Children delivered by caesarean section face twice the risk of developing allergy or intolerance to cow's milk than other children, according to a report in the medical journal Allergy.

The thinking is that compared to infants born normally, children delivered by c-section are exposed less to maternal bacteria and their digestive tracts are consequently not colonized in the normal way. Their immune system in turn overreacts to allergenic substances.

Dr. Merete Eggesboe from the Norwegian Institute of Public Health, Oslo , and colleagues, who previously reported a similar association between caesarean delivery and egg, fish, and nut allergy, investigated possible links to cow milk allergy in 2656 participants in the Oslo Birth Cohort.

The team found that cow milk allergy or intolerance was twice as common among children delivered by caesarean section compared to children delivered vaginally.

None of the children previously diagnosed with milk allergy or intolerance but deemed tolerant by age 2.5 years had been delivered by caesarean section, the researchers note, suggesting that c-section makes it less likely that children will become tolerant.

"The results of the present study cannot be explained by differences between predisposed and not predisposed children and thus provides support for early intestinal colonization playing a role in the etiology of food allergy," the investigators conclude.