Does breastfeeding protect against asthma?

A nurse assists a mother as she attempts to breast feed her newborn baby at the government-run Fabella hospital in Manila March 24, 2010.

NEW YORK (Reuters Health) - Sticking to a strict diet of mom's milk during the first 4 months of life may reduce a child's risk of developing asthma by their eighth birthday, according to a new study.

"Breast milk is the optimal food for infants during the first months of life," lead researcher Dr. Inger Kull of the Karolinska Institute in Stockholm, Sweden, told Reuters Health in an email. "But whether or not breastfeeding reduces the risk of asthma has been debated."

Through her milk, a mother transfers "good" bacteria, antibodies and proteins that can help thwart infection. But the evidence for how breastfeeding might influence the later development of asthma remains confusing, with various studies suggesting protective, neutral and even detrimental effects.

Kull and her colleagues decided to look into the relationship a little differently than previous studies. They followed nearly 4,000 Swedish children over 8 years, noting which received short- or long-term, as well as exclusive or partial, breastfeeding.

The team found that about 12 percent of children who were exclusively breastfed for at least the first 4 months of life developed asthma by age 8, compared with 18 percent of those breastfed for a shorter amount of time.

This translated into a 37 percent lower risk of asthma for those breastfed for 4 months or longer, after adjusting for other risk factors such as maternal smoking and birth weight.

The difference appeared to be driven mostly by allergy-related cases of asthma.

The extended and exclusive diet of breast milk also resulted in better lung function at age 8, the researchers report in the Journal of Allergy and Clinical Immunology.

Of course, not all mothers have the desire or ability to solely breastfeed for their child's first months. For these women, Kull suggests that even partial breastfeeding can somewhat reduce the risk of asthma.

But what about all the other conflicting research? Dr. Malcolm Sears of McMaster University in Ontario, Canada, led one of these studies. His team found that New Zealand children breastfed for more than 4 weeks actually developed more asthma later in childhood.

Sears says one possible explanation for his result is the "hygiene hypothesis" -- the theory that the increasingly germ-free surroundings of modern life are actually contributing to an increase in allergies and asthma.

"If you're absolutely protecting children from everything, and breastfeeding protects them to a certain extent, this may allow the immune system to switch to becoming allergic rather than fighting infections," Sears told Reuters Health.

He also suggested the possibility that while breastfeeding might initially increase the risk of asthma, if a mother keeps at it for a long time, the risk can be reduced again - even to the point where breastfeeding becomes protective. Interestingly, his study used an earlier cut-off between short- and long-term breastfeeding than the Swedish study.

Still, given the inconsistent findings, Sears cautions against using asthma protection as the motivating factor for breastfeeding. "There are 101 good reasons to breastfeed," said Sears. "Whether or not it protects you against asthma, you should choose to breastfeed for all the other good reasons."