Possible mechanisms that may explain the association between vascular risk factors and an increased risk of developing dementia.
Laura E. Middleton, Kristine Yaffe. Promising Strategies for the Prevention of Dementia. Arch Neurol. 2009;66(10):1210–1215. doi:10.1001/archneurol.2009.201
The incidence and prevalence of dementia are expected to increase several-fold in the coming decades. Given that the current pharmaceutical treatment of dementia can only modestly improve symptoms, risk factor modification remains the cornerstone for dementia prevention. Some of the most promising strategies for the prevention of dementia include vascular risk factor control, cognitive activity, physical activity, social engagement, diet, and recognition of depression. In observational studies, vascular risk factors—including diabetes, hypertension, dyslipidemia, and obesity—are fairly consistently associated with increased risk of dementia. In addition, people with depression are at high risk for cognitive impairment. Population studies have reported that intake of antioxidants or polyunsaturated fatty acids may be associated with a reduced incidence of dementia, and it has been reported that people who are cognitively, socially, and physically active have a reduced risk of cognitive impairment. However, results from randomized trials of risk factor modification have been mixed. Most promising, interventions of cognitive and physical activity improve cognitive performance and slow cognitive decline. Future studies should continue to examine the implication of risk factor modification in controlled trials, with particular focus on whether several simultaneous interventions may have additive or multiplicative effects.
By 2050, the number of people with Alzheimer disease (AD) and other forms of dementia in the United States is expected to almost triple. The predicted rise in dementia prevalence can largely be attributed to increasing longevity and the aging of the baby boomer generation. This shift toward an aging population is important because the incidence of all-cause dementia nearly doubles with every 5 years of age.
Not surprisingly, the cost of caring for people with dementia will rise along with the prevalence. In 2008, the US Medicare cost for AD alone was approximately $91 billion.1It is estimated that the cost of caring for people with dementia will grow to $189 billion by 2015.1Furthermore, the indirect cost of dementia to society extends beyond the direct cost and includes lost productivity, increased absenteeism, and replacement expenses of caregivers.1
Given the expected dramatic increase in the incidence and prevalence of dementia, the identification of successful prevention and treatment strategies is critical. However, the current pharmaceutical treatment of dementia can only modestly improve symptoms and cannot cure or prevent dementia. As a result, prevention of dementia through risk factor identification and modification is of the utmost importance until disease-modifying agents prove efficacious. If the onset of AD can be delayed by 5 years, the expected prevalence would decrease by more than 1 million cases after 10 years and more than 4 million cases after 50 years.2In this article, we discuss some of the most promising strategies for the prevention of dementia, including vascular risk factor control, cognitive activity, physical activity, social engagement, diet, and treatment of depression.
Although AD and vascular dementia have traditionally been viewed as distinct disorders, it is now generally agreed that the two rarely occur in isolation. Both types of dementia share many risk factors and pathologic features with atherosclerosis.3In addition, the presence and severity of cerebrovascular pathologic findings appear to increase the risk and stage of AD for any given level of AD neuropathologic findings.3Thus, the modification of vascular risk might reduce the risk of dementia regardless of type.
