Alzheimer’s disease (AD) is a slowly progressive dementia characterized by cognitive decline and behavioral changes. Pathological changes in the brain include atrophy of the cerebral cortex (particularly in the mesial temporal and temporoparietal lobes), the presence of neurofibrillary tangles and senile (amyloid) plaques, a loss of cholinergic neurons in the brain, and reduced activity of choline acetyltransferase (the enzyme responsible for acetylcholine production) in the cerebral cortex and hippocampus. Pathogenesis is not well understood but involves neurotoxicity, inflammation, and, likely, apoptosis.
The disease typically progresses from mild memory impairment to severe cognitive loss with personality/behavioral changes. The patient often experiences language problems (particularly with generation of nouns [dysnomia]), and spatial disorientation is common. AD reduces life expectancy by as much as 50% following initial diagnosis. AD is the sixth leading cause of death among US adults.
Evidence suggests that AD is associated with the following:
Older age. According to estimates for 2010, there were 4.7 million Americans ≥ 65 years of age with AD, with that rate projected to rise to 13.8 million by 2050.
Family history. Risk is inversely proportional to the age of onset in a first-degree relative.
Genetics. Certain genetic abnormalities (particularly with the presenilin and amyloid precursor protein genes) place some individuals at high risk of early onset AD. However, these represent only a small fraction of cases. More commonly, late onset AD is associated with certain subtypes of apo E4, among other genes. Trisomy 21 is also associated with increased risk.
Hypercholesterolemia. There appears to be an association between elevated total or LDL cholesterol concentrations, particularly in middle age and development of AD. Of the 1,037 postmenopausal women enrolled in the Heart and Estrogen/Progestin Replacement Study, those with LDL cholesterol levels in the top 25% had 76% greater odds of developing cognitive impairment, compared with women who had lower LDL levels.
Overweight. Obesity has been shown to be an independent risk factor for development of AD.
Sedentary lifestyle. Observational studies and clinical trials show that regular aerobic exercise (e.g., a 40-minute walk three times per week) reduces the risk for developing dementia, and may reverse shrinkage in the hippocampus and other brain structures. -, , ,
Hypertension, declining blood pressure over time, cerebrovascular and cardiovascular disease, diabetes, smoking, and persistently elevated alcohol use. All these factors are associated with cerebral atrophy. Excess body weight may exacerbate these factors or lead to cerebral atrophy directly.
Brain trauma. A history of traumatic brain injury may be associated with development of AD.
Pathologic findings cannot be demonstrated except by autopsy (or, rarely, by brain biopsy); therefore, definitive diagnosis of AD is not possible in normal clinical practice. Current evaluation of the patient with suspected AD focuses on identifying potentially reversible disorders that can produce cognitive deficits.
Routine neuroimaging, particularly magnetic resonance imaging, can help rule out certain causes of memory loss, such as vascular dementia, normal pressure hydrocephalus, chronic subdural hematomas, and brain tumors. Occasionally, such conditions are identified even in the absence of other clinical indicators. Therefore, neuroimaging should be considered in all cases of progressive cognitive deterioration. Tests of potential metabolic abnormalities (e.g., hypothyroidism, vitamin B 12 deficiency, electrolyte abnormalities) and infection (e.g., neurosyphilis) should be included in the workup. Occasionally, lumbar puncture may be needed, particularly in atypical cases (such as those associated with somnolence or confusion at the outset) or those with rapid progression. Electroencephalography, which is usually normal early in AD, may provide useful clues to alternative diagnosis in unusual cases.
Severe sleep disorders, disorders of liver and renal function, and side effects of various medications can produce cognitive dysfunction, as can depression (pseudodementia). Neuropsychiatric testing may be useful to aid in the diagnosis of AD or to evaluate its progression. Such testing may be particularly helpful if the presentation is atypical. The Diagnostic and Statistical Manual for Mental Disorders, Fifth Edition ( DSM5) criteria for diagnosis of AD were updated in 2013.
Overlap with other causes of dementia does occur (e.g., Lewy body dementia and frontotemporal dementia). However, because treatment is generally based on symptoms, biopsy is not commonly performed.
People who exercise, participate in intellectually stimulating activities, and remain active in social networks appear to be at lower risk for AD, and those affected may slow its progression through these activities. Occupational therapy has been shown to improve daily functioning for patients with AD and to reduce caregiver strain. Exercise programs can slow physical and functional decline.
Drugs may have a modest effect.
