Symptoms include vomiting, colitis, diarrhoea, anaemia and irritability. Coughing, wheezing, asthma and rhinitis can all be features of soya intolerance. Asthma can result if soya flour or dust is inhaled. Skin problems such as atopic eczema can also result and very occasionally anaphylactic shock can occur in severe cases. Vomiting and coughing and wheezing usually occur within 2 hours of ingestion with more severe reactions, such as gastrointestinal symptoms, following in 24 to 48 hours.
Although soya allergy is less common than other allergies such as peanut or dairy allergy, soybeans are included on the allergen list. As proved by studies among allergic infants referred to allergy clinics, soya allergy is amongst the most common allergenic foods.
In a study of the prevalence of specific food allergies within unselected paediatric patients, soya was again one of the most common offenders. The allergy is comparatively common amongst younger age groups, although it is sometimes outgrown. However it can occur in adulthood. Severe fatal reactions have been associated with soya allergy.
It seems soya protein intolerance is more common in people with cow's milk protein intolerance and this is especially so in children. Diagnosis for allergy is best done with trials of withdrawals and re-introduction of soya because skin prick tests are often unhelpful, as can be the case with cow's milk protein intolerance.
Completely avoiding soya protein in the diet is difficult because an enormous range of manufactured foods contain it. There is now a widespread use of soya protein in flour and therefore in most bread. Any bread sold unwrapped would not declare soya in its ingredients and even some wrapped breads do not declare the soya protein used as they do not have to by law. Soya is one of the ingredients along with iron salts, thiamine and bleaching agents etc. that can be undeclared.
All allergic reactions to soya concern soya protein. It is unknown whether soya lecithin, soya margarine or soya oil contain sufficient protein to provoke allergic reactions. In some studies those with soya allergy could tolerate small amounts of soya oil, lecithin and margarine, but in other studies patients could not.
Only minute amounts of protein were found in soya oil in one study. However it depends on the source of soya used and can vary from one product to another. The implication is that these items must be excluded from the diet if protein needs to be completely avoided - again it depends on the level of allergic reaction.