Soy may principally cause allergies, as any other protein-rich food source. However, when compared with other food, soy is usually found less allergenic as, for instance, cow milk. Soy is used as a milk replacement in infants with allergies to milk proteins. Infants fed with soy formulas subsequently develop less asthma, hay fever or neurodermitis than milk-fed infants. Three to four percent of high risk children where various kinds of hypersensitivity such as hayfever/asthma, neurodermitis or food intolerabilities are to be expected due to a family anamnesis of allergies develop an allergy against soy – as compared with 25 % with cow milk (Cordle 2004).
The diagnosis of an allergy against soy is frequently difficult. Allergy testing is often made with the aid of a blood test called RAST (Radio-Allergen-Sorbent-Test), which allws the detection of antibodies against food constituents. The more antibodies are circulating in the blood, the better the diagnosis of an allergy can be secured. Not with soy, though: In this case the RAST is overproportionally false-positive even when in fact there is no allergy against soy (Cordle 2004). For this reason a positive RAST test should be followed by further tests, such as the DBPCFC (Double blind placebo controlled food challenge), which is performed in specialized hospitals. In this test the patient is orally given increasing doses of either a food allergen or of placebo until there is a reaction. Under such conditions which are close to real life soy rarely triggers a reaction.
In a cohort study 1,272 young adults were tested with a prick test as well as with oral food challenge with various potentially allergenic food plants. 19.6 % of the study participants indicated allergic reactions to certain food items which was independent from allergic reactions to pollen. In fact the oral challenge test confirmed a real allergic reaction in 1.7 % of the study participants. The most frequent allergens were peanuts (0.6 %), followed by food additives (0.5 %), shrimps (0.2%), and fish, cow milk and soy in 0.1 % each (Osterballe et al. 2009).
Even with a diagnostically confirmed allergy against soy the consequences for the patient are less dramatic than with other allergies, especially allergies against nuts or eggs. With such allergies the patient already reacts to traces of the allergen in food. E.g., the threshold for reactions to peanuts is 0.1 mg in the food item, with hazelnuts 1 mg – quantities which justify the mandatory warning label „may contain traces of nuts” on industrially processed food. In contrast, the threshold for reactions to soy is approximately 400 mg (Cordle 2004) – and thus above the limit of potential impurities in food due to cross-contaminations, the occurrence of traces of a food ingredient in food which does officially not contain this ingredient, e.g., caused by packaging different food items on the same machine.
Allergies are not necessarily a reaction to proteins naturally occurring in the plant. A part of the soy allergies may well be related to genetic modification of the plant. E.g., in experiments aimed on increasing the content of methionine in soy protein a gene of the Brazil nut was transferred to soy – a gene with the code for a protein which was subsequently found highly allergenic (Lack 2002). Presumably the experiments were terminated thereafter, which does, however, not guarantee that other genes transferred to soy will not increase the allergenic potenca of the soy plant.
References
Lack, G. (2002). Clinical risk assessment of GM foods. Toxicol. Lett. 127 (1-3): 337-340.
Safety 