The findings are based on a study of 865 babies, none of whom were premature. All of the babies were fed only breast milk until they were four months old. All were born into families with a history of allergies and were being monitored as part of the German Infant Nurtitional Intervention Program (GINI) study.

The babies were monitored at one, four, eight and 12 months of age. Blood samples were also taken after 12 months to check for antibodies to food allergens, including eggs, cows' milk proteins and soy protein. During the first six months, their mothers completed weekly diaries on their children's health and feeding.

In all, 147 of the 865 babies were born by C-section -- a rate of 17%. Pregnancy risk factors and maternal illnesses were no different between babies born by C-section and those born vaginally. But delivery by C-section was four times as likely among mothers who had undergone the surgery with previous births. Poor fetal positioning was also more likely among those delivering by C-section.

Normal Bacterial Colonization Delayed

Neither colicky pain nor atopic dermatitis during the first four months of life were associated with type of delivery. But babies born by C-section were significantly more likely to have diarrhea up to the age of 12 months.

They were also twice as likely as babies born vaginally to be sensitized to cows' milk and any of the five food allergens at 12 months of age.

The authors suggest that their findings are in keeping with other research indicating the importance of gut bacteria in the development of the immune system response, and that C-section alters or delays "normal" bacterial colonization of the baby's gut.

They suggest that vaginally delivered babies acquire the mother's vaginal, intestinal and perianal bacteria, whereas babies born by C-section acquire bacteria from the hospital environment.