Vascular Risk Factor Reduction
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.3
In addition, the presence and severity of cerebrovascular pathology appears to increase the risk and stage of AD for any given level of AD neuropathology.3
Thus, 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 the dementia in old age, with several possible mechanisms ().In over 20 observational studies, older adults with diabetes have approximately double the risk of developing dementia and mild cognitive impairment compared to those who do not have diabetes.4,5
Initial 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
Possible mechanisms that may explain the association between vascular risk factors and increased risk of developing dementia.
People with hypertension in midlife consistently have increased risk of AD and all-cause dementia in several observational studies.7,8
In 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.9
However, other studies have found no relationship between late life hypertension and the risk of dementia.10
Some 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 increased risk of cognitive impairment and dementia with stroke.7
This 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 mid-life and possibly later in life have an increased risk of developing dementia.6,7
Despite this relationship, people with dementia are more likely to have low body mass than be obese.6
Low body mass may, however, 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 co-existence, 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, high cholesterol, or were smokers at mid-life were more likely to develop dementia later in life. The effects of each vascular risk factor were approximately additive.12
Similarly, the ‘metabolic syndrome’, which is a clustering of disorders that include abdominal obesity, hypertriglyceridemia, low high density lipoprotein, hypertension and/or hyperglycemia, has been associated with 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 (). The direct relationship between hypertension, cerebrovascular disease (in its most severe case, stroke), and subsequent dementia is well-established,11
and cerebrovascular disease is also likely to link obesity, diabetes, dyslipidemia to cognitive impairment. The degenerative changes in the cerebrovascular vessels may also cause the dysfunction of both the endothelium and blood-brain barrier.11
Consequently, 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 pathology. Specifically, in vitro
studies indicate that insulin causes a significant increase in extracellular β-amyloid levels.6
Consequently, people with insulin resistance, such as type II diabetics or those with precursor hyperinsulemia, 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.8
In particular, the secretion of inflammatory adipocytokines may be involved in neurodegenerative pathways.8
It is unclear, however, whether adipose tissue is directly linked to cognitive impairment or whether the 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 preventative interventions. Interventions could include the behavioral (as discussed below) as well as medical treatment of vascular risk factors. Several studies indicate that people who receive treatment for hypertension, both in midlife and latelife, have reduced risk of developing cognitive impairment compared to those with untreated hypertension;7
however, other studies have not confirmed this.7
Similarly, statin therapy in observational studies is associated with reduced risk of dementia and AD.7
Randomized 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 to a placebo.7,13,14
Preliminary trials of diabetes medications including rosiglitazone and insulin have indicated that these drugs may be beneficial to cognition in patients with AD6,15,16
and mild cognitive impairment.15
From initial reports that elders with more education have 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.17
Several prospective observational studies have reported that people who engage in mentally stimulating activities—such as learning, reading or playing games–at younger ages18
and older ages19
are less likely to develop dementia compared to those people who do not engage in these activities.
Elders who are cognitively active may have higher degrees of neuropathology without exhibiting the symptoms of dementia compared to those who do not engage in cognitively stimulating activity.17
This 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 pathology.17
The 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.20
The 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,20
cognitive training did not generalize across domains and did not affect everyday functioning.21
In addition, elders with memory impairment may be less able to make gains from memory training than those without impairment.22
Consequently, the impact of cognitive training in elders on the risk of dementia is unclear but several trials are underway.
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.23
In addition, physically active people may have a lower incidence of AD, though the association is less consistent for vascular dementia23,24
(). Although fewer studies have investigated the association between mid-life physical activity and cognitive impairment, most have found that mid-life activity is associated with a lower incidence of both AD and all-cause dementia.23
Summary of evidence and suggestions for future directions on risk factors for dementia.
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 four months.25
Furthermore, 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 multi-factorial. People who exercise have higher levels of brain neurotrophic factors,27
which are implicated in neurological repair. Physical activity also modifies vascular risk27
and vascular risk factors are, as discussed above, associated not only with increased risk of vascular dementia but also of AD.4
In 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 to those with socially rich lives.19
Social engagement through visits with friends and relatives, going to movies, clubs, centers, church/synagogues, and volunteering may be protective against developing cognitive impairment.19
Some 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 neuropathology.19
Another study, however, found that the relationship between low social engagement and the risk of dementia was restricted to those subjects who experienced a decline in social engagement from mid-life to late-life.28
This suggests that low social engagement, rather than a risk factor, may be an early symptom of cognitive impairment. 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 three 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.29
As a result, interventions that include cognitive, social, and physical components might best strategy to reduce the risk of cognitive impairment and 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 anti-oxidants 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, elders consuming a Mediterranean diet and higher fruit and vegetable intake may have lower risk of developing dementia.30
Other studies found that that people with high consumption fish have lower risk of dementia and cognitive decline.30
The association between high fish, fruit, and vegetable diets 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 pathology. This lead to the hypothesis that a high dietary intake of antioxidants might slow cognitive decline and lower the risk of dementia. Indeed, in some studies, people with higher intakes of vitamin E and C (both antioxidants), either through diet or supplements, have slower cognitive decline and a lower risk of AD in old age.30
However, the relationship has not been consistent and other large, prospective observational studies found no association between vitamin intake and dementia risk.30
Furthermore, randomized controlled trial evidence has, at best, been inconsistent with most studies finding no relationship between vitamin E supplementation and cognitive performance.30,31,32
Therefore, it remains unclear whether the observed association 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.33
While several observational studies, though not all, reported that people with high polyunsaturated fatty acid consumption had lower risk of dementia and AD, randomized controlled trials have not confirmed the results.33
Furthermore, polyunsaturated fatty acid supplementation had no effect on memory and attention in cognitively healthy elders.33
As 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 anti-oxidants 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.27
In addition, many people with dementia also have depression.34
It is unclear, however, whether depression is 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 to those without these symptoms.35,36
Some studies, however, found that depressive symptoms coincided with or followed dementia onset rather than preceded it. 34
There are several hypotheses that 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; 3) those with early cognitive deficits may be more likely to become depressed because of the earliest cognitive symptoms. 35
Mechanistically, depression is associated with elevated cortisol levels, which may directly damage the hippocampus and increase the risk of dementia.34
Furthermore, 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,37
but 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.