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Low HDL Cholesterol Is a Risk Factor for Deficit and Decline in Memory in Midlife. The Whitehall II Study
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Arteriosclerosis, Thrombosis, and Vascular Biology June 30, 2008
Archana Singh-Manoux *; David Gimeno ; Mika Kivimaki ; Eric Brunner ;and Michael G. Marmot
From INSERM u687-IFR69, the Department of Epidemiology and Public Health (A.S.-M., D.G., M.K., E.B., M.G.M.), University College London, UK; and the Centre de Gerontologie, Hopital Ste Perine (A.S.-M.), AP-HP, France.
"The National Cholesterol Education Program29 has stressed the importance of lowering LDL-C to reduce the burden of cardiovascular disease. Our results suggest that increasing HDL-C might also be important, for cognitive outcomes in particular....
......This study of middle-aged adults suggests a robust association between low HDL-C and poor memory. Furthermore, decreasing HDL-C was associated with decline in memory over a 5-year follow-up. These associations remained after adjustment for the effects of education, occupational position, prevalent disease, or medication use, and they were independent of APOE e4 status. Serum concentrations of total cholesterol and triglycerides show no association with memory deficit or decline. Thus, our results identify HDL-C as being important for memory......There are a number of plausible mechanisms connecting low levels of HDL-C and memory, as HDL-C is the prominent lipoprotein in the human brain27 and is involved in the regulation of amyloid beta protein metabolism and deposition in the brain.34 Deficit in HDL-C could also affect memory through its influence on atherosclerotic disease and stroke,35 or subclinical vascular injury not reflected in the covariates examined. Other possible mechanisms linking low levels of HDL-C to neurodegenerative processes might involve its antiinflammatory36 or antioxidant37 properties.....Among the elderly, there is also some evidence of a link between HDL-C and poor memory10 and Alzheimer disease. However, some previous studies on the elderly have found low HDL-C to be associated with vascular dementia but not with Alzheimer disease.8,14.....The association between low levels of HDL-C and poor memory is unlikely to be simply an accidental finding in our data as the cross-sectional findings were consistently replicated across 2 study phases. (see Discussion).....causality cannot be inferred from observational data, and a randomized controlled trial with treatment specifically targeted at elevating HDL-C levels and measurements of change in memory performance would be necessary to establish causality......it is possible that some unmeasured variable causes decline in both HDL-C and memory..... it is possible that the association between lipids and cognition is underestimated in our sample."
ABSTRACT
Objective-The purpose of this study was to examine the relationship between fasting serum lipids and short-term verbalmemory in middle-aged adults.
Methods and Results-Total cholesterol, high-density lipoprotein cholesterol (HDL-C), triglycerides, and memory were measured twice, at mean ages 55 and 61, in 3673 male and female participants of the Whitehall II study. Short-termverbal memory was assessed using a 20-word list. Logistic regression was used to model associations between ATP-III categories of lipids and memory deficit (recall of 4 words) and decline (decrease of 2 words). Analyses were adjusted for education, occupational position, coronary heart disease, stroke, hypertension, use of medication, diabetes, smoking, and alcohol consumption. Compared to high HDL-C (60 mg/dL), low HDL-C (<40 mg/dL) was associated with greater odds of memory deficit at the first (OR=1.27; 95% confidence interval [CI]=0.91 to 1.77) and second wave of this study (OR=1.53; 95% CI=1.04 to 2.25) in fully adjusted analysis. Decrease in HDL-C over the 5-year follow-up period was associated with decline in memory in the adjusted analysis (OR=1.61; 95% CI=1.19 to 2.16); no interaction with APOE e4 status was present.
Conclusions-HDL-C levels are potentially modifiable, and our results suggest that low HDL-C is associated with poor memory and decline in memory in middle-aged adults.
