Via Slate.com:

Parenting forums are teeming with moms discussing their babies’ so-called dairy and soy “sensitivities” (one mom helpfully recommends gummy bears, oreos, and potato chips as dairy-free snacks), and many of my friends swear that after giving up certain foods, their unhappy babies turned into cooers overnight. But anecdotal reports are one thing; what does science have to say? The research on this issue is complex, but the consensus is that, most of the time, a baby’s crying—even when incessant—isn’t caused or exacerbated by what you or he is eating. Babies cry, and it sucks, but it’s normal. Moreover, mothers who give up foods while breast-feeding can put themselves or their children at risk for nutritional deficiencies.

That said, a small percentage of babies are indeed allergic to food proteins—somewhere between 2 to 8 percent are allergic to cow’s milk, but among babies who have only ever been fed breast milk, the risk is only about 0.5 percent; an infant’s risk of being allergic to soy is estimated at 0.7 percent—and these babies, who usually have other warning signs in addition to crying a lot, do fare better after dietary changes. Food allergies and intolerances are notoriously difficult to diagnose in young kids. (An allergy is not the same thing as an intolerance: The former usually involves an immune-mediated reaction, whereas an intolerance can involve different physiological responses, not usually immune.) Some children do not respond to skin prick tests when they are allergic, while others who aren’t allergic do. Blood antibody tests (which look for evidence that the body is primed to react to certain proteins) aren’t always useful either, in part because some food intolerances don’t involve antibodies.

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