DOI: 10.2337/dc06-0915 © 2006 by the American Diabetes Association
Hyperinsulinemia and Cognitive Decline in a Middle-Aged Cohort
1 Department of Family Medicine, Medical University of South Carolina, Charleston, South Carolina Address correspondence and reprint requests to Sara E. Young, MD, Department of Family Medicine, Medical College of Georgia, HB-3032, Augusta, GA 30912-3500. E-mail: sayoung{at}mcg.edu
OBJECTIVEDetermining modifiable risks factors for cognitive decline and dementia are a public health priority as we seek to prevent dementia. Type 2 diabetes and related disorders such as hyperinsulinemia increase with aging and are increasing in the U.S. population. Our objective was to determine whether hyperinsulinemia is associated with cognitive decline among middle-aged adults without type 2 diabetes, dementia, or stroke in the Atherosclerosis Risk in Communities (ARIC) cohort. RESEARCH DESIGN AND METHODSMiddle-aged adults (aged 4564 years at baseline) in the ARIC cohort had fasting insulin and glucose assessed between 1987 and 1989. Subjects with dementia, type 2 diabetes, or stroke at baseline were excluded from analysis. Three tests of cognitive function available at baseline and 6 years later were delayed word recall (DWR), digit symbol subtest (DSS), and first letter word fluency (WF). Cross-sectional comparisons and linear regression models were computed for cognitive tests at baseline and change in cognitive test scores to determine whether cognitive function was associated with two measures of insulin resistance, fasting insulin and homeostasis model assessment (HOMA). Linear regression models controlled for age, sex, race, marital status, education level, smoking status, alcohol use, depression, hypertension, and hyperlipidemia. RESULTSIn unadjusted and adjusted analyses, hyperinsulinemia based on fasting insulin and HOMA at baseline was associated with significantly lower baseline DWR, DSS, and WF scores and a greater decline over 6 years in DWR and WF. CONCLUSIONSInsulin resistance is a potentially modifiable midlife risk factor for cognitive decline and dementia.
Abbreviations: ARIC, Atherosclerosis Risk in Communities DSS, digit symbol subtest DWR, delayed word recall HOMA, homeostasis model assessment WF, first letter word fluency
Determining modifiable risks factors for cognitive decline and dementia is a public health priority as we seek to prevent dementia, which affects at least 4 million individuals in the U.S. (1). Current therapies, such as cholinesterase inhibitors, may slow cognitive decline in some patients, although the absolute changes in disease trajectory are modest (24). Type 2 diabetes and related disorders such as hyperinsulinemia increase with aging and are increasing in the U.S. population (5,6). Previous studies have produced conflicting evidence linking diabetes, glucose intolerance, and hyperinsulinemia to cognitive decline and Alzheimers dementia (7). Previous epidemiologic studies relate hyperinsulinemia or type 2 diabetes to Alzheimers dementia cross-sectionally in older adults (812), whereas only a few studies have evaluated the longitudinal association between hyperinsulinemia and Alzheimers dementia (1315). None of these studies investigated an association between hyperinsulinemia and cognitive decline in middle-aged adults. Insulin resistance is known to be associated with the development of age-related diseases including hypertension, coronary heart disease, stroke, cancer, and type 2 diabetes (16). People with insulin resistance are at an approximately fivefold risk for diabetes, but this risk can potentially be modified by modest weight loss and lifestyle modifications (17). It has been proposed that inflammation caused by oxidative stress is a common mechanism that exists for chronic progressive diseases including Alzheimers dementia (18). The mechanisms that link type 2 diabetes and cognition are still debated (19). Insulin receptors are concentrated in the hippocampus, a brain region important in memory and learning (20). Animal and human models suggest that elevated insulin levels increase the amyloid ß level (19), and accumulation of amyloid ß has been implicated in development of Alzheimers dementia. The relationship of hyperinsulinemia to cognitive decline has not been previously examined in the Atherosclerosis Risk in Communities (ARIC) cohort (21,22). The primary aim of our current study was to determine whether hyperinsulinemia is associated with cognitive decline among middle-aged adult participants without type 2 diabetes, dementia, or stroke in the ARIC cohort.
Database The ARIC cohort is a large, multiethnic, multisite longitudinal observational study of risk factors for vascular diseases that was initiated in 1987. This study is based on the publicly available ARIC dataset. Initially, 15,732 men and women aged 4564 years were recruited from area sampling of four locations using population-wide lists for probability sampling (23). Four U.S. communities were sampled; three of the communities in the cohort were probability sampled. The fourth community (Jackson, MS) only sampled nonwhites.