Traditional cardiovascular risk factors such as hypertension, dyslipidemia, and diabetes appear to increase the risk of developing dementia in old age, with several possible mechanisms (Figure). More than 20 observational studies have shown that older adults with diabetes have approximately double the risk of developing dementia and mild cognitive impairment compared with those who do not have diabetes.4,5Initial observations indicated that diabetes was most strongly associated with the risk of vascular dementia, but more recent research has confirmed that people with diabetes also have a high risk of developing AD.6
Several observational studies have shown that people with hypertension in midlife consistently have increased risk of AD and all-cause dementia.7,8In contrast, the association between late-life hypertension and dementia is more controversial. Both very high systolic blood pressure and very low diastolic blood pressure in late life have been associated with increased risk of dementia and AD.9However, other studies have found no relationship between late-life hypertension and the risk of dementia.10Some suggest that while hypertension is a risk factor for dementia, blood pressure may decrease with the onset of AD.11
Other vascular risk factors also seem to increase the risk of dementia. People with high serum levels of total cholesterol and low-density lipoproteins in late life have an increased risk of cognitive impairment and dementia with stroke.7This relationship has, however, been less consistent than the relationship between hypertension or diabetes and dementia. In addition, several studies have found that people who are obese in midlife and possibly later in life have an increased risk of developing dementia.6,7Despite this relationship, people with dementia are more likely to have low body mass than be obese.6However, low body mass may be a sign of frailty, which predisposes the person to AD, or may be an early symptom of AD itself.6
Given the individual relationship of vascular risk factors with dementia and the frequency of their coexistence, it is not surprising that studies evaluating the effect of multiple or composite vascular risk factors on the risk of dementia have found that subjects who had diabetes, hypertension, or high cholesterol levels or were smokers at midlife were more likely to develop dementia later in life. The effects of each vascular risk factor were approximately additive.12Similarly, the metabolic syndrome, which is a clustering of disorders that include abdominal obesity, hypertriglyceridemia, low high-density lipoprotein levels, hypertension, and/or hyperglycemia, has been associated with an increased risk of cognitive impairment and cognitive decline, especially in subjects with high levels of inflammation.12
Since people are more likely to have multiple vascular risk factors than just one, it is difficult to establish mechanistic links between individual risk factors and dementia. The mechanisms linking vascular risk factors to cognitive impairment are likely numerous and complex (Figure). The direct relationship between hypertension, cerebrovascular disease (in its most severe case, stroke), and subsequent dementia is well established,11and cerebrovascular disease is also likely to link obesity, diabetes, and dyslipidemia to cognitive impairment. The degenerative changes in the cerebrovascular vessels may also cause the dysfunction of both the endothelium and the blood-brain barrier.11Consequently, endothelial cells may produce an excess of free radicals and cause subsequent oxidative stress with increased blood-brain barrier permeability to proteins leading to β-amyloid accumulation.11
There is also a growing body of work that suggests a direct link between insulin and AD pathologic findings. Specifically, in vitro studies indicate that insulin causes a significant increase in extracellular β-amyloid levels.6Consequently, people with insulin resistance, such as those with type 2 diabetes mellitus or those with precursor hyperinsulinemia, may have insulin-caused increases in β-amyloid levels. Furthermore, cholesterol is a key regulator of neuronal function that may regulate β-amyloid plaque deposition in the brain.7
In addition, adipose tissue secretes both metabolic and inflammatory factors.8In particular, the secretion of inflammatory adipocytokines may be involved in neurodegenerative pathways.8It is unclear, however, whether adipose tissue is directly linked to cognitive impairment or whether adipose tissue is a marker of insulin resistance and hyperinsulinemia.8
Recognition of vascular risk factors may help to identify those people who are most at risk (and should be regularly screened) for dementia and who are most likely to benefit from preventive interventions. Interventions could include the behavioral (as discussed later) as well as medical treatment of vascular risk factors. Several studies indicate that people who receive treatment for hypertension, both in midlife and in late life, have a reduced risk of developing cognitive impairment compared with those with untreated hypertension7; however, other studies have not confirmed this.7Similarly, statin therapy in observational studies is associated with a reduced risk of dementia and AD.7Randomized controlled trials of both antihypertensive medications and statins have not, however, consistently shown that treatment results in improved cognitive performance or less cognitive decline compared with a placebo.7,13,14Preliminary trials of diabetes medications including rosiglitazone maleate and insulin have indicated that these drugs may be beneficial to cognition in patients with AD6,15,16and mild cognitive impairment.15
From initial reports that elderly people with more education have a lower incidence of dementia, research has emerged to suggest that cognitive activity, more generally, is associated with a reduced risk of developing cognitive decline and dementia.17Several prospective observational studies have reported that people who engage in mentally stimulating activities—such as learning, reading, or playing games—at younger ages18and older ages19are less likely to develop dementia compared with those who do not engage in these activities.
