Check into the Doris Rapp books or web site.  We have found that my
son's ability to concentrate and comprehend is directly affected by
foods and particularily the allergic ones.  I can say for myself, as a
person with multiple food and chemical intolerances, when you are not
feeling good everything is affected.  Beyond that certain foods can
affect your mental state, concentration etc.

Larissa Blechman

Also there is a link between ADHD and celiac disease/gluten intolerance,
certainly here learning is affected as well.