Because food antigens both cross the placenta and are excreted in breast milk, there was biological plausibility to the hypothesis that maternal avoidance of certain foods during both pregnancy and breastfeeding could impact the development of allergies in their children. Initially the hypothesis was that avoidance would help prevent the development of allergies. The data do not support that hypothesis.
There is a good Cochrane review of the data, which concludes that there is no benefit, though they do note one trial with a non-significant trend toward decreased severity of eczema symptoms. They reviewed trials of avoidance both during pregnancy and during lactation. Their conclusions:
Prescription of an antigen avoidance diet to a high‐risk woman during pregnancy is unlikely to reduce substantially her child's risk of atopic diseases, and such a diet may adversely affect maternal or fetal nutrition, or both. Prescription of an antigen avoidance diet to a high‐risk woman during lactation may reduce her child's risk of developing atopic eczema, but better trials are needed.
I seem to recall an argument for the opposite intervention, but I can't seem to find the reference. As I recall, it was not very convincing either. Though initial exposure and sensitization are required for the development of allergic disease, the placenta and the breast are very particular immunologic environments. Personally, I would expect exposure at these sites would be more likely to induce tolerance than sensitization, but I don't believe the data support that either.
