Risk Factors:

Alzheimer's disease is the seventh leading cause of death in American adults. It affects about 5 million Americans and 8 million more people worldwide. According to the U.S. Alzheimer’s Association, 1 in 8 people age 65 and older (and nearly 1 in 2 people over age 85) have Alzheimer’s disease.

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 about 16 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 Population 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 in America.

Risk Factors for Cardiovascular Disease

High blood pressure and unhealthy cholesterol levels -- the same important risk factors for heart disease and stroke -- have been suggested as risk factors for 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. Patients may have many small mini- strokes but don't notice them. Dementia that develops as a result of these strokes is called multi-infarct dementia. The role of high blood pressure in the development of Alzheimer's disease is inconclusive.

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.

Diabetes. Patients with diabetes often have high blood pressure, lipid imbalances, and circulatory disorders that affect the heart and vascular system, which have been suggested to increase the risk for Alzheimer’s. However, the evidence to support a role of diabetes in the development of Alzheimer's disease is inconclusive.

High Homocysteine Levels. Homocysteine, an amino acid, 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. However, research has not shown that giving patients vitamin B6 and B12 prevents or treats Alzheimer's disease.

Other Risk Factors Associated with Alzheimer's Disease

Lower Education and Economic Groups. Several 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. 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.

Head Injury. Some studies have suggested an association between serious head injuries in early adulthood and the development of Alzheimer's. While a dramatic brain injury can certainly alter cognitive function, its role in the development of Alzheimer's disease is inconclusive.