Why a Soy Allergy Develops

Food allergies are quite common, with approximately 8% of all children and 2% of all adults having an allergy to at least one food. The most common foods causing food allergies particularly in children include egg, milk, wheat, peanut and soy.

Soybean food and drink products
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Overview of Soy

Soybeans are a member of the legume family, which includes other foods such as peanuts, beans and peas. Soybeans are commonly used in the commercial processing of foods, since they provide a low-cost, high-quality form of protein that is widely available. Soy protein is therefore commonly encountered in daily life, with children being exposed at a young age. Soy protein is a common substitute for milk protein in infant formulas, and is often touted as "gentler" for the gastrointestinal tract of babies.

Soy milk is widely available and frequently consumed by adults, especially those with a dairy allergy, lactose intolerance, or other form of milk intolerance. Soy is also commonly used in Asian foods, including soy sauce, miso soup, and tofu. For these reasons, avoidance of soy protein is extremely difficult, for both children and adults.

Soy Allergy

Soy allergy is quite common, affecting approximately 4 in 1,000 children. Soy allergy may result in a number of different types of allergic symptoms, from atopic dermatitis to urticaria and angioedema to anaphylaxis. Soy allergy has the potential to cause serious, life-threatening reactions, but not as commonly as other food allergies such as peanut and shellfish allergy. Soy allergy is typically diagnosed with the use of allergy skin testing, although blood testing for allergic antibodies directed against soy protein may also be performed.

Soy protein may also cause a non-allergic protein intolerance in young children, called food protein-induced enterocolitis syndrome (FPIES), which results in nausea, vomiting, diarrhea, dehydration, weight loss and even shock. A milder form of FPIES caused by soy formula is food protein-induced proctitis, which causes bloody stools in affected infants. Children with FPIES have negative allergy testing to soy since there is no allergic antibody involved in the disease process. Interestingly, approximately 50% of children with soy-induced FPIES will have a similar reaction to cow’s milk.

Likelihood of Outgrowing Soy Allergy

Soy allergy appears to be mostly a problem for young children, as there are many reports of children outgrowing their soy allergy by the time they reached 3 years of age. A study published by Johns Hopkins University in 2010 found that 70% of children had outgrown their soy allergy by age 10. The study further showed that the amount of allergic antibody against soy could help predict whether a child had outgrown their allergy. However, determining if a child has outgrown a soy allergy should always include an oral food challenge to soy performed under medical supervision.

Soy Allergy and Risk of Developing Other Food Allergies

Soy does share similar proteins with other legumes (such as peanuts, peas, beans and lentils), although most people with soy allergy can eat other legumes without problems. However, many people often are told to avoid all legumes because allergy tests often show positive results to more than one legume. This is a result of cross-sensitization, meaning that the similar proteins found in legumes bind to the same allergic antibodies directed against soy proteins. However, many studies show that true cross-reactivity among various legumes, meaning that allergic reactions actually occur in soy-allergic people when other legumes are eaten, is low—probably around 5%.

If you are told that you have positive allergy tests to multiple legumes, you should check with your healthcare provider before eating any of these foods. While cross-reactivity rates among legumes are low, your healthcare provider will likely perform an oral food challenge to the legume that you are interested in eating to ensure that you are not allergic.

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