And nobody knows why. Popular culprits range from genetically modified foods to vaccines to roasted, rather than boiled, peanuts. The dominant theory, dubbed the hygiene hypothesis holds that as our lives have gotten cleaner-more antibiotics, immunizations, and antibacterial soaps-our immune systems aren't practicing on the right microbes and attack foods instead.
The second major hypothesis, according to Donald Leung, head of pediatric allergy and immunology at the National Jewish Medical and Research Center in Denver, is "food-induced tolerance"early exposure to certain foods (even in utero or through breast milk) triggers allergic reactions.
Contradictory evidence abounds: In a June paper published in Pediatrics, children who ate wheat before six months of age actually had lower allergy rates than those who avoided it. Some studies show C-section delivered babies to be more allergic; others, the opposite. Some evidence suggests homogenized peanut butter is the culprit, other studies point to an increased use of vegetable oils. Dr. Hugh Sampson, professor of pediatrics and immunology at the Mount Sinai School of Medicine and widely considered the top expert in the field, says: "It's all still speculative. The answer probably lies in multiple factors-a combination of the hygiene hypothesis and environmental and dietary changes-but my sense is that we still have not gotten to the bottom of why this dramatic increase is happening."