A profile of impaired insulin degradation in relation to late-life cognitive decline: a preliminary investigation.

OBJECTIVE: Insulin degradation pathways may be related to Alzheimer's disease pathology. In preliminary analyses, we considered the relation of combined lower insulin secretion (c-peptide) and higher insulin--possibly a phenotype for impaired insulin degradation--to cognitive decline.
METHOD: Fasting plasma c-peptide and insulin were measured in 1,187 non-diabetic Nurses' Health Study participants (mean age = 64 years). Cognitive testing began 10 years later. Participants completed three repeated assessments (over an average span of 4.4 years) of verbal memory, a strong predictor of Alzheimer disease development. C-peptide and insulin distributions were dichotomized at their medians to create four cross-tabulated categories. Multivariable linear mixed effects models were used to relate c-peptide/insulin categories to cognitive decline.
RESULTS: Compared to the lower c-peptide/lower insulin group, women with lower c-peptide/higher insulin had a significantly faster rate of verbal memory decline: the mean difference was -0.05 units/year (95% CI -0.09,-0.01). This mean difference was similar to that which we found for women 5 years apart in age, indicating that having a profile of lower c-peptide/higher insulin appeared cognitively equivalent to aging by five years on tests of verbal memory. For women with higher c-peptide/higher insulin, the estimated mean difference in decline compared to those in the lower c-peptide/lower insulin group was statistically significant, but slightly lower, at -0.04 units/year (95% CI: -0.07,-0.02).
CONCLUSION: These preliminary analyses of a possible phenotype of impaired insulin degradation provide supportive evidence that deficits in insulin degradation may be related to late-life verbal memory decline. (c) 2008 John Wiley & Sons, Ltd.