Elderly people who are cognitively active may have higher degrees of neuropathologic findings without exhibiting the symptoms of dementia compared with those who do not engage in cognitively stimulating activity.17This phenomenon is referred to as cognitive reserve, a concept that arose from the initial observation that people with higher levels of education had greater cognitive performance for equal loads of AD pathologic findings.17The expanded construct now includes occupation and leisure activity among cognitive activities that maximize cognitive reserve.17
Recent trials have demonstrated that cognitive interventions may reduce the risk of cognitive impairment and slow cognitive decline.20The benefit of cognitive training, however, seems to be domain specific. Several trials found that while cognitive training can improve memory, reasoning, and mental processing speed in older adults,20cognitive training did not generalize across domains and did not affect everyday functioning.21In addition, elderly people with memory impairment may be less able to make gains from memory training than those without impairment.22Consequently, the effect of cognitive training in elderly people on the risk of dementia is unclear, but several trials are under way.
Recent attention is being paid to the role of physical activity as a potentially protective factor against the risk of dementia. In observational studies, people who are physically active often demonstrate less cognitive decline and a lower risk of dementia than people who are sedentary.23In addition, physically active people may have a lower incidence of AD, although the association is less consistent for vascular dementia23,24(Table). Although fewer studies have investigated the association between midlife physical activity and cognitive impairment, most have found that midlife activity is associated with a lower incidence of both AD and all-cause dementia.23
Interventional studies have confirmed that even short durations of exercise training can improve cognitive performance. A meta-analysis concluded that people who were not previously physically active can show improved cognitive functioning after exercising for as little as 4 months.25Furthermore, exercise interventions may also reduce the rate of cognitive decline in people with cognitive impairment.26
The mechanisms by which physical activity affects cognition are also complex and likely multifactorial. People who exercise have higher levels of brain neurotrophic factors,27which are implicated in neurological repair. Physical activity also modifies vascular risk,27and vascular risk factors are associated not only with increased risk of vascular dementia but also of AD as discussed earlier.4In addition, rats with high levels of voluntary physical activity also have less β-amyloid plaque formation.27
People with limited social networks and low social engagement may be more likely to develop dementia compared with those with socially rich lives.19Social engagement through visits with friends and relatives, going to movies, clubs, centers, and church or synagogues, and volunteering may be protective against developing cognitive impairment.19Some have proposed that social activity, similar to cognitive and physical activity, might reduce the risk of dementia by increasing cognitive reserve so that people can better maintain their cognitive performance even with neuropathologic findings.19
Another study, however, found that the relationship between low social engagement and the risk of dementia was restricted to subjects who experienced a decline in social engagement from midlife to late life.28This suggests that low social engagement may be an early symptom of cognitive impairment rather than a risk factor. Further studies are needed to determine whether social interventions might curb cognitive decline.
The interaction between social activity, cognitive activity, and physical activity is also difficult to disengage. Many leisure activities contain all 3 components. By evaluating each leisure activity for cognitive, social, and physical components, one study concluded that each component is equally important in the protection against dementia.29As a result, interventions that include cognitive, social, and physical components might be the best strategy to reduce the risk of cognitive impairment; research should investigate this possibility.
Many of the risk factors for dementia, such as hypertension, diabetes, and obesity, may be modified by diet. In addition, a diet high in antioxidants may reduce inflammation, which is associated with the risk of dementia. Thus, it is reasonable to suggest that the risk of dementia itself could be modified by diet. Several observational studies support this hypothesis. For example, elderly people consuming a Mediterranean diet and having higher fruit and vegetable intake may have a lower risk of developing dementia.30Other studies found that people with high consumption of fish have a lower risk of dementia and cognitive decline.30
The association between diets high in fish, fruit, and vegetables and a lower risk of dementia has largely been attributed to antioxidants and polyunsaturated fatty acids. The interest in antioxidants in relation to dementia stemmed from the observation that oxidative stress may contribute to AD pathologic findings. This led to the hypothesis that high dietary intake of antioxidants might slow cognitive decline and lower the risk of dementia. Indeed, in some studies, people with higher intake of vitamins E and C (both antioxidants) through either diet or supplements have slower cognitive decline and a lower risk of AD in old age.30However, the relationship has not been consistent and other large, prospective observational studies found no association between vitamin intake and dementia risk.30Furthermore, randomized controlled trial evidence has at best been inconsistent, with most studies finding no relationship between vitamin E supplementation and cognitive performance.30- 32Therefore, it remains unclear whether the observed associations between antioxidant use and dementia are causal or are due to uncontrolled confounding or other biases.
