Fasting insulin and incident dementia in an elderly population of Japanese-American men

Objective: To evaluate the association of fasting insulin level to incident dementia in a cohort of elderly men. Methods: Data are from the Honolulu-Asia Aging Study, a community-based study of Japanese-American men, aged 71 to 91 years in 1991. Serum insulin was measured in 1991 and participants were grouped based on their insulin levels. Dementia was ascertained in 1991, 1994, and 1996 according to international guidelines. The 2,568 men dementia-free in 1991 were reexamined in 1994 and 1996; 244 new cases of dementia were diagnosed. Survival analysis with age as the time scale was used to estimate the risk ( hazard ratio [HR] and 95% CI) for incident dementia associated with levels of insulin. Results: The risk of dementia was increased at the two extremes of the insulin distribution ( lower and upper 15th percentiles). Compared to the rest of the cohort subjects in the lowest 15th percentile and highest 15th percentile had an increased risk for dementia ( HR = 1.54, CI 1.11 to 2.11 and HR = 1.54, CI 1.05 to 2.26). In men with insulin levels < 22.2 mIU/L the risk for dementia decreased with increased levels of insulin ( HR = 0.76, CI 0.72 to 0.79 for each increase of one logarithmic unit - 2.72 mIU/L of insulin). In men with insulin levels ≥ 22.2 mIU/L the risk for dementia increased with increasing levels of insulin ( HR = 1.64, CI = 1.07 to 2.52 for each 2.72 mIU/L). Conclusions: Both low and high levels of insulin are associated with an increased risk of developing dementia.