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Miia Kivipelto a Department of
Neuroscience and Neurology, University of Kuopio, PO Box 1627, 70211 Kuopio, Finland, b Department
of Public Health and General Practice, University of Kuopio, c Department of Clinical Radiology,
Kuopio University Hospital, PO Box 1777, 70211, Kuopio, Finland, d Department of Neurology, Kuopio University Hospital, e North Karelia Central
Hospital, 80210 Joensuu, Finland, f National Public Health
Institute, 00300 Helsinki, Finland
Correspondence to: M Kivipelto
miia.kivipelto{at}uku.fi
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Abstract |
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Objective:
To examine the relation of midlife
raised blood pressure and serum cholesterol concentrations to
Alzheimer's disease in later life.
Design:
Prospective, population based study.
Setting:
Populations of Kuopio and Joensuu, eastern Finland.
Participants:
Participants were derived from random,
population based samples previously studied in a survey carried out in
1972, 1977, 1982, or 1987. After an average of 21 years' follow up, a
total of 1449 (73%) participants aged 65-79 took part in the re-examination in 1998.
Main outcome measures:
Midlife blood pressure and
cholesterol concentrations and development of Alzheimer's disease in
later life.
Results:
People with raised systolic blood
pressure (
160 mm Hg) or high serum cholesterol concentration (
6.5
mmol/l) in midlife had a significantly higher risk of Alzheimer's
disease in later life, even after adjustment for age, body mass index, education, vascular events, smoking status, and alcohol consumption, than those with normal systolic blood pressure (odds ratio 2.3, 95%
confidence interval 1.0 to 5.5) or serum cholesterol (odds ratio 2.1, 1.0 to 4.4). Participants with both of these risk factors in midlife
had a significantly higher risk of developing Alzheimer's disease than
those with either of the risk factors alone (odds ratio 3.5, 1.6 to
7.9). Diastolic blood pressure in midlife had no significant effect on
the risk of Alzheimer's disease.
Conclusion:
Raised systolic blood pressure and
high serum cholesterol concentration, and in particular the combination
of these risks, in midlife increase the risk of Alzheimer's disease in
later life.
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What is already known on this topic
What this study adds
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Introduction |
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Risk factors for vascular disease may also be risk factors for
Alzheimer's disease. Some cross sectional studies have investigated the relation of blood pressure and cholesterol concentration to cognitive function in late life, but with conflicting results. Furthermore, one limitation of cross sectional studies is that they
cannot readily determine causality. This shortcoming can to some extent
be overcome with longitudinal, population based studies. Two such
studies have found that raised blood pressure precedes the development
of Alzheimer's disease
one conducted in a Swedish population aged 70 years and followed for 9-15 years and the other in Japanese-American
men with a mean age of 53 followed for 25 years.
1 2
In
addition, Finnish men aged 70-89 with Alzheimer's disease were found
to have had raised serum cholesterol concentrations 15-25 years before
the onset of the disease.3
It is important to identify early risk factors for Alzheimer's
disease because the neurodegenerative processes of Alzheimer's disease
may begin in midlife.4 Identification of these risk factors may shed some light on the pathophysiology of Alzheimer's disease and also provide new potential avenues for its prevention and
treatment. The preliminary findings showing an association between
vascular risk factors and Alzheimer's disease need to be replicated in
independent populations, and no population based study has yet
evaluated the association of both midlife blood pressure and
cholesterol concentrations with Alzheimer's disease in later life in
both sexes. We investigated the putative impact of raised blood
pressure and cholesterol concentrations in midlife on the subsequent
development of Alzheimer's disease in a population based sample.
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Participants and methods |
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Participants were derived from four separate independent population based samples studied within the framework of the North Karelia project and the FINMONICA study in 1972, 1977, 1982, and 1987.5 Participants who were still alive, aged 65-79 by the end of 1997, and living in two geographically defined areas in or close to the towns of Kuopio and Joensuu were the target of this study. From these 2293 people, a random sample of 2000 participants were invited to the re-examination carried out during 1998. Altogether 1449 (72.5%) people participated (figure on website). The mean length of follow up was 21 (SD 4.9) years (range 11-26 years; 26 years for 34.9%, 21 years for 38.1%, 16 years for 15.6%, and 11 years for 11.4% of participants). The study was approved by the local ethics committee, and written informed consent was obtained from all participants.
Examination and re-examination
The protocol for the midlife examination is described in greater
detail elsewhere.5 In brief, the survey included a self
administered questionnaire on medical history, cerebrovascular and
cardiovascular events, and vascular conditions diagnosed by a
physician. Height and weight were measured. Blood pressure was measured
in the right arm after participants had been seated for five
minutes. Venous blood specimens were taken for determination of serum
cholesterol concentrations.5
Statistical analyses
Differences between the study groups were analysed with
Student's t test and the
2 test as
appropriate. Midlife blood pressure values were classified into normal
(<140 mm Hg), borderline (140-159 mm Hg), and high (
160 mm Hg) for
systolic pressure; and normal (<90 mm Hg), borderline (90-94 mm Hg),
and high (
95 mm Hg) for diastolic pressure. Midlife serum
cholesterol concentrations were classified into high (
6.5 mmol/l)
and normal (<6.5 mmol/l). The association between midlife blood
pressure and cholesterol concentration and subsequent Alzheimer's disease was investigated with multiple logistic regression analyses, using normal blood pressure and cholesterol as the reference category (model 1). As blood pressure and cholesterol concentration are influenced by age and body mass index, the analyses were replicated controlling for these factors (model 2). Further analyses were carried
out controlling also for education, history of myocardial infarction
and cerebrovascular symptoms (used as indicators of vascular events),
and smoking status and alcohol consumption (model 3).
