IQ and somatic health in late adolescence

Intelligence quotient (IQ) is associated with mental health in youth onwards, as well as somatic health and longevity later in life. However, little is known about the association with somatic health in youth. We aimed to investigate the cross-sectional association between IQ and a range of somatic diagnoses and health indicators in late adolescence. In a cohort comprising 49 321 Swedish men, IQ test performance and health status were recorded at conscription in 1969-70, at ages 18-20. Information on socioeconomic factors in childhood was obtained from the national census. Odds ratios (ORs) were estimated by logistic regression. With adjustment for socioeconomic background, each step decrease in IQ test performance on a nine-point scale was associated with an increased OR for the following somatic diagnoses: impaired hearing (1.14, 1. 1.12-1.16), endocrine disorders (1.13, 1.10-1.17), symptoms and ill-defined conditions (1.11, 1.08-1.14), back pain (1.10, 1.08-1.12), digestive system diseases (1.08, 1.05-1.10) and injuries (1.02, 1.00-1.05); and a decreased OR for hayfever (0.87, 0.85-0.90) and refractive errors (0.87, 0.86-0.88). IQ was also associated with increased ORs for low physical capacity (1.12, 1.10-1.14), signs of inflammation (1.07, 1.06-1.09) and low self-rated health (1.03, 1.02-1.05). Several diagnoses were not associated with IQ. In conclusion, lower IQ at conscription was associated with a higher risk for several diagnoses and indicators of poor health, but the risk was decreased for a few of the diagnoses. The mechanisms underlying the associations presumably differ. However, socioeconomic factors in childhood could not explain the associations.