Seven studies are reviewed that have examined the impact of vitamin/mineral supplementation on children's performance on intelligence tests. Five studies have reported that the taking of supplements was associated with improved performance, although not all children respond, rather those whose diet supplies low levels of vitamins and minerals benefit. In the studies that have reported positive findings, in each case the response was primarily in terms of non-verbal measures of intelligence, a change that is predicted theoretically. There have been suggestions that supplementation may also be associated with an improved mood and the ability to sustain attention, although it is unclear whether this completely accounts for the improved performance on non-verbal intelligence tests.

http://orthomolecular.org/library/jom/1992/articles/1992-v07n01-p031.html