Researchers reviewed the medical records of several hundred children in Olmsted County, in southeastern Minnesota, and found that new diagnoses of peanut allergy rose from two out of every 10,000 kids in 1999 to nearly seven out of every 10,000 in 2007.

Overall, 65 of every 10,000 children in the county - home to a sophisticated countywide electronic health records system that provided data for the study - had a verified peanut allergy in 2007.

Though exact percentages of kids with peanut allergies may vary from place to place and study to study, Dr. Ruchi Gupta, an associate professor of pediatrics at Northwestern University's Feinberg School of Medicine in Chicago, who was not involved in the research, said, "What's consistent with this article is it does look like peanut allergies, especially among the younger children, are increasing over time."

Most, though not all, studies trying to gauge how widespread food allergies are among kids have tracked an increase in recent decades.

Food allergies are thought to affect eight percent of U.S. kids, with the most common culprits being cow's milk, wheat, egg, soy, peanuts, tree nuts and some seafood.

Peanut allergies typically appear early in life, when toddlers are first encountering solid foods, and about 20 percent of cases go away on their own as a child gets older.

Maria Rinaldi, lead author of the new report and an epidemiologist at the University of Minnesota in Minneapolis, said estimates of peanut allergy vary because of the way peanut allergy is defined for the purposes of research studies.

Some rely on parents' reports of allergy, which may or may not be validated by a doctor.

Rinaldi and her colleagues took a conservative approach, gathering data from the medical records of more than 500 Olmsted County children with suspected peanut allergies between1999 and 2007.

Rinaldi said her team, who published their findings in The Journal of Allergy and Clinical Immunology, used a strict definition of allergy, and only included children who had laboratory-confirmed peanut allergy, narrowing the group down to 171 kids.

They found that fewer children had been diagnosed with peanut allergy in 1999 compared to later years.

For instance, just 10 children in the county were diagnosed in 1999, and 30 were diagnosed in 2007.

"No matter how we're defining peanut allergy, we're seeing this consistent increase," Rinaldi told Reuters Health.

Over three quarters of new diagnoses were in children under two years old, and about 70 percent were boys.

It's not clear why the food allergy has become more common.

"The leading theory is about hygiene - with less infection thanks to city living, smaller families, vaccines, sanitation, antibiotics, etc., the immune system is less 'busy' with germs and may become more prone to attack harmless food proteins," said Dr. Scott Sicherer, a pediatrics professor at Mount Sinai School of Medicine in New York, in an email.

Sicherer, who did not participate in the latest research, said another theory is that kids receive less vitamin D from sunlight because they are indoors or use sunscreen, and that could alter their immune system responses.

But these remain theories, and no one has proven why allergies are increasing.

Because peanut reactions can be severe, even lethal, measures to protect allergy sufferers from accidental exposure are becoming more common too, with peanut bans on airplanes, peanut-free sections of baseball stadiums and "school safe" packaged snacks manufactured in nut-free environments.

Gupta said it's important for parents to be aware of the potential signs of a food allergy, including hives, respiratory symptoms and digestion problems.

She also urges parents to see a doctor if they suspect their child has an allergy.

SOURCE: bit.ly/NXCIYX The Journal of Allergy and Clinical Immunology, online September 3, 2012.

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