Vitamin-mineral intake and intelligence: A macrolevel analysis of randomized controlled trials

Context: Two independent groups suspected that poor diets in school children might impair intelligence. Because dietary changes produce psychological effects, both groups conducted randomized trials in which children were challenged with placebo or vitamin-mineral tablets. Both reported significantly greater gains in intelligence among the actives. The findings were important because of the apparent inadequacy of diet they revealed, and the magnitude of the potential for increased intelligence. However, 5 of 11 replications were not significant, leaving the issue in doubt. Objective: To determine if school children who receive low-dose vitamin-mineral tablets produce significantly higher IQ scores than children who receive placebo. Design: A macrolevel analysis of the 13 known randomized, double-blind trials was undertaken. Setting and Subjects: A total of 15 public schools in Arizona, California, Missouri, Oklahoma, Belgium, England, Scotland, and Wales participated, with 1477 school children, aged 6 to 17 years, and 276 young adult males, aged 18 to 25 years, in 2 American correctional facilities. Main Outcome Measures: All studies used 1 of 3 standardized tests of nonverbal intelligence: the Wechsler Intelligence Scale for Children-Revised, the Wechsler Adult Intelligence Scale, or the Calvert Non-verbal test. Results: The activities in each study performed better, on average, than placebo in nonverbal IQ, regardless of formula, location, age, race, gender, or research team composition. The probability of 13 randomly selected experimental groups always performing better than 13 randomly selected independent control groups is one-half to the 13th power (p = 0.000122). The mean difference across all studies is 3.2 IQ points. Furthermore, the standard deviation in the variable "IQ change" was also consistently larger in each active group when compared to its controls. This confirms that a few children in each study, presumably the poorly nourished minority, were producing large differences, rather than a 3.2 point gain in all active children. Conclusion: There are important health risks when school children's dietary habits depart substantially from government guidelines; poor dietary habits may lead to impaired intelligence. Low-dose vitamin-mineral supplementation may restore the cognitive abilities of these children by raising low blood nutrient concentrations. However, there is also evidence that supplementation has no measurable effect on the intelligence of well-nourished children with normal blood nutrient concentrations.