What is a soy allergy? Symptoms, diagnosis & treatment explained

Soy is a protein-rich plant used in a wide variety of foods. But did you know that some people are allergic to this seemingly harmless protein? Given the growing popularity of plant-based diets, this information may be particularly relevant to you. Read on to learn more about soy allergies, their causes, symptoms, and how they are diagnosed. 

Table of Contents

• What is soy and in which foods is it found?
• What happens in the body during a soy allergy?
• Are soy allergies common?
• Can you develop a soy allergy later in life?
• What are the causes of a soy allergy?
• What symptoms occur in a soy allergy?
• Are there cross-reactions between soy and other foods or allergens?
• Is soy lecithin safe for someone with a soy allergy?
• How is a soy allergy diagnosed?
• What treatments are available for soy allergies?
• Summary

What is soy and in which foods is it found?

Soy, also known as Glycine max, is a plant species from the legume family. Originally from East Asia, it has established itself as one of the most important crops worldwide due to its high protein content and versatility. The main component of soy is the soybean, which is used to produce a wide range of foods and products. Because of its high protein content and versatility, soy is a key ingredient in many vegetarian and vegan foods. The most common soy-based foods include:

• Tofu
• Soy milk
• Soy sauce
• Tempeh (fermented soybeans)
• Miso (savory paste made from fermented soybeans and grains)
• Edamame (young, green soybeans)
• Soy oil
• Soy lecithin (a by-product of soy oil production, often used as an emulsifier in foods and other products)

Source: Britannica, 2023

Interesting to know: Soy contains phytoestrogens, plant compounds that have a similar structure to the human hormone estrogen. Although their effect is weaker than that of natural estrogen, they can still influence hormonal processes in the body (Cleveland Clinic, 2022).

What happens in the body during a soy allergy?

Soy allergy is classified as a food allergy and represents an excessive and often harmful immune reaction to substances that are normally harmless—in this case, to proteins found in soy.

When a person who is allergic to soy consumes soy or soy-based products, the immune system mistakenly identifies the soy proteins as harmful invaders. In response, the body produces specific antibodies known as immunoglobulin E (IgE). Upon subsequent exposure to the allergen—i.e., the next time soy is consumed—the soy proteins bind to the IgE antibodies. This triggers the release of histamine and other chemicals. These chemicals, especially histamine, are responsible for causing allergic symptoms.

It is important to understand that not every reaction to soy is necessarily an allergic one. Some individuals may also have a food intolerance to soy, in which case the mechanisms and symptoms are different and do not involve the IgE antibody. While allergic reactions typically occur quickly, intolerance reactions can be slower and less acute (Cleveland Clinic, 2022).

Are soy allergies common?

The prevalence of soy allergies in the Western world varies depending on the study and the population group examined. A review of 40 studies found that the average prevalence of soy allergies in the general population ranges between 0 and 0.5% (Katz et al., 2014).

Can you develop a soy allergy later in life?

Yes, soy allergy can develop later in life, although it is more common in children. Allergies can emerge at any age, including adulthood, due to changes in the immune system or repeated exposure to soy. Symptoms can range from mild to severe (Asero et al., 2016).

What are the causes of a soy allergy?

The exact causes of developing a soy allergy are not fully understood, but there are several factors that may increase a person’s risk:

• Genetics: People with a family history of allergies—whether food allergies, hay fever, asthma, or eczema—have a higher risk of developing allergies themselves.
• Age: Soy allergies occur more frequently in children. However, many children outgrow this allergy by the time they reach school age.
• Other allergies: Individuals who already have another food allergy may have a higher risk of developing a soy allergy as well.
• Early exposure to soy: Some studies suggest that early exposure to soy products during infancy may increase the risk of developing a soy allergy later in life.

It is important to note that not everyone exposed to these risk factors will develop a soy allergy. The exact mechanism explaining why some people react allergically to soy and others do not is still under investigation (Cleveland Clinic, 2022).

What symptoms occur in a soy allergy?

The symptoms of a soy allergy can resemble those of other food allergies. The most common include:

• Skin reactions: Itching, hives, or swelling
• Gastrointestinal issues: Nausea, abdominal pain, or diarrhea
• Respiratory reactions: Shortness of breath, coughing, or sneezing
• Anaphylactic shock: This is an acute, potentially life-threatening reaction of the body to an allergen—in this case, soy. Symptoms may include difficulty breathing, a tight feeling in the chest and throat, rapid heartbeat, and a sudden drop in blood pressure that can lead to fainting or shock.

