Soybeans are a good source of protein, and soy-based products are ingredients in many vegetarian, vegan and processed foods, as well as Asian dishes. However, people with a soy sensitivity should avoid soy products because they produce a harmful or uncomfortable physiological response in the body. Soy intolerance causes gastrointestinal symptoms, while a soy allergy causes a potentially serious immune response.
Soy intolerance causes gastrointestinal symptoms such as abdominal pain, bloating, nausea and diarrhea.
Food Intolerance Versus Food Allergies
In some individuals, eating certain foods can cause a reaction in the body accompanied by a specific set of symptoms. The terms food allergy and food intolerance are sometimes used interchangeably to describe a reaction to a specific food, but they have different physiological effects on the body. A food intolerance and allergy may share some symptoms, but allergies can be life-threatening while intolerances are not fatal.
Having a food allergy means that even a small amount of exposure triggers a response from the immune system. The presence of an allergenic substance in the food — such as soy protein — is diagnosed with skin and blood tests. Food allergies are often genetic and are more likely to occur in individuals who have parents with allergies.
An intolerance does not cause the immune system to respond to the offending food. Instead, the body's reaction to a food intolerance is restricted to the gastrointestinal system, because the gut is unable to process a food or a specific component of food.
Soy Intolerance Symptoms
Unlike allergies, there are no blood or skin tests to detect a food intolerance. The offending substance is found by eliminating foods from the diet to narrow down the list of suspects until the culprit is identified, often by trial and error. Keeping a food diary can help with tracking symptoms.
Symptoms of soy intolerance may present as soon as 30 minutes after eating, or may take up to 48 hours to appear. Symptoms include nausea, abdominal pain, gas and bloating. Substances in soy that aren't properly digested cause inflammation in the gut which causes diarrhea.
Individuals with a moderate intolerance can usually eat a small amount of soy without experiencing unpleasant symptoms, but should avoid consuming a full serving of soy. However, if the intolerance is severe, even a tiny amount can cause discomfort.
Soy Allergy Symptoms
A minute amount of soy protein can quickly trigger an immune response in people with a soy protein allergy. Symptoms can manifest immediately or within two hours from exposure.
People with this allergy may experience respiratory difficulty, hives, swelling of the tongue or throat, vomiting or loss of consciousness. Soy allergy gastrointestinal symptoms may also occur, such as gas, diarrhea and nausea due the release of histamines in the intestine. In extreme cases, exposure to soy protein can cause anaphylaxis— a life threatening condition that requires an injected dose of epinephrine.
Sources of Soy Protein Allergens
Soybeans, soy milk and many soy-derived products contain soy protein allergens. Common allergy-inducing products include edamame, tofu, soy cheese and yogurt, miso, soy sauce, soy flour and soy protein isolate. Fats derived from soybeans such as soybean oil and soy lecithin generally do not trigger a reaction.
The best way to prevent an allergic reaction or symptoms of intolerance is to avoid consuming soy products. Sometimes, avoidance can be challenging because soy is a component in many ingredients in processed foods. Some hidden sources of soy include:
- Vegetable broth
- Vegetable starch
- Vegetable gum
- Hydrolyzed vegetable protein
- "Natural" flavorings
Gas Caused by Soy Products
Complex carbohydrates in some plant-based foods such as vegetables, whole grains and legumes (like soybeans) can cause gas and bloating. People who lack a specific enzyme that is necessary to break down complex carbohydrates in the gut have complex carbohydrate intolerance.
Soybeans and soy products that contain these carbohydrates are not properly broken down and produce gas when they ferment in the gut. Taking an over-the-counter alpha-galactosidase enzyme product helps the small intestine process the carbohydrates to avoid excess gas and bloating.
- Canadian Society of Intestinal Research: "Food Allergy vs. Intolerance"
- University of Rochester Medical Center: "Soy Allergy Diet"
- Canadian Society of Intestinal Research: "Intestinal Gas From Complex Carbohydrates"
- National Institute of Allergy and Infections Diseases: "Identifying Causes of Food Allergy & Assessing Strategies for Prevention"
- Children's Hospital of Philadelphia: "Food Intolerance"
- Harvard Health Publishing: "Is it a Food Intolerance, Allergy or Something Else?"