Parents with children a decade apart in age were often given opposite advice about when to introduce common allergenic foods. The reversal followed a change in the underlying evidence.
The original advice was precautionary
Earlier guidance in several countries recommended delaying foods such as peanut, egg and fish, particularly for infants considered at higher risk.
The reasoning was intuitive rather than tested: postponing exposure to a potential allergen would allow the immune system to mature before encountering it.
The recommendations were adopted widely and were not accompanied by evidence that delay reduced allergy, because that evidence did not exist at the time.
The rates moved in the wrong direction
Food allergy became more common in countries that adopted delayed introduction, and comparisons between populations with different feeding practices pointed the same way.
Populations where allergenic foods were routinely given early tended to have lower rates of the corresponding allergies, which was difficult to reconcile with the delay hypothesis.
These observations prompted trials that tested introduction timing directly rather than inferring it, and the results led to guidance being rewritten.
Tolerance and sensitisation are different routes
The current explanation distinguishes between the immune system meeting a protein through the gut and meeting it through damaged skin.
Exposure through the gut tends to promote tolerance, in which the immune system learns to accept the protein as food rather than treating it as a threat.
Exposure through inflamed or broken skin, without corresponding exposure through eating, appears more likely to produce sensitisation. This is why eczema is closely linked to food allergy.
Guidance now generally favours earlier introduction
Most current guidance recommends introducing common allergenic foods during the first year alongside other complementary foods rather than deferring them.
The details differ between countries, including which foods are named, the ages given, and how frequently continued exposure is recommended once a food has been introduced.
Guidance also continues to change as evidence accumulates, so information from a previous pregnancy or from another country may no longer reflect local practice.
Higher-risk infants are a separate conversation
Infants with severe eczema, an existing food allergy, or a strong family history are handled differently, and several guidelines recommend clinical assessment before introduction.
Any suspected reaction to a food, and any decision to introduce a specific allergen in a higher-risk infant, should involve a doctor or allergy service rather than general guidance.
Removing foods from a child's diet without assessment carries its own risks, both nutritional and in reducing the exposure that maintains tolerance once it has been established.