Between 1 and 2 percent of children in the U.S. and Canada may have allergies to peanuts or tree nuts, with the U.S. rates seemingly on the rise, two new studies suggest.

Peanuts and tree nuts such as almonds and walnuts are the most common causes of severe, sometimes life-threatening allergic reactions to food in the U.S. Recent research has suggested that while the percentage of children with these allergies remains relatively low, rates may be on the upswing.

In the new U.S. study, researchers found that among 5,800 households surveyed in 2008, 1.4 percent of children younger than 18 had a peanut allergy, based on parents’ reports. That was triple the rate the same researchers found in a comparable survey done in 1997.

The Canadian study, of more than 3,600 households surveyed in 2008 and 2009, found a somewhat higher rate - with 1.7 percent of children having a “probable” peanut allergy. Similar patterns were seen with allergies to tree nuts.

In the U.S. survey, 1 percent of children were reported to have tree-nut allergies - up from 0.2 percent in 1997. Meanwhile, in the Canadian study, 1.6 percent of children were considered to have probable tree-nut allergies.

Allergies were considered “probable” when parents reported a “convincing” history of allergy symptoms - such as a rash, hives, swelling or wheezing within 2 hours of their child having the suspect food - or when they said a doctor had diagnosed the problem.

The Canadian researchers also tried to confirm the reported allergies by asking participants for permission to contact their doctors. However, many refused to give it, and even when they did, the doctors often failed to provide the test results.

As a consequence, the rates of “confirmed” allergies in the study were lower. Just 1 percent of children had a confirmed peanut allergy; among adults, the rate of probable peanut allergy was 0.7 percent, while the rate of confirmed cases was less than 0.3 percent.

“We think the ‘probable’ prevalence is probably more reflective of the true prevalence,” lead researcher Dr. Moshe Ben-Shoshan, of McGill University in Montreal, told Reuters Health.

The study is the first to look at the national rate of potentially severe food allergies in Canada, according to Ben-Shoshan. So it is unclear whether, as in the U.S. study, some allergies might be on the rise there.

Both studies are published in the Journal of Allergy & Clinical Immunology.

The U.S. study adds to evidence that peanut allergies may be more common among kids now than a decade or two ago. But it also squares with a research review published last week in the Journal of the American Medical Association (JAMA) finding that estimates of the true rate of food allergies - and whether they are increasing - have been hampered by a “lack of uniformity for criteria” for diagnosing such allergies.

“The study is unique because we used the same (survey) methods three times over an 11-year period and assessed specific food allergies in the general U.S. population. Nothing like this has been done before,” lead researcher Dr. Scott H. Sicherer, of Mount Sinai School of Medicine in New York, told Reuters Health in an email.

A limitation of the study, though, is that it relied on self-reports to estimate the prevalence of food allergies. However, Sicherer said that factors such as increased public awareness of peanut allergies would be unlikely to explain the increase between surveys. Peanut allergies, he pointed out, are “usually fairly evident” in that they lead to sudden reactions like swelling and trouble breathing.

Sicherer and his colleagues also found that self-reported peanut and tree-nut allergies did not increase over time among adults; in 2008, 1.3 percent reported an allergy to peanuts, tree nuts or both, versus 1.6 percent in 1997.

The “gold standard” for diagnosing the allergies is the food challenge - where the patient consumes the suspect food under medical supervision to see if it triggers an allergic reaction. But doctors are often reluctant to perform food challenges, in part because they worry about the risk of setting off a serious allergic reaction. Other tests, such as skin-prick tests and blood tests, are less precise.

Relying on symptoms alone is also problematic. Ben-Shoshan pointed out that often what people think is a food allergy is actually a food intolerance. The difference is that allergies involve an immune system reaction that can, in some cases, cause severe symptoms. Food intolerance is not an immune system reaction, and people may be able to eat small amounts of the culprit food with no symptoms or only mild problems, like indigestion.

In their study, Ben-Shoshan and his colleagues found that none of those with possible food allergies said they had received a food-challenge test. Often, they had received no tests at all.

Of all adults and children who reported peanut allergies, for example, 79 percent either said they had undergone confirmatory testing or were not sure if they had.

No one is sure why peanut and tree-nut allergies may be on the rise among children. One hypothesis has to do with how peanuts are processed; in roasted form, Sicherer and his colleagues write, peanuts may be more likely to trigger allergies.

Some researchers also suspect that the so-called “hygiene hypothesis” may be contributing to allergies more generally. This theory holds that today’s clean living - creating less exposure to germs from early life on - may in some people make the immune system more prone to attacking normally benign substances, like food proteins and pollen.

Sicherer noted that the JAMA review found little good evidence on how to prevent or best manage food allergies. “The current results highlight the need for more research on food allergy prevention and treatment strategies,” he said.

SOURCES:  Journal of Allergy & Clinical Immunology, online May 10, 2010.