Older siblings are more likely to suffer from food allergies than their younger brothers and sisters, according to a new study.
Among the study participants, the prevalence of food allergies was 4 percent in firstborn children, 3.5 percent in second-born children, and 2.6 percent in those born later.
Firstborns were also more likely to have allergic rhinitis (an itchy, running nose) and conjunctivitis (inflammation of the eyelids) than their younger siblings. However, birth order did not seem to impact children’s susceptibility to asthma or atopic dermatitis (a type of eczema ).
Previous studies have shown a link between birth order and vulnerability to allergies, but the new study examined how this effect differs depending on the type of allergy. The study is the first to show this “birth order effect” occurs specifically in food allergies, the researchers say.
The findings are based on surveys of more than 13,000 children ages 7 to 15. The researchers compared the prevalence of each allergy to the children’s birth order.
Researchers aren’t sure why younger siblings appear to have some protection against allergies compared to older siblings. It may have something to do with changes in the womb or prenatal environment. For instance, multiple pregnancies may cause changes to the mother’s immune system, which may affect the fetus, said study researcher Takashi Kusunoki, of the Shiga Medical Center for Children in Shiga, Japan.