Background
The distinction between vascular dementia and Alzheimer disease has become blurred, partly because of similar associations of cardiovascular disease and its risk factors with different types of dementia1 and lower cognitive functioning. 2,3 Among the many risk factors investigated, the association between cholesterol and cognition appears to be the most elusive. High cholesterol is a proven risk factor for cardiovascular disease4 but the association with cognition appears complicated. Some studies have shown high lipid levels to be a risk factor for impaired cognition or dementia, 3,5-14 whereas others either show no association15-17 or a protective association.18,19 Findings from lipid lowering
agents are also mixed: some studies show a protective effect on dementia20 and others no effect.21
There is some consensus to suggest that dementia itself modifies lipid levels either through changes in diet or metabolism, leading to low total or low-density lipoprotein cholesterol levels (LDL-C) (ie, a more favorable profile) among those with dementia.1,6,7,22 Thus, examination of the effect of lipids on cognition in the elderly either in crosssectional analysis or in analysis using short follow-up periods is likely to yield spurious results. The associations between midlife lipid levels and late life dementia appear to be robust.5-7,23 However, the precise lipid that might be important remains unclear, with studies implicating high levels of LDL-C3,13 or total cholesterol (TC)5-7,24 or low levels of high-density lipoprotein cholesterol (HDL-C).8 -12,14,25 HDL-C is critical for the maturation of synapses and the maintenance of synaptic plasticity.26 It can influence the formation of amyloid beta, the main constituent of amyloid plaques.27 Low HDL-C has also been shown to be associated with lower hippocampal volume.11
We investigate the association between lipids and shortterm verbal memory by examining the cross-sectional associations and associations between changes in lipids with changes in memory over 5 years in middle age individuals. We examine whether these associations are independent of morbidities related to lipid levels, such as cardiovascular disease, stroke, and hypertension, and inherited apolipoprotein E epsilon 4 status (APOE e4), shown to be important in the association between lipids and cognition.1,5
Results
Data for the longitudinal analysis were available on 3673 individuals; the cross-sectional analyses are also shown on the same individuals. Table 1 shows the characteristics of these individuals at Phases 5 and 7.
The cross-sectional associations between the lipids and poor memory are presented in Table 2. At Phase 5, 34.8%, 11.9%, and 9.9% of the sample was in the high risk category for total cholesterol, HDL-C, and triglycerides, respectively.
Women were less likely to be present in the high-risk categories of HDL-C (9.8%) and triglycerides (13.0%). Nevertheless, men and women were combined in these analyses as the interaction term with TC (P 0.49), HDL-C (P 0.07), and triglycerides (P 0.37) did not provide strong evidence for sex differences in the association between lipids and memory deficit. Table 2 shows no association between TC and memory deficit, and the same is true for triglycerides. However, low level of HDL-C was associated with memory deficit in the fully adjusted model (OR 1.27; 95% confidence Interval [CI] 0.91 to 1.77). Table 3 shows results from analysis using Phase 7 data when the participants were 5 years older. The interaction term for TC (P 0.44), HDL-C (P 0.53), and triglycerides (P 0.54) again allowed us to combine men and women in the analysis. Here again, low HDL-C was associated with memory deficit (OR 1.53; 95% CI 1.04 to 2.25).
Table 4 shows the results for the effect of change in lipid levels on decline in memory. For a majority of individuals, lipid risk levels remained the same over the 2 measures that were 5 years apart: 59.1% for TC, 69.8% for HDL-C, and
78.8% for triglycerides. Men and women were again combined in these analyses as the interaction term with TC (P 0.63), HDL-C (P=0.63), and triglycerides (P=0.09) did not provide strong evidence for sex differences. Results show only changes in HDL-C to be associated with decline in memory. Compared to those with high levels of HDL-C, individuals with decreasing HDL-C had a greater risk of memory decline in fully adjusted analysis (OR 1.61; 95% CI 1.19 to 2.16). Changing the reference category to those with decreasing HDL-C (results not shown but available on request) revealed that all groups except the low-low HDL-C group had lower odds of decline in memory. Statin use was not associated with decline in memory (OR 0.98; 95% CI 0.73 to 1.32). It should be noted that the decreasing HDL-C category contains individuals who decreased from high to low or intermediate level or from intermediate to low level.
Sensitivity Analysis
Of the 3673 participants in this study, APOE e4 data were available on 3326 individuals. The interaction term (P=0.53) between APOE e4 status and change in HDL-C did not suggest any evidence of a stronger association in different
APOE e4 groups. 2397 participants were APOE e4 negative and Table 5 (subsample 1) shows the association between change in HDL-C levels and memory decline in this group. Decreasing HDL-C was associated with decline in memory (OR 1.73, 95% CI 1.20 to 2.50) after adjustment for all covariates.
