Soybeans are in the legume family, which also includes foods such as kidney beans, peas, lentils, and peanuts. Whole, immature soybeans are also known as edamame. Although primarily associated with tofu, soy is found in many unexpected, processed foods in the United States, such as:

  • condiments like Worcestershire sauce and mayonnaise
  • natural and artificial flavorings
  • vegetable broths and starches
  • meat substitutes
  • fillers in processed meat, like chicken nuggets
  • frozen meals
  • most Asian foods
  • certain brands of cereal
  • some peanut butters

Soy is one of the most difficult products for people with allergies to avoid.

A soy allergy occurs when the body’s immune system mistakes the harmless proteins found in soy for invaders and creates antibodies against them. The next time a soy product is consumed, the immune system releases substances such as histamines to “protect” the body. The release of these substances causes an allergic reaction.

Soy is one of the “Big Eight” allergens, along with cow’s milk, eggs, peanuts, tree nuts, wheat, fish, and shellfish. These are responsible for 90 percent of all food allergies, according to the Cleveland Clinic. Soy allergy is one of several food allergies that begins early in life, usually before age 3, and often resolves by age 10.

Symptoms of a soy allergy may range from mild to severe and include:

Soy lecithin

Soy lecithin is a nontoxic food additive. It’s used in foods that require a natural emulsifier. Lecithin helps control sugar crystallization in chocolates, improves shelf life in some products, and reduces spattering while frying certain foods. Most people who are allergic to soy may tolerate soy lecithin, according to the University of Nebraska Food Allergy Research. This is because soy lecithin typically doesn’t contain enough of the soy protein responsible for allergic reactions.

Soy milk

It’s estimated that about fifteen percent of infants who are allergic to cow’s milk are also allergic to soy. If a child is on a formula, parents must switch to a hypoallergenic formula. In extensively hydrolyzed formulas, proteins have been broken down so they’re less likely to cause an allergic reaction. In elemental formulas, the proteins are in the simplest form and unlikely to cause a reaction.

Soy sauce

In addition to soy, soy sauce also usually contains wheat, which may make it difficult to decipher whether allergic symptoms were caused by soy or by wheat. If wheat is the allergen, consider tamari instead of soy sauce. It’s similar to soy sauce but usually made without adding wheat products. A skin prick test or other allergy testing should be used to determine which allergen — if any — was behind any allergic symptoms.

Soybean oil typically doesn’t contain soy proteins and is generally safe to consume for those with soy allergies. However, you should still discuss it with your doctor before consuming it.

According to experts, it’s unusual for people with a soy allergy to only be allergic to soy. People with soy allergies often also have allergies to peanuts, cow’s milk, or birch pollen.

There are at least 28 possible allergy-causing proteins in soybeans that have been identified. However, most allergic reactions are only caused by a few. Check labels for all forms of soy if you have a soy allergy. You may spot several forms of soy, including:

  • soy flour
  • soy fiber
  • soy protein
  • soy nuts
  • soy sauce
  • tempeh
  • tofu

There are several tests available to confirm soy and other food allergies. Your doctor may use one or more of the following if they suspect you have a soy allergy:

  • Skin prick test. A drop of the suspected allergen is put on the skin and a needle is used to prick the top layer of skin so a tiny amount of the allergen can enter the skin. If you are allergic to soy, a red bump similar to a mosquito bite will appear at the spot of the prick.
  • Intradermal skin test. This test is similar to a skin prick except a larger amount of the allergen is injected underneath the skin with a syringe. It may do a better job than a skin prick test at detecting certain allergies. It may also be used if other tests do not provide clear answers.
  • Radioallergosorbent test (RAST). Blood tests are sometimes done on babies less than a year old because their skin doesn’t react as well to prick tests. A RAST test measures the amount of the IgE antibody in the blood.
  • Food challenge test. A food challenge is considered to be one of the best ways to test for food allergies. You are given increasing amounts of the suspected allergen while under direct observation of a doctor who can monitor symptoms and provide emergency treatment if necessary.
  • Elimination diet. With an elimination diet, you stop eating a suspected food for a couple of weeks and then slowly add it back into your diet, while recording any symptoms.

The only definitive treatment for a soy allergy is complete avoidance of soy and soy products. People with soy allergies and parents of children with soy allergies must read labels to familiarize themselves with ingredients that contain soy. You should also ask about ingredients in items served in restaurants.

Research is ongoing as to the potential role of probiotics in preventing allergies, asthma, and eczema. Laboratory studies have been hopeful, but there are not enough studies in humans yet for experts to make any specific recommendations.

Consider talking to your allergy specialist about whether probiotics may be useful to you or your child.

Children who have a soy allergy may outgrow this condition by the age of 10, according to the American College of Allergy, Asthma and Immunology. It’s important to recognize the signs of a soy allergy and take precautions to avoid a reaction. Soy allergy often occurs alongside other allergies. In rare cases, a soy allergy can cause anaphylaxis, a potentially life-threatening reaction.