Soy Allergy

Soy allergy is an immune reaction to soybean proteins. It is commonest in infants, usually mild, and most children outgrow it by school age.

Pathophysiology. Allergens include the storage proteins glycinin and beta conglycinin (Gly m 5, Gly m 6) in IgE-mediated disease, and Gly m 4 (a birch pollen Bet v 1 homologue) in birch allergic patients. Non-IgE forms (FPIES, allergic proctocolitis) occur in infants with cow's milk allergy.

It affects roughly 0.5% of children. Cross-reactivity with peanut and other legumes is common on testing but seldom causes clinical reactions.

Highly refined soy oil and soy lecithin contain little protein and are usually tolerated.

• Urticaria, angio oedema, vomiting and wheeze within minutes to 2 hours
• Anaphylaxis, which is uncommon
• Eczema flares in infants
• Delayed vomiting, pallor and lethargy (FPIES)
• Blood streaked stools in infants (proctocolitis)
• Oral itching in birch allergic adults who drink soy milk (Gly m 4), occasionally with systemic reactions

• History of reactions and soy exposure, including infant formula and processed foods.
• Skin prick test and specific IgE to soy and Gly m 4, Gly m 5, Gly m 6: positive tests are common and poorly predictive, so the history and challenge decide. Gly m 5 and Gly m 6 positivity predicts systemic reactions, and Gly m 4 positivity with birch sensitisation predicts mild oral symptoms or reactions to soy milk and soy protein supplements.
• Supervised oral food challenge to confirm allergy or resolution.
• Growth chart, and non-IgE workup in infants with gastrointestinal disease.

Differential diagnosis.

• Cow's milk protein allergy (frequent coexisting problem)
• Peanut and other legume allergy
• Pollen food allergy syndrome from birch
• FPIES and allergic proctocolitis
• Soy intolerance and irritable bowel symptoms
• Eosinophilic oesophagitis

• Avoidance of soy protein. Read labels for soy, soya, soybean, tofu, tempeh, miso, edamame and textured vegetable protein. Soy lecithin and highly refined soy oil are usually permitted, but individual tolerance should be tested.
• Infants: soy formula is not used under 6 months. Infants with cow's milk allergy use extensively hydrolysed or amino acid formula.
• Adrenaline auto injector for IgE-mediated disease with systemic reactions: 150 microgram for 7.5 to 25 kg, 300 microgram for 25 to 50 kg, 500 microgram for over 50 kg, with a written action plan and training.
• Anaphylaxis: adrenaline 0.01 mg/kg intramuscularly (maximum 0.5 mg; 0.5 mg in adults) into the anterolateral thigh, repeated after 5 minutes if no improvement, plus oxygen and intravenous fluids, and observation for at least 6 hours.
• Mild reactions: cetirizine (children 1 to 5 years 2.5 mg twice daily; 6 to 11 years 5 mg twice daily; over 12 years and adults 10 mg once daily).
• Reintroduction: supervised challenge every 12 months, since most children tolerate soy by school age.
• Birch related soy reactions: avoid unheated soy milk and concentrated soy protein products, and eat processed soy if tolerated.

Special groups.

• FPIES to soy: acute episodes with dehydration or lethargy need ondansetron 0.15 mg/kg intravenously (maximum 16 mg), isotonic saline 20 mL/kg as a bolus (repeated as needed), and methylprednisolone 1 mg/kg intravenously (maximum 60 mg) for severe episodes.

Monitoring. Growth and weight at each visit. Review every 6 to 12 months and plan challenge. Check auto injector dose and expiry.

Referral and escalation. Refer to paediatric allergy for suspected IgE-mediated disease, FPIES, multiple food allergy, or faltering growth. Emergency for anaphylaxis, or repeated vomiting with pallor and lethargy.