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Diagnosing food allergies properly is hard, and many children with suspected allergies don't really have them. In surveys, almost 25 percent of parents believe their children have food allergies, but only 4 to 7 percent are substantiated by formal testing. That means that most suspected allergies may be nonexistent. Unfortunately, some parents self-diagnose allergies based on vomiting or rash after their babies eat certain foods, and don't seek medical care. That's a problem, since a 2003 study found that children told they had a severe food allergy had more anxiety and felt more physically restricted than those with diabetes.
The "gold standard" test is a blinded food challenge, where a child is fed either the suspected food or a placebo in a clinical setting to see if a reaction occurs. But because of the time and expense of the test, it's not often done. The alternatives--blood and skin-prick tests--can be imprecise. According to a 2001 study from Clinical and Experimental Allergy, for example, only 1 in 4 children with positive skin tests (that is, those who develop a small hive when injected with a tiny dose of peanuts) had positive food challenges.
Bottom line: Don't assume your child has a food allergy unless a board-certified allergist or pediatrician with excellent allergy experience confirms the diagnosis.
You don't need a special diet to prevent allergies in your child, especially if you don't have a strong family history of food allergies. First of all, avoiding certain foods during pregnancy and withholding foods from infants does no good in healthy infants. Further, it's not clear whether "high-risk" infants (those with a strong family history) benefit either. The best study of this practice, published in the Journal of Allergy and Clinical Immunology in 1995, showed that an extremely strict maternal elimination diet coupled with delayed introduction of allergenic foods to kids had no impact on allergy symptoms by the time a high-risk child was 7-years-old.
For unclear reasons, the American Academy of Pediatrics (AAP) still warns pregnant and nursing mothers to "consider eliminating eggs, cow's milk, fish, and perhaps other foods from their diets while nursing." But in 2006, a systematic review in the Archives of Pediatrics and Adolescent Medicine concluded, "Maternal avoidance of allergenic foods during lactation, particularly milk, egg, and peanut, has been studied extensively without conclusive results."