Even people who do not intentionally consume soya products often ingest it in ‘hidden’ form – for example, as soya oil in margarine and cooking fats or as lecithin (E322), a soya oil product found in sweets. Soya protein is also added to many processed foods such as sausages. But soya protein, whether isolated or in a soya product, can also trigger allergies. Around 0.3 per cent of the European population is allergic to soya.
The most common form of soya allergy is probably pollen-food syndrome. Referred to as a secondary food allergy, it occurs in individuals who have previously been allergic to birch, hazel or alder pollen. Certain proteins in tree pollens, such as the birch pollen allergen Bet v 1, are similar in structure to a particular soya protein (Gly m 4). Therefore the cells that are responsible for the immune defence and hypersensitive to pollen allergens can also dock onto the soya protein and trigger allergy symptoms.
A soya allergy can be caused not only by ingesting soya as food but also by breathing in dust containing soya particles. Food industry workers who handle or process soya beans may breathe in large quantities of the dust and develop allergic reactions in the airways. In the baking industry, soya flour dust can provoke these respiratory allergies.
A soya allergy can cause mild or severe reactions in various organs. Symptoms range from what is called oral allergy syndrome, in which the mucosa in the mouth and throat start to tingle or swell a few minutes to hours after exposure, to anaphylactic shock accompanied by difficulty breathing and circulatory collapse. The risk of anaphylactic shock appears to increase if the individual quickly consumes large quantities of a protein-rich, minimally processed soya product – for example, a shake made with a soya drink or soya protein powder.
The digestive system can also be affected, resulting in abdominal pain, vomiting or diarrhoea. The skin may also develop allergic reactions. Apart from itchiness, redness and hives, a soya allergy can also trigger or worsen an eczema flare-up. In still other cases, the airways can be affected, with reactions ranging from allergic rhinitis to asthma symptoms.
Soya Allergy in Childhood
People who experience an immune response directly to a particular food (in the intestinal mucosa) rather than indirectly via pollen are referred to as having a primary food allergy. This form of soya allergy occurs more frequently in infants than in adults, since the protective function of the intestinal mucosa may not yet be fully developed in infancy.
Soya is one of the most common food allergy triggers in children and adolescents, along with milk and chicken egg proteins, wheat, peanuts and tree nuts. In many people, this form of soya allergy disappears spontaneously by the age of ten, which is why a doctor should check at regular intervals whether the allergy is still present.
As a first step in diagnosing a soya allergy, the doctor asks the patient about his or her eating habits and symptoms. Some patients are advised to keep a food diary in which they keep track of what they eat and any symptoms that occur. If an allergy is suspected, skin and/or blood tests are conducted in order to confirm it. These tests reveal an allergic disposition to certain substances but do not provide evidence of an actual allergy. For this reason, an additional diagnostic procedure, the provocation test, is conducted to rule out any doubts. For this test, the patient eliminates the allergen from the diet for a certain period. He or she is then given small amounts of soya protein under medical supervision. The results confirm whether soya triggers an allergic reaction or whether it can be removed from the list of suspected allergens.
It is not yet possible to effectively treat the causes of a soya allergy. Allergic individuals must therefore try to avoid soya allergens.
It is also important to know that the allergen content of soya products can vary depending on the extent to which it has been processed and the amount of protein they contain. The allergen Gly m 4, which triggers reactions in people with a pollen-related soya allergy, is therefore found in soya protein powder at a very high concentration. Soya beverages, tofu and soya flakes also contain greater amounts. On the other hand, soya sauces, miso and roasted soya beans have not been shown to contain the allergen, which is destroyed through fermentation and heating.
Protein or protein-free soya oils and derived products such as soya lecithin are often well tolerated by individuals with a pollen-related soya allergy. However, people with a primary soya allergy also react to low-protein and highly processed soya products and should therefore avoid soya entirely. In accordance with European food regulations, all foods containing soya as an additive must be labelled as such. But this does not protect against foods contaminated with soya protein during the manufacturing process.
Medications may also contain ingredients derived from soya (the anaesthetic Propofol, for example). Patients with a diagnosed soya allergy should therefore inform their attending physicians about their allergy. But according to the allergology work group of the Austrian Society of Dermatology and Venereology, the protein content of medications containing ingredients derived from soya is insufficient to trigger allergic reactions. However, more comprehensive studies are needed in order to draw a solid conclusion.
People who are highly allergic to soya and at risk of anaphylactic shock from exposure to even the smallest amounts of soya protein must carry an emergency kit with them containing an adrenaline auto-injector, a corticosteroid (in liquid or tablet form) and an antihistamine (in liquid or tablet form).
Prof. Dr. med. Dr. h.c. T. Zuberbier
Last changes made: July 2016