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Results |
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Sociodemographic and clinical characteristics
Table 1 shows the sociodemographic and clinical characteristics of the study population. Mean age was 50.4 years (SD
6.0, range 40-64 years) in the original survey and 71.3 years (4.0, 65-80) at re-examination. Patients with Alzheimer's disease were
significantly older, and they also had less formal education than
participants without dementia. Patients with Alzheimer's disease had
higher body mass index, systolic blood pressure, and serum total
cholesterol in midlife than did participants without dementia, but at
re-examination these values were similar in the two groups. Diastolic
blood pressure was similar in patients with Alzheimer's disease and
participants without dementia both in midlife and at
re-examination.
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Blood pressure, cholesterol, and risk of Alzheimer's
disease
High systolic blood pressure in midlife was a significant risk for
Alzheimer's disease in later life (table 2), and this remained true
after all the adjustments (models 2 and 3). Borderline high systolic
blood pressure in midlife also increased the risk (model 1), but after
adjustments (models 2 and 3) this association was no longer
significant. Midlife diastolic blood pressure had no significant effect
on the risk of Alzheimer's disease in any of the models. High serum
cholesterol concentration in midlife was a significant risk for
Alzheimer's disease, even after all the adjustments (models
1-3).
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Medical history and Alzheimer's disease
Table 3 shows the medical history of the participants.
Patients with Alzheimer's disease were more likely to have been
treated with antihypertensive drugs in midlife, but at re-examination
later in life there was no difference between the groups. At
re-examination, patients with Alzheimer's disease were significantly
more likely to have a history of myocardial infarction and
cerebrovascular symptoms (almost invariably expressed as transient
ischaemic attack) and less likely to be alcohol users than participants
without dementia.
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Discussion |
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This study showed that raised systolic blood pressure and high serum cholesterol concentrations in midlife increased the risk of Alzheimer's disease in later life. The combination of these risk factors in midlife, even when participants with borderline high systolic blood pressure were included, increased the risk to a greater extent than either of the risk factors on its own. Diastolic blood pressure in midlife was not associated with Alzheimer's disease in later life.
The role of blood pressure
Two previous longitudinal, population based studies have shown an
association between raised blood pressure and subsequent Alzheimer's
disease.
1 2
These studies, however, suggested that the
risk was related to raised diastolic blood pressure rather than raised
systolic pressure. Differences in the study settings and populations
could account for the discrepancy. For instance, treatment patterns may
have contributed to our finding. Participants with Alzheimer' s
disease were more likely to have received antihypertensive drug
treatment in midlife but, despite the treatment, still had higher
systolic blood pressure in midlife than their counterparts without
dementia. This is in line with findings that a considerable proportion
of treated hypertensive patients do not achieve the target blood
pressure and the fact that diastolic blood pressure has traditionally
been the main indication for antihypertensive
treatment.
2 9 10
The role of cholesterol
In this study, clinical indicators of atherosclerosis were more
common in patients with Alzheimer's disease than in participants
without dementia. A population based, cross sectional study has
previously indicated an increased risk of Alzheimer's disease in
patients with atherosclerosis.12 Hypertension and hypercholesterolaemia may increase the risk of dementia by inducing atherosclerosis and impairing blood flow, but they may also directly induce the neurodegeneration of Alzheimer's disease.13
Controlling for vascular events did not change the association between
midlife high systolic blood pressure and cholesterol concentration and subsequent Alzheimer's disease, suggesting that hypertension and hypercholesterolaemia themselves pose a risk for Alzheimer's disease. Furthermore, the combination of hypertension and hypercholesterolaemia in midlife was a particularly strong predictor of Alzheimer's disease;
these factors may accelerate the development of Alzheimer's disease
partly through different pathophysiological mechanisms.
Study methodology
The design of our study as a population based, longitudinal study
with a large cohort of participants and substantial response rate
increases the credibility of these findings. No autopsy data were
available to confirm the clinical diagnosis, although the accuracy of
the clinical diagnosis of Alzheimer's disease at Kuopio University
Hospital, verified by neuropathology, has been reported to be
96%.15
24 in the mini-mental state examination in the screening phase underwent the exhaustive examinations needed for the diagnosis of
dementia. Some dementia cases may have been lost because of this
cut-off score, and this may also have resulted in underestimation of
the prevalence of dementia.
Conclusion
As the proportion of elderly people in the population increases,
Alzheimer's disease will become an enormous public health problem.
Interventions that could delay the onset of the disease, even modestly,
would therefore have a major impact on public health.17
The observed relation between midlife vascular risk factors and
Alzheimer's disease later in life may have implications for the
prevention of dementia as both hypertension and hypercholesterolaemia can be treated.
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Acknowledgments |
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We thank Veli Koistinen, Veikko Jokela, and Pirjo Halonen for statistical help and Liisi Saarela for technical assistance.
Contributors: MK and E-LH were the principal investigators. MK analysed the data and drafted the paper. E-LH assisted in analyses and writing. E-LH, AN, JT, and HS contributed to the conception and design of the study, and AN and JT were also involved in the baseline surveys for the study. MH and KA participated in the diagnosis of dementia. MPL and TH participated in analysing the data and writing the paper. All the authors took part in planning the study and interpreting the data and commented on the manuscript. MK is the guarantor.
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Footnotes |
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Funding: Academy of Finland grants 37573 and 63645 and EVO grant 477268.
Competing interests: None declared.
Figure showing the study design
appears on the BMJ's website
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References |
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(Accepted 19 March 2001)
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