It is critical to seek immediate medical attention at the first signs of anaphylactic shock, as this can be life-threatening (MedlinePlus, 2018; Mayo Clinic, 2021).

How do you know if you have a soy allergy?

A soy allergy can be identified by symptoms that appear shortly after consuming soy—such as rash, itching, swelling, upset stomach, nausea, diarrhea, breathing problems, or, in severe cases, an anaphylactic reaction. The diagnosis is often confirmed through allergy tests such as a skin prick test or a blood test.

Are there cross-reactions between soy and other foods or allergens?

Cross-reactions occur when a person’s immune system becomes confused due to structural similarities between certain proteins in different substances. In the case of soy, this means that a person who reacts allergically to soy may also react to other foods or substances whose proteins resemble those of soy (Worm et al., 2014). However, not everyone with a soy allergy will experience cross-reactions with substances such as peanuts, other legumes, or birch pollen. If cross-reactions are suspected, it is advisable to consult a doctor to avoid unexpected allergic responses.

In relation to soy, there is also the pollen-food syndrome, also known as oral allergy syndrome. This is a type of cross-allergy in which individuals who are allergic to birch pollen may occasionally experience symptoms when consuming soy. This occurs because the proteins in birch pollen and soy are structurally similar and may be misidentified by the immune system (NHS, 2023).

Is soy lecithin safe for someone with a soy allergy?

Soy lecithin is a by-product obtained during the production of soy oil. In foods, it is primarily used to improve consistency and help mix ingredients that would otherwise separate. Although soy lecithin is derived from soybeans, it contains only trace amounts of the soy proteins that typically trigger allergic reactions. Therefore, the risk of an allergic reaction to soy lecithin is relatively low for individuals with a soy allergy. Nevertheless, people with a severe soy allergy should exercise caution and discuss the risk with their doctor (ACAAI, 2019).

How is a soy allergy diagnosed?

A correct diagnosis is essential for understanding and managing a soy allergy. If a soy allergy is suspected, doctors usually perform a series of tests to confirm the allergy and determine its severity. Common diagnostic methods include:

• Skin prick test: A small amount of the allergen is applied to the skin, usually on the forearm or back. If a reaction such as a raised bump (wheal) occurs within a short time, it may indicate an allergy.
• Blood test: This test checks for specific antibodies against soy proteins. Elevated antibody levels can suggest an allergy.
• Elimination diet: The person removes soy and soy-based products entirely from their diet for a certain period. If symptoms disappear and then return when soy is reintroduced, this may be an indication of an allergy.
• Oral food challenge: Under medical supervision, the affected person is given a controlled amount of soy to trigger an allergic reaction. This is the definitive test but should only be performed in a safe clinical setting.

Source: Cleveland Clinic, 2022

Want to gain insights yourself?

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What treatments are available for soy allergies?

General tips

• Avoid soy products: This is the most effective way to prevent allergic reactions. Always read food ingredient labels and be especially cautious when eating out.
• Be aware of cross-reactions: Some individuals allergic to soy may also react to other foods. It's important to be aware of these potential cross-reactions.
• Emergency plan: Always have an up-to-date allergy emergency plan and make sure you know how and when to use medications, especially epinephrine auto-injectors.
• Inform friends, coworkers, and family: It's important that people around you are aware of your allergy. They can help in an emergency and are better prepared to accommodate your needs.
• Wear a medical alert bracelet: Such a bracelet can be life-saving in emergencies by providing first responders with crucial information about your allergy.

Medical treatment

• Antihistamines: These medications help relieve mild allergic reactions by blocking the effects of histamine, a substance released during allergic responses.
• Corticosteroids: In the form of creams or tablets, they can help reduce allergic skin reactions.
• Epinephrine (adrenaline): For individuals prone to severe allergic reactions, it is life-saving to always carry an epinephrine auto-injector. This medication can be administered at the first signs of anaphylactic shock.

Sources: NHS, 2023; Mayo Clinic, 2021

It is important to remember that everyone responds differently to allergies. When choosing the right treatment, one should always seek advice from an allergist or other medical professional.

Summary

Soy allergies are particularly common in children and can cause a wide range of symptoms. By understanding the causes, recognizing the symptoms, and knowing which foods contain soy, affected individuals can make safer choices.