Soy is so widely distributed in processed foods that avoidance of soy in one’s diet is very difficult. Thus, it is a particularly insidious hidden allergen. As with many other allergens, reactions may occur to very small quantities of soy protein, and anaphylaxis to soybean protein has been reported. Soybean lectin is also an important allergen and has been associated with allergic reactions.

Soybeans may be ingested as whole beans, as flour or as oil. In addition, soy can be used in the manufacture of food in a vast variety of ways, including as a texturizer, emulsifier and protein filler. Soy may thus be listed on the ingredient panel according to its use, e.g., hydrolyzed protein or lecithin (see table below).

Soybean flour is often added to cereal flour and is used extensively in the baking industry. The majority of breads contain some soy flour. Pastries, cakes, biscuits and baby foods may contain soy flour. It is also used in the manufacture of sausages, processed meats, hamburgers and other meat products. Fermented soybean may be used in the preparation of soy sauce or Worcestershire sauce. Fermented soy is in wide use as a food in the Far East.

Soy may also find its way into a food product when added as a compound ingredient. For example, if margarine is added to a food product it will be listed as such, but soy present in the margarine itself will not be listed on the ingredients panel.

Soy protein isolate or concentrate may be used to emulsify fat in food products and may thus be used in the manufacture of ice cream, mayonnaise, and a variety of other liquid fat- or oil-containing foods. The concentrate or isolate may also be used in soymilk and as a protein concentrate added to health foods and high-protein biscuits. Other foods that may contain soy include pureed and cereal baby foods, margarine, and white and brown bread.

Other uses for soy include the manufacture of tofu (soybean curd), which may in turn be used for the manufacture of soy-based ice cream. Soy may be converted into products having a meat-like texture. This textured vegetable protein is used in simulated meat products or may be added to meat as an extender. These products are often used as meat substitutes in vegetarian products and in catering establishments, including hospital and army food services, and feeding programs.

The seeds of soybeans are widely used as a source of oil. The oil has many uses. e.g., in salad dressings, margarine, baby foods, industrial components, linoleum, paint, plastics, soap, and glue for plywood (see table below). Although soybean oil was initially thought to be safe for soy-sensitive individuals, it is now evident that soy protein may occur in soybean oil. Thus the allergenicity of soybean oil would depend on its purity, which in turn depends on the extraction process. Recent evidence has demonstrated that although oxidized soybean oil may not show allergenicity, proteins in soybeans are capable of interacting with oxidized lipid to form products that are allergenic to soybean-sensitive patients. Indeed, Hiyama et al. report a case of urticaria associated with parenteral nutrition with an intravenous 10% lipid emulsion containing a soybean oil base. Such reactions, however, appear to be uncommon, and there are very few reports of this nature in the literature.  

Soy products are often purchased by those specifically avoiding cow's milk, often with the assumption that a soy-based product is free of cow's milk protein. This may not always be true, and caution is recommended. Again, labels should be read carefully, and they should, of course, contain the correct declaration.

Thickeners, stabilizers, emulsifiers and bulking agents may be manufactured from a variety of other members of the legume family in addition to soybeans. On the basis of in vitro studies, Barnett et al. suggests there may be cross-reactions between soy and other members of the legume family (see table below). Further evidence for broad cross-reactivity has been provided by RAST and skin prick tests; however, it is rare to have symptomatic reactivity to more than one member, and clinical hypersensitivity to one legume does not require elimination of the entire legume family. Carob, derived from the carob bean, is used commonly as a chocolate substitute, and one should possibly guard against cross-reactivity to this legume.