As a population, Americans are becoming more allergic, and scientists don’t know why.

The incidence of food allergies has skyrocketed, doubling in the last 10 years. One in 25 Americans has a food allergy, and for children younger than 3, the ratio is 1 in 17.

“All types of allergies are increasing,” said Anne Munoz-Furlong, founder and CEO of the Food Allergy & Anaphylaxis Network. “We’re better at diagnosing food allergies.”

A food allergy occurs when the body’s immune system mistakenly attacks the proteins in a particular food, releasing chemicals (histamines) that cause symptoms that can include hives or gastrointestinal or respiratory distress. Symptoms, whether mild or severe, occur quickly, within a few minutes to two hours of eating. In the most severe cases, they progress to anaphylaxis, a potentially fatal condition in which the allergic reaction overtakes the entire body, according to the Food Allergy & Anaphylaxis Network.

“We’ve done such a good job eliminating childhood diseases that we grew up with that our immune systems are looking for something to do. We don’t understand why our immune system selects one food over another,” Munoz-Furlong said.

Allergists say any food can cause an allergic reaction, but 90 percent of the time it’s one of the “big eight” foods: milk, eggs, wheat, soy, peanuts, tree nuts (almonds, cashews, pecans, pistachios, walnuts), shellfish (shrimp, crab, lobster) and fish (tuna, salmon, catfish).

Years ago, doctors didn’t recognize food allergies as such and simply labeled children as “sickly” or said they had a weak stomach. Some health professionals think that food allergies are increasing because we’re all exposed to more of the “big eight” allergens through processed foods.

“As soon as you stray into the aisles of prepared and processed foods, life becomes much more complicated,” said Alice Sherwood, whose son is allergic to eggs and nuts.

Sherwood wrote “Allergy-Free Cookbook” (DK Publishing, $25) because allergy-free food was something she couldn’t find when she needed it. Although there are 12 million Americans affected by food allergies, there aren’t many allergy-friendly products on the market, she said.

For the cook, four of these food groups (gluten, eggs, nuts, dairy) are particularly difficult to avoid because they are used in so many dishes. Research shows that most people are allergic to only one or two foods. In her book, Sherwood chose to develop alternative versions of each recipe to cater to each of those four food allergens. Each recipe has versions that are free of gluten, dairy (milk), eggs or nuts.

Children with food allergies, and their parents, always have to be on guard to prevent a reaction.

“The best defense is education,” Munoz-Furlong said. “Learn what the symptoms are, and talk to your doctor and get a diagnosis. Make sure you read the ingredient label, and talk to anyone who is giving you food. Ask about the ingredients that are in that food. Accidents are never planned. Be ready when one occurs.”

The Food Allergen Labeling and Consumer Protection Act, which went into effect in January 2006, requires food manufacturers to declare food allergens in plain language on their ingredient lists. For example, before the law was passed, milk could be listed on a label as “ammonium caseinate.”

The only way to prevent an allergic reaction, Munoz-Furlong said, is by strictly avoiding the food.

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