The initial visit, occurring between 1987 and 1989, included comprehensive clinical and laboratory examination including insulin level in 15,027 subjects who fasted
Data abstraction
Cognitive testing
Hyperinsulinemia
Control variables
Statistical methods Multiple variable linear regression models with the dependent variable of baseline cognitive test score or cognitive test change score were computed to evaluate associations with measures of hyperinsulinemia. Control variables, as described above, were used in all linear regressions to adjust for potential confounders. In these forced inclusion models, the dependent variable of either baseline cognitive test score or cognitive test change score was a continuous variable. Age, alcohol use, and depression score were treated as continuous variables in linear regressions. Additional multiple linear regression models included the control variables as described above as well as a control variable for later development of diabetes to assess the effects of change over time in diabetes status and treatment status.
The corresponding percentiles for the clinically informed cutoffs of fasting insulin >12.2 mU/l and HOMA >2.6 were 68.2 and 56.8, respectively, in all subjects in the ARIC cohort who had fasting insulin measured. Characteristics of ARIC participants without type 2 diabetes, dementia, or stroke/transient ischemic attack at baseline are presented in Table 1. DWR at baseline, available in the 7,148 individuals included in subsequent analyses, was 6.78 ± 1.43 (means ± SD). Minimum baseline DWR was 3, and maximum DWR was 10. The greatest decline in DWR score at 6-year follow-up was 7, and 5 was the greatest improvement. At follow-up cognitive testing, 72 participants had developed DWR scores <3. DWR change score, available in 7,008 individuals, was 0.153 ± 1.54. DSS at baseline, available in the 7,136 individuals included in subsequent analyses, was 46.9 ± 13.4. Minimum baseline DSS was 0, and maximum DSS was 93. The greatest decline in DSS score at the 6-year follow-up was 59, and 63 was the greatest improvement. DSS change score, available in 6,986 individuals, had a mean of 2.55 ± 6.94. WF at baseline, available in the 7,142 individuals included in subsequent analyses, was 34.6 ± 12.2. Minimum baseline WF was 0, and maximum WF was 86. The greatest decline in WF score at the 6-year follow-up was 44, and 66 was the greatest improvement. WF change score, available in 6,991 individuals, was 0.657 ± 7.91.
In unadjusted analyses, utilizing both clinical cutoffs and highest quartiles of both fasting insulin and HOMA, hyperinsulinemia at baseline was associated with significantly lower baseline DWR, DSS, and WF scores. DWR change scores for participants with hyperinsulinemia based on highest quartile fasting insulin, highest quartile HOMA, and HOMA with a cutoff of 2.6 had significantly greater decline than those without hyperinsulinemia (Table 2). WF change scores for participants with hyperinsulinemia based on highest quartile fasting insulin, fasting insulin with a cutoff of 12.2 mU/l, and highest quartile HOMA had significantly greater decline than those without hyperinsulinemia. DSS change scores were not significantly different between those with and without hyperinsulinemia. Variables for age, sex, race, and education level were statistically significant in most of the fully adjusted multiple linear regression models and explained a majority of the variance in cognitive test scores and change scores. R2 values for the linear regression models of baseline cognitive function tests with all control variables, but before addition of any hyperinsulinemia variable, were 0.125 for DWR, 0.492 for DSS, and 0.224 for WF. Multiple variable linear regression models adjusted for potential confounders showed lower baseline DWR scores for those with hyperinsulinemia, although only in the model utilizing fasting insulin with 12.2 mU/l as the cutoff was the hyperinsulinemia variable statistically significant (Table 3). Similarly, adjusted models showed lower baseline DSS scores for those with hyperinsulinemia, although the hyperinsulinemia variable was statistically significant only when utilizing highest quartile fasting insulin or highest quartile HOMA. Adjusted multiple variable linear regression models showed lower baseline WF scores using both empirical and both clinical cutoffs for hyperinsulinemia, and all hyperinsulinemia variables were statistically significant in their models of baseline WF scores.
