Allergens in Breast Milk: how much, for how long and does it matter?
In clinic, we are commonly asked about food allergens in breast milk and whether this is beneficial or harmful for their infants’ allergies.
Food allergens eaten by a breastfeeding mother do pass into breast milk, typically peak within 2 hours of maternal ingestion, but the amounts are typically very low.
For example, cow's milk protein, the highest reported median β-lactoglobulin level across studies was 4.2 ng/mL, which is 1 million times lower than the level in fresh cow's milk.
Food allergens in breast milk
Concentration Levels
Protein fragments (peptides) can be detected in breast milk but are typically present in very low amounts (nanograms to low micrograms per millilitre). For example, common allergens such as ovalbumin (egg) and beta-lactoglobulin (cow's milk) usually reach peak concentrations between 100 and 500 ng/mL.
These levels are significantly lower than the protein concentrations found in the original food source!
Timing and Duration
The appearance of food proteins in milk is rapid but transient:
Initial Detection: Proteins often appear within 1 to 6 hours after ingestion.
Peak Concentration: Most proteins peak between 2 and 4 hours.
Clearance: Concentrations usually decline sharply and become undetectable within 8 to 24 hours, depending on the mother's metabolism and the specific protein type.
Relevance for the Baby
The presence of these proteins serves several biological functions:
Immune Tolerance: Low-dose exposure to food antigens via breast milk is generally considered beneficial and represents ‘nature's way’ of first introducing infants to allergens. It helps "train" the infant's immune system to recognise these proteins as safe, potentially reducing the risk of developing future food allergies. These proteins travel alongside maternal Secretory IgA (sIgA) antibodies, which may help neutralise potential allergens or pathogens within the infant’s gut. It is therefore surprising that studies do not show a strong protective effect of breastfeeding against the development of food allergy.
Allergic reactions to proteins in breast milk: These low levels of proteins are very unlikely to ever cause an immediate-onset food reaction. However, in rare cases, highly sensitive infants may react to trace amounts of allergens, leading to delayed symptoms such as eczema, colic, or gastrointestinal distress. We can only diagnose the delayed allergies through elimination and then reintroduction diets.
What does the BSACI say wrt an elimination diet due to the baby's allergy symptoms (blood in stool is not included in this advice; in this condition, a trial of strict maternal cow’s milk avoidance must be trialled)?
Summary of BSACI Recommendations
Maternal Dairy Exclusion: For breastfed infants with suspected non-IgE-mediated cow's milk allergy, BSACI recommends a 2–4 week trial where the mother excludes all cow's milk and dairy products from her diet.
Confirmed Diagnosis: Diagnosis must be confirmed by the reappearance of symptoms upon the reintroduction of dairy into the maternal or infant diet.
Nutritional Safeguards: Mothers on a milk-exclusion diet are at risk of calcium deficiency; BSACI recommends assessment by a dietitian and, where appropriate, calcium and vitamin D supplementation.
Specialist Review: If symptoms are severe or do not improve after the exclusion period, referral to a specialist allergy service is advised to investigate other potential triggers like soya or egg.
Frequently Asked Questions
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Current science-based consensus in the UK indicates that restrictive diets during breastfeeding do not offer a protective effect against the development of allergic diseases in infants. Indeed, research suggests that maternal consumption of diverse foods may actually help induce oral tolerance in infants. Restricting your diet without a medical reason is discouraged
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Yes, infants can develop Non-IgE mediated food allergies, such as FPIES or Allergic Proctocolitis, through breast milk. Common culprits include cow's milk, soy, and eggs, which may cause symptoms like eczema, hives, or bloody stools. Small amounts of food proteins (such as beta-lactoglobulin from cow’s milk, or proteins from egg and peanut) can be detected in breast milk.
Cow's Milk Protein Allergy (CMPA): This is the most common allergy affecting breastfed infants.
Prevalence: Clinical IgE-mediated (immediate) reactions are extremely rare in exclusively breastfed babies. Non-IgE (delayed) sensitivities are more common but still affect only a small percentage of the population.
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Unless you are personally allergic to these foods, there is no scientific evidence supporting their removal from a breastfeeding diet. In fact, leading health organisations suggest that avoiding common food allergens does not prevent childhood food allergeis. Maintaining a varied, nutrient-dense diet is the best approach for both your health and the baby's development.
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Watch for immediate signs like hives and swelling (this is very rare in this setting). Unlike normal "fussiness," allergic reactions often involve the skin or significant gastrointestinal distress.
Proving that food allergens inbreastmilk are causing delayed symptoms, such as eczema flares, colic, reflux.. will require an elimination reintroduction diet. Ideally this should be supervised by a dietitian.
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In summary: breastfeeding provides modest protection against eczema in the first 2 years of life, but does not clearly prevent food allergies. The evidence for allergy prevention is limited and inconsistent.
More specifically:
Eczema:
Exclusive breastfeeding for 3-4 months decreases the cumulative incidence of eczema in the first 2 years of life. However, this protective effect does not extend beyond age 2 years, and there is no additional benefit from exclusive breastfeeding beyond 3-4 months for eczema prevention.
A large prospective cohort study found that shorter breastfeeding duration was associated with only a weak overall increased risk of eczema, with no effect on allergic sensitisation at age 10 years.
Food Allergy
The evidence does not support breastfeeding as a strategy to prevent food allergies. The American Academy of Pediatrics states that no conclusions can be made about the role of any duration of breastfeeding in either preventing or delaying the onset of specific food allergies.
A 2024 meta-analysis of 68 studies (772,142 children) found no significant overall association between breastfeeding patterns and food allergy, though breastfeeding >6 months was paradoxically associated with a potential increased risk (OR 1.69, 95% CI 1.27-2.25) - likely reflecting reverse causality or confounding.
Asthma and Wheezing
Any breastfeeding beyond 3-4 months is protective against wheezing in the first 2 years of life, and longer breastfeeding may protect against asthma even after 5 years of age.description
Worried About Allergies? Let’s Help You Get Answers
If your child is showing signs of a food, pollen, or skin allergy, early diagnosis is key. At London Allergy Consultants, our expert team provides trusted, evidence-based care tailored to your child’s needs. From testing to treatment plans, we guide you every step of the way.
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