For most people, soy allergy is uncomfortable but not serious. Rarely, an allergic reaction to soy can be frightening and even life-threatening. Signs and symptoms of a food allergy usually develop within a few minutes to hours after eating a food containing the allergen.
Soy allergy symptoms can include:
- Tingling in the mouth
- Hives; itching; or itchy, scaly skin (eczema)
- Swelling of lips, face, tongue and throat, or other body parts
- Wheezing, runny nose or breathing difficulty
- Abdominal pain, diarrhea, nausea or vomiting
- Skin redness (flushing)
A severe allergic reaction (anaphylaxis) is rare with a soy allergy. It's more likely to occur in people who also have asthma or who are allergic to other foods besides soy, such as peanuts.
Anaphylaxis causes more-extreme signs and symptoms including:
- Difficulty breathing, caused by throat swelling
- Shock, with a severe drop in blood pressure
- Rapid pulse
- Dizziness, lightheadedness or loss of consciousness
When to see a doctor
See your primary care doctor or a doctor who specializes in treating allergies (allergist) if you experience food allergy symptoms shortly after eating. If possible, see your doctor during an allergic reaction.
Seek emergency treatment if you develop signs or symptoms of anaphylaxis, such as:
- Difficulty breathing
- Rapid, weak pulse
- Dizziness or lightheadedness
- Drooling and inability to swallow
- Full-body redness and warmth (flushing)
An immune system reaction causes food allergies. With a soy allergy, your immune system identifies certain soy proteins as harmful, triggering the production of immunoglobulin E (IgE) antibodies to the soy protein (allergen). The next time you come in contact with soy, these IgE antibodies recognize it and signal your immune system to release histamine and other chemicals into your bloodstream.
Histamine and other body chemicals cause a range of allergic signs and symptoms. Histamine is partly responsible for most allergic responses, including runny nose, itchy eyes, dry throat, rashes and hives, nausea, diarrhea, difficulty breathing and anaphylactic shock.
Food protein-induced enterocolitis syndrome (FPIES)
A food allergen can also cause what's sometimes called a delayed food allergy. Although any food can be a trigger, soy is one of the most common. The reaction, commonly vomiting and diarrhea, usually occurs within hours after eating the trigger rather than minutes.
Unlike some food allergies, FPIES usually resolves over time. As with typical soy allergies, preventing a reaction involves avoiding foods with soy.
Certain factors may put you at greater risk of developing a soy allergy:
- Family history. You're at increased risk of allergy to soy or other foods if other allergies, such as hay fever, asthma, hives or eczema, are common in your family.
- Age. Soy allergy is most common in children, especially toddlers and infants.
- Other allergies. In some cases, people who are allergic to wheat, beans (legumes), milk or other foods can have an allergic reaction to soy.
There's no way to prevent a food allergy. If you have an infant, breast-feeding instead of using a soy-based or milk-based formula may help.
If you're allergic to soy, the only way to avoid a reaction is to avoid soy products. It's not always easy to know which foods contain soy, a common ingredient in many foods.
Read food labels carefully. Soy is often present in unexpected foods, including canned tuna and meat, baked goods, crackers, energy bars, low-fat peanut butter, and canned soups. Read labels every time you buy a product, because ingredients can change. Also, check for the statement "contains soy" or "may contain soy" on product labels.
Highly refined soy oil may not cause a reaction because it doesn't contain soy proteins. Similarly, you might not react to foods that contain soy lecithin. But generally, if a label includes the word "soy," avoid it. Products to avoid include:
- Soy milk, soy cheese, soy ice cream and soy yogurt
- Soy flour
- Soy sauce, shoyu and tamari
- Vegetable oil, vegetable gum, vegetable broth and vegetable starch
Besides "soy," "soya" and "soybeans," other words on food labels may indicate that the product contains soy, including:
- Glycine max
- Hydrolyzed vegetable protein (HVP)
- Textured vegetable protein (TVP)
- Monosodium glutamate (MSG)