The results for subsample 2 (n 3326) in Table 5 also show a robust association between change in HDL-C and decline in memory (OR 1.58; 95% CI 1.17 to 2.13), revealing LDL-C not to change the association between change in
HDL-C and decline in memory.
Discussion
This study of middle-aged adults suggests a robust association between low HDL-C and poor memory. Furthermore, decreasing HDL-C was associated with decline in memory over a 5-year follow-up. These associations remained after adjustment for the effects of education, occupational position, prevalent disease, or medication use, and they were independent of APOE e4 status. Serum concentrations of total cholesterol and triglycerides show no association with memory deficit or decline. Thus, our results identify HDL-C as being important for memory.
Many previous investigations into the association between lipids and memory in the elderly have focused on total or LDL-C,5,33 perhaps because of their status as proven risk factors for cardiovascular disease. Our findings emphasize the need to expand the focus to HDL-C. In our study on middle aged adults, the associations of low levels of HDL-C with memory deficit and decline were independent of other lipids and robust to adjustments for a number of potential
confounding factors. There are a number of plausible mechanisms connecting low levels of HDL-C and memory, as HDL-C is the prominent lipoprotein in the human brain27 and is involved in the regulation of amyloid beta protein metabolism and deposition in the brain.34 Deficit in HDL-C could also affect memory through its influence on atherosclerotic disease and stroke,35 or subclinical vascular injury not reflected in the covariates examined. Other possible mechanisms linking low levels of HDL-C to neurodegenerative processes might involve its antiinflammatory36 or antioxidant37 properties.
Although memory deficits are critical to the diagnosis of mild cognitive impairment38 and Alzheimer disease, the association between lipids and memory remains little explored in midlife. Dementia occurs late in life but it is increasingly recognized that there is a long preclinical phase characterized
by progressive neuropathological changes that become clinically detectable later. The "life-long" view of dementia stresses the importance of risk factors in midlife.39 Our findings on individuals aged 55 and 61 at the 2 phases of data
collection suggest that low levels of HDL-C may be an important risk factor. Among the elderly, there is also some evidence of a link between HDL-C and poor memory10 and Alzheimer disease. However, some previous studies on the elderly have found low HDL-C to be associated with vascular dementia but not with Alzheimer disease.8,14 The inconsistency in findings needs to be viewed in light of the fact that dementia itself modifies lipid levels6,7; necessitating further research where lipids are measured before the diagnosis of
dementia.
The association between low levels of HDL-C and poor memory is unlikely to be simply an accidental finding in our data as the cross-sectional findings were consistently replicated across 2 study phases. There is some evidence of increase in association over time as the Phase 7 data show stronger associations between HDL-C and memory deficit. Furthermore, decreasing HDL-C level was also predictive of decline in memory. We undertook further analysis in subsamples first to assess whether this association held in APOE e4 negative subjects. APOE e4 is widely regarded as being implicated with adverse outcomes for dementia.1,5,23,24,40 Our results on APOE e4 negative individuals show decreasing levels of HDL-C to be associated with greater odds of memory decline. In the second analysis on a subsample we reran the longitudinal analysis by replacing total cholesterol with LDL-C. Here again, results were not much different.
There are a number of potential limitations to this study. First, causality cannot be inferred from observational data, and a randomized controlled trial with treatment specifically targeted at elevating HDL-C levels and measurements of change in memory performance would be necessary to establish causality. A further possibility would be to examine this issue using brain imaging data. Second, despite extensive adjustments for a variety of potential confounding factors, it is possible that some unmeasured variable causes decline in both HDL-C and memory. Third, data here are drawn from the 5th and 7th phase of a study,
implying both survival and selection effects. Therefore, it is possible that the association between lipids and cognition is underestimated in our sample.
In conclusion, our results show low levels of HDL-C (<40 mg/dL) to be associated with poor memory. Furthermore, decline in HDL-C was associated with declines in memory over a 5-year period. The National Cholesterol Education Program29 has stressed the importance of lowering LDL-C to reduce the burden of cardiovascular disease. Our results suggest that increasing HDL-C might also be important, for cognitive outcomes in particular.
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