Results of a prospective cohort study [Arch Neurol. doi:10.1001/archneurol.2012.1117] show that well-functioning older adults with diabetes mellitus (DM) and those with DM with poor glucose control have worse cognitive function and greater decline.
Although evidence suggests an increased risk of both DM and cognitive impairment with increased age, the association between DM and cognitive function in older adults is less clear, and little is known about cognitive function in older adults with newly diagnosed DM. In addition, the data are inconsistent on whether poor blood glucose control is a risk factor for cognitive decline among older adults with DM.
To assess the association between DM and increased risk of cognitive decline in elderly adults, as well as whether poor glucose control in elderly adults with DM is related to worse cognitive performance, investigators prospectively evaluated data from 3069 participants enrolled in a study called the Health ABC (Health, Aging, and Body Composition) study. The study, which began in 1997, included black and white adults 70 to 79 years of age living in 2 communities. People who had difficulties with daily living, walking a quarter of a mile, or climbing 10 steps without resting were excluded from the study.
The status of DM in each participant was determined at baseline and during follow-up visits, and defined as use of hypoglycemic medication, a fasting glucose level of
≥126 mg/dL, or a 2-hour glucose tolerance test level of >200 mg/dL.
To evaluate the association between DM and cognitive functioning, 2 tests were administered at baseline and at follow-up visits to assess cognitive functioning: (1) the Modified Mini-Mental State Examination (3MS), which evaluated components of orientation, concentration, language, praxis, and immediate and delayed memory; and (2) the Digit Symbol Substitution Test (DSST), which measured attention, psychomotor speed, and executive function. To evaluate the association between glucose levels in patients with DM and cognitive functioning, the glycoslyated hemoglobin A1c (HbA1c) level was measured from fasting whole blood at baseline and at follow-up visits.
The study found that 717 of the 3069 participants (23.4%) at baseline had prevalent DM and 3252 (76.6%) did not have DM. Of these 3252 participants without DM at baseline, 159 (5.2%) developed DM during subsequent follow-up visits.
Compared with participants without DM at baseline, those with DM had significantly lower baseline 3MS (90.9 vs 88.8, P=.001) and DSST (36.3 vs 32.5, P=.001) scores. After an average of 9 years, participants with prevalent DM continued to show a significantly greater decline than those without DM on both the 3MS test (-6.0 vs -4.5 point decline, P=.008) and the DSST (-7.9 vs -5.7 point decline, P=.001).
The study also found that among participants with prevalent DM, higher levels of HbA1c were associated with lower cognitive functioning. Significantly lower mean (SE) scores on the 3MS test were found in participants with mid or high HbA1clevels (86.2 [0.4] and 85.7 [0.5], respectively) compared with those with low HbA1clevels (87.1 [0.4]) (P=.003).
Similarly, significantly lower mean (SE) scores were found on the DSST in participants with mid or high HbA1clevels (29.0 [0.6] and 28.0 [0.7], respectively) compared with those with low HbA1clevels (29.5 [0.6]) (P=.04). These scores remained significantly different after adjusting for age, race, sex, and education, as well as for myocardial infarction, hypertension, and body mass index.
Several limitations of the study were noted by the investigators, including the inability to generalize the results to elderly persons with disabilities, the cognitive tests could only assess general cognitive functioning, the relatively small differences in cognitive scores between the 2 groups that made the clinical relevance of the difference unclear, and lack of information on duration or severity of DM in patients with prevalent DM at baseline.
The investigators concluded that older adults with DM have reduced cognitive function, and that poor glycemic control may contribute to this reduced cognitive function in these adults. They suggest further studies to determine “if early diagnosis and treatment of DM lessen the risk of developing cognitive impairment and if maintaining optimal glucose control helps mitigate the effect of DM on cognition.”