Several epidemiological studies have examined associations between diet and the risk of AD and cognitive decline., , The following factors are under study for a possible role in reducing risk:
Cardiovascular risk factors
Elevations in cardiovascular risk factors during the midlife period, including hypercholesterolemia, hypertension, obesity, and diabetes, are associated with a significantly greater risk for AD. Individuals with high cholesterol have a greater than 70% greater risk, while diabetes and high blood pressure carry a 40% and 31% greater risk, respectively. While observational studies of drug treatment of these risk factors in patients with existing AD has suggested possible benefits, clinical trial data do not support this hypothesis.
Reduced Saturated Fat, Cholesterol, and Trans Fats
Ecological data have shown that the availability of animal products is more highly correlated with AD than any other types of foods. Previous research revealed more than twice the risk for developing this disease in persons consuming the most saturated fat compared with the lowest amount, noting similar findings for the trans fats commonly found in fast foods. Other studies have also implicated saturated fats in the cognitive decline that precedes AD. 31
The mechanism may relate to the influence of dietary fat on blood cholesterol concentrations. As noted above, midlife hypercholesterolemia is associated with risk for AD. Cholesterol forms the core of the neuritic plaques that are a well-know feature of this disease; indeed, a primary role for the amyloid precursor protein is the clearance of excess cholesterol from the brain. 29 In addition, animal products are chief dietary sources of advanced glycation end products that are now thought to play a key role in the pathogenesis of AD.
In contrast, higher intakes of unsaturated fats and a higher ratio of unsaturated to saturated fat are associated with a significantly lower risk for either cognitive decline or AD. A meta-analysis of fish and omega-3 fatty acid intake found that higher compared with lower intake of fish was associated with a 36% lower risk of AD.
Both longitudinal studies and meta-analyses have revealed cognitive benefits from what is referred to as a “Mediterranean diet.” , Definitions of such a diet vary from one study to another, but the diets are generally lower in animal products, higher in plant-derived foods, and may include some oil, wine, or other regional products. Studies have shown a similar benefit from both traditional Japanese diets and diets that focused on higher intakes of vegetables, fruits, nuts, fish, and poultry and a lower intake of high-fat dairy animal foods. 35 Soy foods, used in Japanese and other Asian types of meals, may be particularly beneficial as a replacement for animal foods when considering protein sources. These are not only free of the cholesterol and high saturated fat content of animal products, but have been associated with significant benefits on cognitive function in postmenopausal women.
Presumed explanations for the benefits of these diets include their low content of saturated and trans fats, their high content of of antioxidant-containing foods, and, in Mediterranean regions, the use of olive oil, which contains oleocanthal, a phytochemical that appears to inhibit the fibrillization of tau protein, deposits of which are a characteristic of AD.
Maintaining Healthy Weight
Obesity before age 65 years is associated with a roughly 40% greater risk for dementia compared with normal-weight individuals; however, obesity after this age is associated with a lower risk.
Consuming Vitamin E-Rich Foods
The Chicago Health and Aging Project reported that, in older people followed over a four-year period, AD developed in 14.3% of those whose vitamin E intake was low, but in only 5.9% of those in the highest category of dietary (nonsupplement) vitamin E.
Similarly, in a Dutch study including 5,395 people aged 55 and older, those who got the most vitamin E cut their risk of developing AD and other forms of dementia over the next decade by about 25 percent.
Studies have shown that patients with AD and cognitive impairment have lower blood levels of all forms of vitamin E found in foods (four tocopherols and four tocotrienols) when compared with cognitively normal persons. While each form shows different biological properties that may offer neuroprotection effects in AD, α-tocotrienol (not alpha-tocopherol, the types used in AD studies) is the most neuroprotective form of vitamin E.
Lower concentrations of γ- and α-tocopherols are also found in CNS tissue of AD patients, compared with normal controls, and may be associated with AD neuropathology. A controlled clinical trial of high-dose vitamin E (alpha tocopherol, 2000 I.U./day for roughly two years) found a significant benefit for AD patients. This study used the Alzheimer’s Disease Cooperative Study/Activities of Daily Living (ADCS-ADL) Inventory score, which assesses ability to perform activities of daily living in AD patients. Investigators found that the ADCS-ADL scores declined by roughly 3 units less in the vitamin E than the placebo group, a difference that was superior to memantine as well.
