Is obesity associated with a decline in intelligence quotient during the first half of the life course?

Cross-sectional studies have found that obesity is associated with low intellectual ability and neuroimaging abnormalities in adolescence and adulthood. Some have interpreted these associations to suggest that obesity causes intellectual decline in the first half of the life course. We analyzed data from a prospective longitudinal study to test whether becoming obese was associated with intellectual decline from childhood to midlife. We used data from the ongoing Dunedin Multidisciplinary Health and Development Study, a population-representative birth cohort study of 1,037 children in New Zealand who were followed prospectively from birth (1972-1973) through their fourth decade of life with a 95% retention rate. Intelligence quotient (IQ) was measured in childhood and adulthood. Anthropometric measurements were taken at birth and at 12 subsequent in-person assessments. As expected, cohort members who became obese had lower adulthood IQ scores. However, obese cohort members exhibited no excess decline in IQ. Instead, these cohort members had lower IQ scores since childhood. This pattern remained consistent when we accounted for children's birth weights and growth during the first years of life, as well as for childhood-onset obesity. Lower IQ scores among children who later developed obesity were present as early as 3 years of age. We observed no evidence that obesity contributed to a decline in IQ, even among obese individuals who displayed evidence of the metabolic syndrome and/or elevated systemic inflammation.