Soybeans are in the legume family, which also includes foods such as kidney beans, peas, lentils, and peanuts. Soybeans, also called edamame, are used in a lot of processed foods in the United States. Although primarily associated with tofu, soy is found in many unexpected, processed foods, such as:
- condiments like Worcestershire sauce
- natural and artificial flavorings
- vegetable broths and starches
- frozen meats
- frozen meals
- most Asian foods
- certain brands of cereal
- some peanut butters
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 release chemicals such as histamines to “protect” the body. The release of these chemicals causes an allergic reaction.
Soy, along with cow's milk, eggs, peanuts, tree nuts, wheat, fish, and shellfish, make up the “big eight” allergens. These are responsible for 90 percent of all food allergies, according to the Cleveland Clinic. Soy allergies are one of several food allergies that begin early in life and may resolve by age 3.
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
- conjunctivitis (pink eye)
- skin reactions including hives and eczema
- itching and swelling
- anaphylactic shock (very rarely in the case of soy allergies)
Additional symptoms in infants may include crying, irritability, and soy avoidance.
Soy and soy products can trigger a variety of allergy types. These include:
Soy Lecithin Allergy
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 National Food Management Institute.
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 a hypoallergenic formula. 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 in the simplest form and unlikely to cause a reaction.
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, according to Food and Chemical Toxicology. This may result in histamine poisoning, which causes symptoms similar to an allergic reaction, including inflammation around the mouth and dermatitis. A skin prick test or other test should be used to determine which allergen — if any — was behind the symptoms.
Soybean oil typically doesn’t contain proteins and is generally safe to consume for those with soy allergies. However, you should still discuss it with your doctor before consuming it.
There are at least 15 proteins in soybeans that have been found to cause allergic reactions. 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
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 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: This test is similar to a skin prick 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.
- 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 the best way to test for food allergies. A person is given increasing amounts of the suspected allergen while a doctor 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.
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. New research from Beneficial Microbes Journal reveals a link between probiotics, specifically Lactobacillus, and allergy prevention and management. More research is needed in this area to be able to provide specific recommendations.
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. In rare cases, a soy allergy can cause throat swelling, a potentially life-threatening condition.