I just encountered this study of the link between dietary patterns in early childhood and eight-year-old IQ.
Methods: "The current study, based on the Avon Longitudinal Study of Parents and Children, uses data on children’s diet reported by parents in food-frequency questionnaires at 3, 4, 7 and 8.5 years of age. … IQ was assessed using the Wechsler Intelligence Scale for Children at 8.5 years. Data on a number of confounders were collected, and complete data were available for 3966 children."
Results: "After adjustment, the ‘processed’ (high fat and sugar content) pattern of diet at 3 years of age was negatively associated with IQ assessed at 8.5 years of age—a 1 SD increase in dietary pattern score was associated with a 1.67 point decrease in IQ (95% CI −2.34 to −1.00; p<0.0001). The ‘health-conscious’ (salad, rice, pasta, fish, fruit) pattern at 8.5 years was positively associated with IQ: a 1 SD increase in pattern score led to a 1.20 point increase in IQ (95% CI 0.52 to 1.88; p=0.001)."
Of course, this kind of observational study is ripe for confounding factors. This is what they have to say: "A wide variety of factors were considered as potential confounders or mediating factors in the relationship between diet and IQ. The following variables were taken into account: gender; age at WISC assessment; the WISC administrator; the number of stressful life-events experienced by the child; breastfeeding duration (ascertained at 6 months of age), estimated energy intake at each time point, a measure of parenting (HOME score) assessed at 18 months of age, maternal education, housing tenure and social class recorded during pregnancy and maternal age at birth of the study child. Finally, maternal consumption of oily fish during pregnancy was included, as this has been shown to be associated with IQ in this cohort."
Unfortunately, they don’t have a measure of parent IQ. What is smarter parents have smarter kids AND give their kids better diets?