An estimated 2.5 percent of Americans – 7.5 million people – have at least one food allergy and young black children with asthma appear to be at the highest risk, according to findings from what is believed to be the largest food allergy study to date. The research was conducted by investigators at Johns Hopkins Children’s Center, the National Institutes of Health and other institutions.

The findings, published in the October issue of the Journal of Allergy and Clinical Immunology, are based on blood samples and interviews with more than 8,200 participants, ages 1 to older than 60, in whom investigators searched for the prevalence of four food allergies and for links between food allergies and asthma, eczema and hay fever.

Previous research has found slightly higher numbers of national food allergy prevalence, but the researchers say a true comparison between the new and the previous findings would not be meaningful because of different methodologies, different criteria and different population sizes.

Besides the sheer size of this new study, one of its strengths, the researchers say, was the use of blood levels of antibodies as an indicator of actual disease rather than theoretical risk, making it the first of its kind to use that standard in thousands of participants. Indeed, only people with levels high enough to suggest clinical disease were classified as allergic.

Overall, 2.5 percent of the people in the study had a blood test indicating a food allergy. The most common allergy was to peanuts, with 1.5 percent of people testing highly positive for peanut antibodies, the proteins made by the immune system in response to allergens.

These were followed by shrimp (1 percent), eggs (0.4 percent) and milk (0.2 percent). Many (1.3 percent) had more than one type of allergy. Overall, allergies were most common in children 5 years old or younger, with 4.2 percent of them testing highly positive for one, followed by those between ages 6 and 19 (3.8 percent).

“This study is comprehensive in its scope and is the first to use specific blood serum levels and look at food allergies across the whole life spectrum,” says study senior investigator Darryl Zeldin, M.D., acting clinical director at the National Institute of Environmental Health Sciences (NIEHS).

In the study, children under the age of 5 were more than twice as likely as those older than 20 to have a food allergy and black people were three times as likely as white people to have one, while men were nearly 1.9 times more likely than women to be affected.

Black boys were more than four times as likely as white women over 20 to have a food allergy.

The findings also show that food allergies were more common in those with asthma. While the researchers did not study cause and effect between food allergies and asthma, having a food allergy appeared to compound the risk for asthma and vice versa.

Those with asthma had nearly four times the risk of having a food allergy than those without it. Overall, people with food allergies were nearly seven times more likely than those without them to have required ER treatment for their asthma in the 12 months leading up to the study.

“Our findings confirm a long-suspected interplay between food allergies and asthma, and that people with one of the conditions are at higher risk for the other,” says investigator Robert Wood, M.D., director of Allergy and Immunology at Hopkins Children’s.

Wood notes that many children experience an “allergic march,” developing a food allergy first and getting asthma and hay fever later.

While people with food allergies were somewhat more likely to be diagnosed with hay fever, the link between the two was not particularly strong, and they did not appear to have higher risk for eczema, the investigators found.

Andrew H. Liu, M.D., of National Jewish Health hospital and the University of Colorado, was the lead investigator on the study.

How does allergy develop?

When a baby is born, the child’s immune system is skewed toward allergy and is in what scientists call the TH2 mode. As the child is exposed to germs, bacteria, viruses and infection in the first few days, week and month of life, the immune system begins to mature and figure out what is harmful and what is not, “switching” over from the TH2 to TH1 mode.

But for some children, this switchover doesn’t happen properly, leaving them predisposed to developing an allergic response to proteins in foods, pollens or certain insects. Genetics play a role as well; allergy is usually an inherited condition.

A food allergy develops when the person with the tendency toward allergy fails to develop a tolerance for one or more specific food proteins. The immune system identifies the specific food proteins, such as those in peanuts, as allergens and begins to create IgE (Immunoglobulin E) antibodies to guard against them. These antibodies attach themselves to cells in the body called mast cells.

When the person again eats the food he or she has developed an allergy to, the proteins from it, in turn, becomes attached to the IgE antibodies. This causes the mast cells to explode, releasing histamine and other powerful chemicals into the body. It is these chemicals that cause the symptoms of allergy. Link to Food Allergy Symptoms

While allergy is a largely inherited condition, it doesn’t always develop into the same form of allergy within one family. For example, a woman with allergies to tree pollen may give birth to a child who develops an allergy to milk and dairy.

Why are allergies more common now?

No one knows for certain why people have more allergies today than they did in the past. One theory that has gained prominence is the hygiene hypothesis. This hypothesis states that kids growing up in industrialized countries are not exposed to the same germs, infections and parasites as kids who grow up in less clean, modern environments.

The idea is that the immune system needs these exposures to develop properly, and without them is underworked – and it begins to develop antibodies to otherwise harmless substances, such as peanuts.

There is evidence that kids growing up on farms (see “Why Allergies Now?”), as well as in larger families, are less likely to develop allergies and asthma.

Other theories that attempt to explain a rise in food allergy include insufficient vitamin D, food processing (ie: roasting and emulsification of peanuts for peanut butter, rather than fried or boiled peanuts), and the delay of exposure to the allergen in infancy.