Cognitive ability in young adulthood and risk of dementia in a cohort of Danish men, brothers, and twins

Abstract Introduction We examined the association between cognitive ability in young adulthood and dementia in Danish men, brothers, and male twins. Methods In total, 666,986 men born between 1939 and 1959 were identified for dementia diagnosis in national registries from 1969 to 2016. The association between cognitive ability from draft board examination and dementia was examined using Cox regression. Results During a 44-year follow-up, 6416 (0.96%) men developed dementia, 1760 (0.26%) and 970 (0.15%) of which were classified as Alzheimer's and vascular dementia, respectively. Low cognitive ability was associated with increased risk of dementia (hazard ratio [HR] per SD decrease 1.33 [95% confidence interval {CI} = 1.30–1.35]) with the strongest associations for vascular dementia (HR per SD decrease 1.47 [95% CI = 1.31–1.56]) and a weaker for Alzheimer's disease (HR per SD decrease 1.07 [95% CI = 1.03–1.13]). The intrabrother and twin analyses (taking shared family factors into account) showed attenuated risk estimates but with wide CIs. Discussion Low early-life cognitive ability increases the risk of dementia before the age of 78 years. The association is partly explained by shared family factors.