A new study consisting of more than 13,000 Japanese school children, and discussed at the Annual Meeting of the American Academy of Asthma, Allergy and Immunology in San Francisco this week has shown an effect of birth order on food allergies in kids. First-born children may be more likely to develop certain types of allergies than their younger brothers or sisters.

Researchers also say these findings suggest that food allergies may have their origins in the prenatal period in the womb.

The results showed there was no significant difference in the prevalence of asthma or atopic dermatitis (a type of eczema) according to birth order, but the prevalence of allergic rhinitis, allergic conjunctivitis, and food allergy decreased based on birth order, from oldest to youngest.

Takashi Kusunoki, MD, PhD, of the Shiga Medical Center for Children in Moriyama, Japan explains:

“Individuals with increased birth order have a smaller risk of allergy. However, the significance of the effect may differ by allergic diseases.”

In the study, 4% of first-borns had some type of food allergy compared with 3.5% of second-borns and 2.6% of third-borns.

An allergy should be suspected if someone has a reaction within minutes or hours of eating a food, according to guidelines. Physicians should then take a detailed medical history, conduct a physical exam and confirm the allergy with a skin-prick test, in which tiny drops of the suspected allergen are pricked into the skin, usually in the forearm, to see if red wheals form. None of those steps is definitive by itself, the recommendations say, which will likely to lead to fewer diagnoses.

Roughly 4% of children under age 18, about three million, reported having food allergies in 2007, an 18% increase from 1997, according to the Centers for Disease Control and Prevention. Milk, eggs, peanuts, wheat, soy, fish and shellfish are the most common culprits, although more than 170 other foods have been reported to cause allergic reactions. Symptoms can range from eczema and hives to asthma, inflammation of the esophagus, diarrhea, vomiting and life-threatening anaphylaxis, in which major body systems quickly shut down.

More common than food allergies are intolerances to certain foods, which can cause vomiting, diarrhea, spitting up, and skin rashes. An example of such a reaction occurs in children with lactose intolerance, which occurs because of a deficiency of the enzyme lactase, which normally breaks down the sugar lactose. Children without this enzyme or who have a decreased amount of the enzyme, develop symptoms after drinking lactose containing food products, such as cow’s milk. However, because this reaction does not involve the immune system, it is not a real food allergy.

The study also showed that in infancy symptoms such as wheezing increased and food allergies decreased as birth order increased. The prevalence of allergic rhinitis (hay fever or seasonal allergies), allergic conjunctivitis (eye inflammation due to allergies), and food allergy were the main categories tracked.

Source: The American Academy of Allergy, Asthma and Immunology

Written by Sy Kraft, B.A.