David Hill, MD, PhD, of the Children's Hospital of Philadelphia, and his colleagues compared the records of 158,422 children to see how the method of birth and feeding practice influenced the number of allergic conditions reported by each young person over a period of 18 years.
Earlier studies have looked at the effect of breastfeeding or birth method for a single condition, Hill said. He presented results from the study -- which looked at eczema, food allergy, allergies, and asthma -- at the American College of Allergy, Asthma & Immunology 2019 Annual Scientific Meeting in Houston.
After adjusting for race and sex, the team looked at the influence of how the baby was born (vaginal vs. cesarean) and feeding (breastfed, bottle-fed, or a combination of the two).
Children delivered vaginally had lower rates of one, two, three, and four allergic conditions.
"The benefits of vaginal delivery were quite broad," Hill said. The protective effect for those who would develop one condition and for those who would develop all four was reduced for this group.
The protective effect likely has something to do with the microbiome, he said. "I think there's some pretty good evidence now -- from animal models and associations in humans -- to suggest that the birth canal is probably the source of microbial introduction."
Children who were exclusively breastfed had a reduced rate of one, two, and three conditions.
Children who were fed a combination of breast milk and formula had a reduced rate of one allergic condition but no significant difference for two or three conditions.
Supplemental breastfeeding protected against the development of a single allergic condition, but for children who got two or more conditions, supplemental breastfeeding had no effect.
Families, for many reasons, might choose to not exclusively breastfeed and might supplement with formula, he pointed out. "They should know this still has a protective effect."
Hill explained that in his own family, his child could not be breastfed for as long as recommended. "It is nice to see that even supplemented-breastfed babies benefit. I was surprised to see the degree of this benefit."
Carina Venter, PhD, a registered dietitian from Children's Hospital Colorado in Aurora, also presented findings from the Healthy Start Study at the meeting.
In their survey of 1,315 women, Venter and her colleagues showed that a poor diet during pregnancy, combined with a history of allergic disease, means that children have a 33% chance of being diagnosed with eczema or food allergy by the age of 2.
"Of the remaining mothers who were classified as having either good dietary diversity, with or without a personal history of allergic disease, or as having poor dietary diversity with no personal history of allergic disease, 21% of their children were diagnosed with eczema and/or food allergy by age 2 years," Venter reported.
"Pregnant women -- especially those with allergies -- should be aware that their diet during pregnancy can affect their child's chances of developing eczema and/or food allergies" co-author David Fleischer, MD, also from Children's Hospital Colorado, said in a news release.