Answer: as soon as possible.
For a number of years, medical societies recommended that the introduction of highly allergenic foods to a child’s diet should be delayed. It was hoped that this would reduce the incidence of food allergies. In reality, it probably did the opposite. There are now numerous published studies showing that food avoidance during pregnancy, nursing, or early childhood does not reduce the risk of developing food allergy. In fact, in countries where peanuts are eaten often at an early age the incidence of peanut allergy is much lower than in countries where peanut avoidance has been practiced. Why is this?
The answer might lie in what is being termed the dual allergen exposure hypothesis. In this hypothesis, the development of food allergy is determined by both the age at which a person is exposed, and the route through which they are exposed. It appears that early cutaneous exposure to food allergens promotes allergy while early oral exposure to allergens promotes tolerance and vice versa.
Cutaneous exposure to food allergens is particularly likely to induce sensitization in children with eczema. A large number of eczema sufferers have a defect in a component of their skin called fillagrin. Fillagrin plays an important role in the barrier function of the skin. When it is missing or defective, more allergens can penetrate deeply enough into the skin to come into contact with cells of the immune system. These cells seem to know that allergens like peanut are not supposed to be there so they create an allergy response to try and get rid of them. So, while we used to think that food allergy caused eczema, it may actually be that eczema predisposes to food allergy.
Much like the immune cells in the skin know that food allergens are not supposed to be there, immune cells in the gut naturally sense that foods are ok to be around. In medical jargon we would say they default to tolerance. So,when these cells see certain foods early in life, the immune system learns to ignore them. But, the longer we delay ingestion of certain foods, the more likely the immune system is to sense them as problematic and develop an allergy to them.
So, if we can modify these two variables- decrease the early cutaneous exposure and increase the early oral exposure we should be able to reduce the incidence of food allergy. There are a couple of prospective trials currently running that examine this very notion. Results in one should be back next year, but for now the best recommendation is early exposure to foods, not avoidance.