Increased Vitamin C
Studies have found that blood levels of vitamin C were both lower in patients with mild cognitive impairment and AD, when compared with controls. In addition, individuals with higher vitamin C intakes (through diet and low-level supplementation of 500 mg/day or less) may have slower rates of cognitive decline than do persons with low intakes, thereby raising the possibility that long-term consumption of diets high in vitamin C-containing foods can reduce the risk for AD.
In AD patients, no benefit was found in a 16-week study of a combination of vitamin C and E, lipoic acid, and Coenzyme Q10. In contrast, a study in which AD patients were given a combination of an antioxidant (lipoic acid) and an omega-3 supplement over 12 months resulted in less decline in the Mini-Mental State Examination (MMSE) and Instrumental Activities of Daily Living IADL).
A Nutrient-Dense Diet
A systematic review and meta-analysis found that AD patients had significantly lower blood levels of vitamins A, B12, C, E, and folate and nonsignificantly lower levels of vitamin D and zinc when compared to individuals without AD. Similar results were found even in AD patients who were not considered malnourished. 46 Given the large number of nutrients AD patients are lacking, a multiple vitamin supplement may be of benefit, provided it omits iron and copper, as noted below.
Avoiding Excess Copper
A meta-analysis examining copper levels in serum, plasma, and cerebrospinal fluid concluded that AD patients have a higher body copper burden than do normal individuals. In addition, a higher than normal amount of this copper is unbound from its carrier (ceruloplasmin) in these patients, as a result of alterations in copper metabolism seen in AD patients. The consequence, apparently, is an increase in unbound copper in the brain. This is of concern because brain copper increases with age and upregulates the expression of the amyloid beta precursor protein (AβPP) protein and increases the aggregation and neurotoxicity of Aβ.
Avoiding Excess Iron
Some evidence suggests that excess iron may contribute to AD risk. Patients with AD have elevated iron levels in several brain areas affected by this disease, and an excess of iron in the brain is associated with beta-amyloid (Aβ) plaque formation.
Iron transport through the blood brain barrier is usually tightly controlled. However, a damaged blood-brain barrier has been implicated in the development of this disease, and may precede the development of clinical symptoms. A Western diet has been linked to a compromised blood-brain barrier, thereby providing a possible mechanism by which iron could accumulate in the brain. In the ADNeuroimaging Initiative cohort study, ferritin levels predicted the conversion from mild cognitive impairment to AD. Ferritin levels were also strongly associated with cerebrospinal fluid levels of apolipoprotein E and were elevated by the APOE-ɛ4allele, revealing that elevated brain iron adversely impacts disease progression.
Copper and iron are often added to multiple vitamin-mineral supplements. It may be prudent to choose supplements that omit these minerals.
Adequate Vitamin D Status
Vitamin D deficiency is associated with global cognitive impairment in adults, and a meta-analysis found a greater than 20% greater risk for AD and dementia in persons whose vitamin D blood levels were in the deficient range (< 50 nmol/l) compared with individuals above this level. In addition, patients with AD are significantly more likely to have vitamin D deficiency than normal individuals.
Aluminum exposures can be neurotoxic, and studies have shown associations between aluminum in drinking water and AD risk. Unlike iron and copper, there is no requirement for aluminum in human biology. Nonetheless, the role of aluminum in AD remains controversial.
Moderate Alcohol Consumption
Although alcohol intake as low as 20 grams per day (1.25 servings) is a known risk factor for certain cancers, hypertension, and several other diseases, a meta-analysis found a roughly 30% lower risk for AD and a 25% lower risk for dementia in light to moderate alcohol consumers, compared with nondrinkers.
Benefits of moderate alcohol consumption, if any, are thought to derive from cardiovascular risk factor reduction (reducing platelet aggregation or modifying serum lipids) or through acetylcholine release in the hippocampus. However, hippocampal insulin resistance has been found in patients with AD, and moderate alcohol consumption is known to enhance insulin sensitivity.
Physical and occupational therapy consultation for home safety evaluation and needs assessment.
A diet low in saturated fat, trans fats, and cholesterol; moderate in iron and copper; and high in dietary fiber and vitamin E may reduce the risk of AD, in addition to helping prevent other age-related debilitating diseases. It is not yet clear that diet changes can alter the course of AD that has already been diagnosed.
Safety precautions for AD patients are important at all times. Connection with social services or a support group may also help ease the burden of care for a person with AD. Although routine genetic testing is not typically recommended, family members may want to be tested for the presence of the apo E4 allele. At minimum, they should be encouraged to mitigate their future risk of AD by noting the above recommendations.