Evidence from observational studies to date suggests that healthy dietary patterns are associated with better cognitive performance in later life. We examined the extent to which childhood intelligence quotient (IQ) and socioeconomic status account for this association.

Analyses were carried out on 882 participants in the Lothian Birth Cohort 1936 Study. Four dietary patterns were extracted using principal components analysis of a food frequency questionnaire, namely “Mediterranean-style,” “health aware,” “traditional,” and “sweet foods.” Cognitive function was assessed at the age of 70 years, including general (g) cognitive ability, processing speed, memory, and verbal ability.

Before adjustment for childhood IQ and socioeconomic status, the “Mediterranean-style” dietary pattern was associated with significantly better cognitive performance (effect size as partial eta-square (ηp2) range = 0.005 to 0.055), and the “traditional” dietary pattern was associated with poorer performance on all cognitive domains measured in old age (ηp2 = 0.009 to 0.103). After adjustment for childhood IQ (measured at the age of 11 years) and socioeconomic status, statistical significance was lost for most associations, with the exception of verbal ability and the “Mediterranean-style” pattern (National Adult Reading Test (NART) ηp2 = 0.006 and Wechsler Test of Adult Reading (WTAR) ηp2 = 0.013), and the “traditional” pattern (NART ηp2 = 0.035 and WTAR ηp2 = 0.027).

Our results suggest a pattern of reverse causation or confounding; a higher childhood cognitive ability (and adult socioeconomic status) predicts adherence to a “healthy” diet and better cognitive performance in old age. Our models show no direct link between diet and cognitive performance in old age; instead they are related via the lifelong-stable trait of intelligence.