Soy Allergy

Soybeans are in the legume family, which also includes foods such as kidney beans, peas, lentils and peanuts. Soybeans are used in most processed foods in the United States—from edamame to tofu—and is found in many unexpected foods, such as Worcestershire sauce, natural and artificial flavorings, vegetable broths and starches, and most Asian foods. Certain brands of cereal and peanut butter also contain soy. Therefore, soy is one of the most difficult products for allergy sufferers 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 will go release chemicals such as histamines to "protect" the body. The release of these chemicals is what causes an allergic reaction. 

Soy, along with cow's milk, eggs, peanuts, tree nuts, wheat, fish, and shellfish, make up the "big eight" allergens which are responsible for 90 percent of all food allergies. Soy allergies are one of several food allergies that begin early in life and may resolve by age three. 


The only 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 become adept at reading labels to familiarize themselves with ingredients that contain soy. They also should be diligent about asking about ingredients in the food served in restaurants.


There are several tests available to confirm soy and other food allergies, including:

  • 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 through the extract. If a person is allergic to soy, a red bump similar to a mosquito bite will appear at the spot of the prick.
  • Intradermal skin test: similar to a skin prick test except a small amount of the allergen is injected underneath the skin with a syringe. It is more sensitive than a skin prick test and may be needed if other tests are negative.
  • Blood test (RAST test): Blood tests are sometimes done on babies less than a year old because their skin doesn’t react as well to prick tests as older children’s and adults’ skin does. A RAST test measures the amount of the IgE antibody in the blood.
  • Food challenge test: A food challenge is considered to be the best way to test for food allergies. A person is given increasing amounts of the suspected allergen while a healthcare provider checks for symptoms.
  • Elimination diet: With an elimination diet, a person stops eating the suspected foods for a couple of weeks and then slowly adds them back into their diet one at a time while recording any symptoms.

Soy Allergy in Infants

A soy allergy will generally show up in infants by three months of age.

Soy Lecithin Allergy

Soy lecithin is a non-toxic 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.

Soy Milk Allergy

Nearly half of children with a slow-onset cow's milk allergy are also allergic to soy. If a child is on a formula, parents must switch to one of two hypoallergenic formulas on the market. In extensively hydrolyzed formulas, proteins have been broken down so they are less likely to cause an allergic reaction. In elemental formulas, the proteins are the simplest forms of proteins and unlikely to cause a reaction.

Soy Protein Allergy

There are at least 21 proteins in soybeans that have been found to cause asthma-related allergic reactions. Soybean plants store proteins in developing seeds as a source of carbon, nitrogen, and sulfur.

Soy Oil Allergy

Soybean oil typically doesn’t contain proteins and is generally safe to consume for those with soy allergies. 

Soy Sauce Allergy

In addition to soy, soy sauce also usually contains wheat, which may make it difficult to decipher whether allergic symptoms were caused by one allergen or the other. In addition, soy sauce also contains histamines. This may result in "histamine poisoning," which causes symptoms similar to an allergic reaction, including inflammation around the mouth and dermatitis. A skin prick or other test should be used to determine which—if either allergen—was behind symptoms. 

Signs and Symptoms  

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

  • abdominal pain
  • diarrhea, nausea, or vomiting
  • runny nose, wheezing, or trouble breathing
  • fever blisters
  • fever
  • conjunctivitis (pink eye)
  • skin reactions including hives and eczema
  • itching and swelling
  • anaphylactic shock (very rarely in the case of soy)

In Infants

Additional symptoms in infants may include crying, irritability, and soy avoidance.