A husband and wife team of nutrition experts, Fred and Alice Ottoboni, have published an excellent article on the theory that ADHD might be a nutritional deficiency disease . They draw analogies with Beriberi and Pellagra which became endemic in previous centuries as a consequence of significant dietary changes within a large population. The wholesale switch from brown to white rice in Asia in the 1800's led to many deaths from Beriberi, and the switch from meat, eggs, and milk to corn led to widespread Pellagra in western nations at the same time.
They argue that ADHD may be a direct consequence of a large shift in dietary practices that has taken place in America over the last 40 years -- a shift towards increased consumption of processed foods containing an overabundance of sugar and starch, along with a dramatic shift in the sources of fat from meat, fish, and eggs to vegetable fats and oils. They express concern about both the high ratio of omega-6 to omega-3 fats (20:1) in vegetable oils, as well as their propensity to become highly damaging trans fats if overheated. They specifically mention the importance of DHA (docosahexaenic acid), found in meats and eggs, and AA (arachidonic acid), found in cold water fish.
These authors cite a number of different studies that have shown a relationship between fatty acid deficiency and ADHD   or a decrease in brain size in ADHD children  . They also suggest that the observed difference in IQ between breast fed and bottle fed infants may be due to the higher concentration of DHA and AA in breast milk. This deficiency is now fortunately being aggressively corrected both in America and elsewhere in the world (Recommendations for AA and DHA in Baby Formula) , and we can hope that this will lead to a decrease in the incidence of ADHD looking forward.
A study by Price on the health of isolated racial groups compared those who remained on their indigenous diet with those who transitioned to a western diet . The indigenous diet invariably contained essentially no empty carbs, and included large amounts of meat and fish. The children who switched to the "modern" diet showed symptoms of both physical and mental degeneration.
Ottoboni and Ottoboni conclude with this rather ominous remark: "The choice seems clear. We can either continue to depend on prescription drugs to mask the symptoms of ADHD, or consider preventing ADHD by modifying the American diet, particularly for child bearing women and children. Should we decide to continue to depend on prescription drugs, which do not remedy the underlying causes of nutritional deficiency diseases, we can look forward to a country in which there will be more and more children with undersized brains who cannot learn, use costly prescription drugs, drop out of school, commit crimes, and cause anguish for their parents."