Soy belongs to the legume family. The seeds or soybeans are the edible part of the plant. They are particularly rich in proteins of high biological value and are therefore an important nutrient. Their protein content is about 35% of dry seeds. Allergenicity of soybean proteins has been documented since the eighties. Soybean exposure is widespread in Asia and the USA but its consumption has increased in Europe during the last years, especially in vegetarian cuisine.
Allergy to soy is caused by proteins in the seeds, in particular by some of the so-called storage proteins. Storage proteins are abundant in seeds and serve as “source material” during the growth of a new plant. At least two storage proteins have been identified as allergens, vicilin and legumin. Storage proteins usually are stable heat-resistant proteins. Heat treatment leaves these allergens largely intact. A non-storage protein which is related to the major allergen for birch pollen hay fever (see “Link to hay fever”) has recently been identified as an allergen in soybean.
Soy may induce all kind of allergic symptoms ranging from itching and swelling of mouth and throat, gastrointestinal reactions (nausea, cramps and diarrhoea), respiratory symptoms (hay fever and asthma), skin reactions (hives also referred to as nettle rash or urticaria) up to severe life-threatening reactions including shortness of breath and drop of blood pressure (anaphylactic shock).
Although no food allergy, it is worth mentioning that inhalation of soy powder and flour by work-related exposure may cause respiratory symptoms such as rhinitis and asthma. Patients, however, developing symptoms after inhalation usually do tolerate ingested soy. Soybean hull dust was responsible of asthma epidemics that occurred between 1981 and 1987 in Spain after unloading soybean seeds in the harbours of Barcelona, Cartagena and Tarragona but also in New Orleans in the 1950s and 1960s.
Related foods (cross-reactions)
As soy is a legume, there has been real concern that reactivity to other legumes notably peanut would be common in soy allergic subjects. This concern is based on the expectation that similar proteins in related foods will also cause reactions. Such reactions are referred to as cross-reactions. Indeed, storage proteins in e.g. peanut and pea are very similar to their counterparts in soy. In contrast to some other well known food allergies, cross-reactive allergy appears to be rare for legumes. Most soy allergic individuals are tolerant of peanut, perhaps with exception of patient with birch pollen allergy (see below). This does not mean that absolutely no patients exist that demonstrate cross-reactive allergy to multiple legumes, but it is rare.
Patients with hay fever in early spring caused by birch pollen frequently have food allergies. The most important foods that are implicated in this phenomenon are fruits like apple and peach and tree nuts like hazelnut. The cause of this marriage between respiratory allergy and food allergy is that these fruits and nuts contain an allergen that is closely related to the major birch pollen allergen. Allergic reactions based on such similarities are called cross reactions. Recently, it has been demonstrated that soy also contains such a birch pollen related allergen. Allergic reactions to a dietary product containing a soy protein isolate have recently been described in Central Europe, i.e. Germany and Switzerland. Most of the patients experiencing allergic reactions to the soy protein isolate were allergic to birch pollen. Interviews among birch pollen allergic subjects in a birch rich area revealed that 10% of highly sensitised birch pollen allergic patients suffer from a concomitant food allergy to soybean.
Who, when, how long and how often?
The prevalence of soy allergy has not been specifically studied so far. To date, food allergy to soy has been described mainly in young children with eczema (atopic dermatitis), often outgrowing their soy allergy after 1-2 years of dietary elimination. In children with eczema and suspected food allergy, 1 to 4% of them react on exposure to soy with allergic symptoms. Allergic reactions to soy-derived foods in adults, however, are less frequent.
How much is too much?
Thresholds for allergic reactions to soy have not been established accurately. In Sweden, soy was reported to be responsible for 6 out of 12 severe or life-threatening allergic reactions caused by foods during 1993-1996 after intake of an estimated amount of 1-10 g of soy. Threshold levels can differ drastically between individuals. Unpublished results from Switzerland suggest that first subjective symptoms may start already at a dose of 8 mg.
Diagnosis of soy allergy starts with recording a clear clinical history to establish a link between allergic reactions and soy. Since soy is frequently used as an “invisible” ingredient in a compound food, this is not always so straight forward. Skin prick tests and measurement of specific IgE levels are used to support a history-based suspicion of soy allergy. Soy shares both similar allergens with peanut and other legumes and pollen. A positive skin test or serum IgE test can easily be based on cross-reactivity to peanut, other legumes or even birch pollen. To distinguish whether such cross reactions have clinical relevance, the only definitive method is a so-called double-blind placebo-controlled challenge. In this procedure, increasing doses of soy are administered to the patient as well as placebo meals not containing soy. Both patient and doctor are unaware of the meals with and without soy. Effective blinding of the taste of soy is essential for such challenge procedures.
Where do I find soy?
Soy beans are usually consumed as processed food although whole beans can also be eaten. Soy products are nowadays widely used in the food industry, in particular as texturizer, emulsifier and protein filler. In these cases soy is used as oil, flour or concentrate. Soy flour is often added to bread, pastry and cookies. Soy also ends up in soy milk, soy drinks and soy flakes. Soy curd or tofu is a common ingredient in Asian cuisine and is the basis for fermented soybean products such as miso, okara, soy sauce, or tempeh. Moreover, soy is used for the production of textured vegetable protein (TVP) which is consumed as a meat substitute. Since soy is a good and cheap source of protein and it may be part of a wide variety of processed foods such as meat products, sausages, chocolate, ice cream and breakfast cereals. It is introduced into the diet early in life as infant formula or infant meals.
Soybean lecithin is used as an emulsifier (E322) in processed foods but also as an ingredient in pharmaceuticals and cosmetics. Furthermore, it is ingested as a health supplement due to its predicted effect on reduction of the cholesterol level. It is derived from the oil fraction, but it may contain allergenic soy proteins at low level. Soy lecithin is mainly used as a food additive in chocolate, bakery goods, spices, ice cream. Commercial soy lecithin preparations are not labelled as containing soy proteins, thus they may represent a source of hidden allergens.
Soy is used in many non-food products including body lotions and crèmes, soaps, pet food, adhesives and lubricants. Soy lecithin is used in pharmaceuticals and cosmetics.
For the treatment of food allergy, at present avoidance is the only solution. Since soybean is an “invisible” ingredient in a broad range of food products, avoidance is difficult. Fortunately, according to recent new legislation (EU Labelling Directive 2003/89/EG and list of Codex Alimentarius Commission on mandatory labelling of pre-packaged food) all products containing soy-derived ingredients should be labelled as such.