and asthma.

All of this remains speculation for now. However, the study “adds to the importance of promoting prolonged breastfeeding for the prevention of overweight and asthma.”

SOURCE: Journal of Allergy and Clinical Immunology, September 2007.