Risk Factors

Alzheimer's disease is the seventh leading cause of death in Americans adults. It affects an estimated 4.5 million Americans and 8 million more people worldwide. Age is the greatest risk factor for Alzheimer's disease. The number of cases of Alzheimer's disease doubles every 5 years in people over 65. By age 85, almost half of all people are afflicted. People with the disease survive, on average, half as long as similarly aged adults without the disease.

With the increasing numbers of aging adults, unless effective methods for prevention and treatment are developed, Alzheimer's disease will reach epidemic proportions, afflicting an estimated 14 million Americans within 50 years. Evidence points to older age, high blood pressure, cholesterol levels, and a family history of the disease as the most important risk factors for Alzheimer's disease.

Gender and Estrogen Loss

Several studies have reported that women have a much higher risk for Alzheimer's disease than men. If there is a gender difference, it is likely to be due estrogen, the primary female hormone, which appears to have properties that protect against the memory loss and lower mental functioning associated with normal aging. Such actions include blocking production of beta amyloid, offering antioxidant protection, and regulating blood sugar (glucose) levels in the brain. The drop in estrogen levels after menopause may explain a higher risk for Alzheimer's disease in older women than in men. (Testosterone, the male hormone, converts to estrogen, which may help protect men.) Studies have been mixed, however, on the association between the decline in natural estrogen levels and mental functioning in older women.

Family History and Populations Differences

People with a family history of the disease are at higher than average risk for Alzheimer's disease. Researchers are identifying important genetic factors, notably the ApoE4 gene, that may be responsible for late- and early-onset cases.

Dietary and other cultural factors that increase the risk for hypertension and unhealthy cholesterol levels may also play a role. For example, a study of Japanese men showed that their risk increased if they emigrated to America. And the disease is much less common in West Africa than in African Americans, who share the same or higher risk with Caucasians Americans.

Risk Factors for Cardiovascular Disease

High blood pressure and unhealthy cholesterol levels -- the same important risk factors for heart disease and stroke -- may also be risk factors for Alzheimer's disease. In fact, they appear to be more important than ApoE4, the genetic factor most commonly associated with Alzheimer's disease.

Blood pressure is the force applied against the walls of the arteries as the heart pumps blood through the body. The pressure is determined by the force and amount of blood pumped and the size and flexibility of the arteries.

High Blood Pressure. Studies have reported an association between Alzheimer's disease and systolic hypertension (the higher and first number in blood pressure measurement). High blood pressure can cause problems with the vascular system, which is responsible for delivering blood to the brain. Recent research suggests that some types of blood pressure medication may lower Alzheimer's risk.

High Cholesterol Levels. Research indicates an association between high cholesterol levels and Alzheimer's disease in some people. One theory is that cholesterol regulates the processing and accumulation of amyloid beta-protein.

Click the icon to see an image of cholesterol.

Diabetes. Patients with diabetes often have high blood pressure, lipid imbalances, and circulatory disorders that affect the heart and vascular system. Research suggests that diabetes can also affect cognitive function and increase the risk of developing Alzheimer's disease.

High Homocysteine Levels. Homocysteine is an amino acid that has been identified as a modest risk factor in heart disease. It has also been associated with a higher risk for Alzheimer's disease. High levels are general due to deficiencies of the B vitamins B6, B12, and folate. Such vitamins are also related to nerve protection. Researchers theorize that homocysteine impairs the ability of DNA to repair nerve cells. The weakened cells are then more vulnerable to the harmful effects of oxidized beta amyloid.

Down Syndrome

Nearly all patients who inherit Down syndrome develop changes in the brain that resemble Alzheimer's if they live into their 40s, although onset varies and can occur as late as age 70. Women under the age of 35, but not older mothers, who give birth to children with Down syndrome are also at much higher risk for Alzheimer's.

Other Risk Factors Associated with Alzheimer's Disease

Lower Education and Economic Groups. A number of studies have reported either a higher risk for Alzheimer's disease in people with less education or a lower risk for Alzheimer's disease in those who remain mentally active. Some experts speculate that learning itself may stimulate more neurons to grow and thus create a larger reserve in the brain so that it takes longer for brain cells to be destroyed. Some evidence suggests that early malnutrition, which is more likely to occur in lower income and educational groups, has been associated with smaller brains and with Alzheimer's disease in old age. Low-birth weight can cause problems in growth factors that could affect both mental and physical health later on in adulthood.

Small Head Size. The size of the skull is fixed by age 7. Brain size approximates the head size until old age, when it begins to shrink. Some evidence has reported an association between small head size (and therefore less brain volume) and Alzheimer's disease, possibly because people who start with larger brains can sustain more injury over time. For example, a 2002 study indicated that it was reduction in overall brain volume, not specific regions, that contributed to mental impairment in older healthy adults. Another study reported that people who had small heads plus the ApoE4 gene had 14 times the risk for Alzheimer's disease than those without this combination. Nevertheless, other studies have found no association between a small head size and Alzheimer's disease.

Some experts suggest that the relationship observed in other research may simply be due to social and economic factors, such as malnutrition or low birth weight, which have been associated with both Alzheimer's disease and small head size. Small head size independent of other factors, they argue, does not pose a higher risk for either Alzheimer's disease or low intelligence

Depression. There is a significant overlap between depression and dementia in the elderly. In fact depression itself is often an early symptom of Alzheimer's disease. In a 2002 study of Catholic nuns, for each of four depressive symptoms, the risk for developing Alzheimer's disease increased by an additional 19%. For example, for a woman with four depressive symptoms the risk increased by 76%. Some evidence suggests that there may even be common genetic factors in people who have both early depression and Alzheimer's disease.

Head Injury. Some studies have found an association between serious head injuries in early adulthood and the development of Alzheimer's. It is not yet known if such injuries directly cause Alzheimer's or simply accelerate the disease in people who are already susceptible to it.

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Review Date: 5/22/2006
Reviewed By: Harvey Simon, M.D., Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.