In adjusted multiple variable linear regression models, greater decline in DWR over 6 years was found in those with hyperinsulinemia, although only in the model utilizing the highest HOMA quartile as the cutoff was the hyperinsulinemia variable statistically significant. Adjusted models showed greater decline in WF change scores for those with hyperinsulinemia, although only in the model utilizing the highest fasting insulin quartile as the cutoff was the hyperinsulinemia variable statistically significant. In adjusted models for DSS change scores, none of the hyperinsulinemia variables were statistically significantly. In terms of the strength of the relationship of hyperinsulinemia and cognitive tests in addition to the statistical significance previously shown, we conducted several additional analyses. The Spearmans rank correlation coefficient, summarizing the strength of the relationship between hyperinsulinemia by highest quartile fasting insulin and cognitive testing, was r = 0.144 (P < 0.0001) for baseline DSS score, r = 0.070 (P < 0.0001) for baseline DWR, and r = 0.084 (P < 0.0001) for baseline WF. Following adjustment for control variables in linear regression analyses of cognitive tests, the relationships remained significant; however, there was an indication of substantial shared variance with some of the control variables. For example, the partial correlation between baseline DSS score and hyperinsulinemia by highest quartile fasting insulin, net of the influence of the other control variables, was significant but had dropped to r = 0.029 (P = 0.0154). With the addition of a variable controlling for later development of diabetes to the previously described multiple linear regression models exploring change in cognitive function scores, none of the previously described statistically significant hyperinsulinemia variables became nonsignificant. The variable controlling for later development of diabetes was not statistically significant in any of the additional models.
Cognitive decline and dementia are typically considered diseases of the elderly, although prevention of cognitive decline may require intercession earlier in midlife. We have described the relationship between hyperinsulinemia and change in cognitive function over time independent of diabetes over 6 years in a middle-aged cohort. This study provides evidence that decline in cognitive function in a middle-aged adult cohort without preexisting dementia, stroke/transient ischemic attack, or type 2 diabetes was greater in participants with higher measures of insulin resistance. With such a large cohort, there is often more than sufficient power to detect significant differences between groups. More relevant to future interpretation is the minimum detectable difference between each groups mean cognitive changes and whether these differences are clinically significant. These performance decrements were small and probably not clinically significant to participants but offer evidence for initiation of cognitive impairment and cognitive decline associated with insulin resistance in midlife. In considering clinical significance, a previous study (22) using the ARIC cohort showed that diabetic subjects did not have a significant decline in DWR compared with nondiabetic subjects, though diabetic subjects had a greater decline in two other tests of cognitive function. This current study does not address the mechanisms by which hyperinsulinemia causes impaired cognition and cognitive decline. Given our findings of significant differences in decline in DWR and WF between those with and without hyperinsulinemia (excluding individuals with type 2 diabetes and after controlling for later development of diabetes), insulin resistance may increase risk of dementia and progression of dementia by mechanisms discrete to insulin resistance independent of the pathogenic course of type 2 diabetes. Assessing what incremental predictive value novel risk markers add to existing models of risk has been a challenge even in research of coronary heart disease prevention (28,29). The utility of this strategy in dementia prevention, particularly with assessment of potentially modifiable risk factors to identify individuals at risk for dementia toward providing preventive action, is in its infancy, and further exploration of the predictive utility of novel risk factors that have modest though independent statistical associations with cognitive decline is warranted. The strengths of this study include its large, population-based, multiethnic, longitudinal nature with repeat cognitive testing and its younger subject age than studies that have previously investigated longitudinal associations between hyperinsulinemia and dementia (14,15). Additional strengths are the use of two measures of insulin resistance, which we explored at both empirical and clinical cutoff levels. Use of the ARIC cohort allowed for examination of the relationship between insulin and cognitive function in both men and women and allowed for assessment of potentially preclinical cognitive decline, unlike the study by Okereke et al. (15), which included only older women in the Nurses Health Study, and Peila et al. (14), which included only elderly Japanese-American men. A limitation of this study is that we were unable to directly link time to development of decline; instead, we were limited to using a 6-year time frame. While some decline may have happened earlier, this time frame for an intermediate indicator of dementia seems like a reasonable assessment. The results of this study lead us to propose that insulin resistance is a potentially modifiable midlife risk factor for dementia. Hopefully, the results of this study will lead to assessments and interventions in middle age that can prevent or reduce the burden of dementia in the aging population. Middle-aged subjects without dementia, type 2 diabetes, or stroke at baseline who have hyperinsulinemia have a greater decline in performance on tests of cognitive function over a 6-year period compared with subjects without hyperinsulinemia.
This study was supported in part by grants 1D12HP00023 and 1D14HP00161 from the Health Resources and Services Administration and grant 1PS0AG021677 from the National Institute of Aging.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C Section 1734 solely to indicate this fact. Received for publication May 4, 2006. Accepted for publication September 17, 2006.
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