Approximately 20 percent of children in the United States respond to supplementation with IQ increases of 9+ points over test-retest increases in a placebo group. However, no effects are found for children with adequate levels of vitamins and minerals in their diets. The concentration of effects is likely to be greatest among disadvantaged children.
Effects of micronutrient supplementation have been demonstrated to continue for one year and may last longer.
One provocative natural experiment of the effect of dietary changes on academic performance took place in New York City public schools in the late 1970s and early 1980s. Schoenthaler et al. (1986a, b) analyzed the results of dietary modifications in the foods supplied to the schools. In school years 1979-1980, 1980-1981, and 1982-1983 there was a gradual elimination of synthetic colors, synthetic flavors, and selected preservatives. High-sucrose foods were gradually eliminated. When Schoenthaler and colleagues compared the student percentile rankings on the California Achievement Test, the results were striking (see Figure 3-8). The average ranking in the 41st percentile in the three years before the changes rose to 47th, 51st, and 55th in each of the three change years. In 1981-1982, when no new changes were introduced, the scores remained flat. Gains were largest for students doing worst academically. In 1979, 12.4 percent of students were performing two or more grades below level. At the end of 1983, that rate had dropped to 4.9 percent. While the precise nutritional change was not measured in this study, the authors argue that the foods eliminated tend to be low in the ratio of essential nutrients to calories, thus increasing the proportion of available foods with a higher ratio of nutrients to calories (Eysenck and Schoenthaler, 1997). The claim, however plausible, was not tested.
Lead, a common element in the earth’s crust, becomes harmful to humans only when it is bioavailable: that is, when it is ingested in paint chips or dust that contain lead, taken into the lungs via pollution from leaded gasoline, absorbed through foods that have been stored in lead soldered cans or ceramics (Rice, 1998), or consumed in drinking water that has flowed through lead-soldered pipes (NRC, 1993). Lead is both carried in the bloodstream and stored in bone and soft tissue. The fetal months and early childhood years of rapid bone and tissue growth therefore constitute a particularly vulnerable period for lead exposure.
Childhood lead poisoning was recognized only in the past century, a period that was marked by dramatic shifts in lead exposure. Widespread exposure to lead first rose, particularly with the addition of lead to gaso-