The investigations regarding polyunsaturated fatty acid consumption in relation to cognitive outcomes have been similarly inconclusive.33While several observational studies, although not all, reported that people with high polyunsaturated fatty acid consumption had a lower risk of dementia and AD, randomized controlled trials have not confirmed the results.33Furthermore, polyunsaturated fatty acid supplementation had no effect on memory and attention in cognitively healthy elderly people.33As a result of these trials, the relationship between polyunsaturated fatty acids and cognition has come under question, although further study is still warranted. Moreover, given that adherence to a Mediterranean diet and high consumption of antioxidants and polyunsaturated fatty acids are associated with reduced risk of cardiovascular disease, people who adopt healthy diets are likely to have positive health outcomes regardless of the effect on cognitive functioning and without any adverse effects.
It is well known that people, especially older adults, with depression have reduced cognitive performance.27In addition, many people with dementia also have depression.34It is unclear, however, whether depression is a risk factor for dementia or whether it is a prodromal symptom.
Some observational studies found that older people with depressive symptoms were more likely to have dementia at follow-up compared with those without these symptoms.35,36Some studies, however, found that depressive symptoms coincided with or followed dementia onset rather than preceded it.34Several hypotheses have been proposed to explain the relationships between depression and dementia: (1) depression may sometimes be an early symptom or prodrome of dementia; (2) the clinical examination required for the diagnosis of depression may make dementia more likely to be detected, especially at an earlier stage; and (3) those with early cognitive deficits may be more likely to become depressed because of the earliest cognitive symptoms.35Mechanistically, depression is associated with elevated cortisol levels, which may directly damage the hippocampus and increase the risk of dementia.34Furthermore, recent studies have suggested that people with depression have enhanced deposition of β-amyloid plaques.34
The treatment of depression also seems to improve cognitive function in people who are depressed,37but it may not return cognition to normal levels. In addition, whether the treatment of depression decreases the risk of dementia among people with depressive symptoms has not yet been studied. Additional research is needed to clarify whether depressive symptoms are a true risk factor for dementia and whether the treatment of depressive symptoms might reduce the risk of dementia.
Understanding which lifestyle and biological factors might alter the risk of dementia is crucial to preventing the disease. Population studies have identified many factors that could be important in reducing the risk of dementia, including factors that identify people at risk for dementia (vascular risk factors, depressive symptoms) and factors that may reduce the risk of dementia (cognitive, physical, and social activity, a diet rich in antioxidants and polyunsaturated fatty acids, vascular risk factor control). While early interventional studies have been less conclusive (Table), future trials should continue to examine the effect of risk factor modification on cognitive outcomes. In particular, interventions that combine a number of factors, such as healthy nutrition along with cognitive, social, and physical activity, should be investigated. In the most optimistic view, dementia could be delayed or even prevented by these interventions. At worst, people will improve their overall health, especially their cardiovascular health, and enjoy a more cognitively and socially engaging life.
Correspondence:Kristine Yaffe, MD, Departments of Psychiatry, Neurology, and Epidemiology and Biostatistics, University of California, San Francisco, 4150 Clement St, Box 181, San Francisco, CA 94121 (email@example.com).
Accepted for Publication:February 15, 2009.
Author Contributions:Drs Middleton and Yaffe drafted various aspects of the manuscript and revised the manuscript to its final version. Study concept and design: Middleton and Yaffe. Analysis and interpretation of data: Middleton and Yaffe. Drafting of the manuscript: Middleton. Critical revision of the manuscript for important intellectual content: Middleton and Yaffe. Administrative, technical, and material support: Middleton and Yaffe. Study supervision: Yaffe.
Financial Disclosure:None reported.
Funding/Support:This work was supported in part by grant AG 031155 from the National Institute on Aging (Dr Yaffe).