While researchers are unsure of the exact causes of Alzheimer’s disease, there are several known risk factors that increase your likelihood of developing the disease:

Age

The single greatest risk factor for developing Alzheimer’s disease is increasing age. While early-onset Alzheimer’s disease can occur in those aged 30 to 60, the vast majority of Alzheimer’s cases occur after age 65. Experts estimate that the risk for Alzheimer’s disease doubles every five years after age 65 and by age 85, nearly half of all people have the disease.

Sex

Women are more likely than men to develop the disease, in part because they live longer and as stated above, the risk of the disease increases greatly with advanced age.

Family history

Having a close relative such as a parent or sibling with Alzheimer’s disease makes you more likely to develop the disease yourself. The risk is even greater if more than one relative has Alzheimer’s disease.

Genetics

Genetics are undoubtedly responsible for at least some portion of the familial risk mentioned above. There are two basic categories of known Alzheimer’s disease genes: “risk” genes that increase the likelihood of developing Alzheimer’s disease, and “familial Alzheimer’s disease” genes that, when inherited, virtually guarantee that a person will develop Alzheimer’s disease.

Risk gene: APOE-e4

One gene that confers an increased risk of developing Alzheimer’s disease is a variant of the apolipoprotein E gene called apolipoprotein E-e4 (APOE-e4). APOE-e4 is one of three common forms of the APOE gene, along with APOE-e2 and APOE-e3. APOE normally makes a protein that helps to ferry cholesterol from place to place throughout the body in the bloodstream. If you inherit one copy of the APOE-e4 gene you are at increased risk of developing Alzheimer’s and if you inherit two copies, your risk is even greater. Some estimates indicate that 40% of people with Alzheimer’s disease have the APOE-e4 gene. Researchers don’t know why having the APOE-e4 gene makes Alzheimer’s disease more likely but studies are in progress to find out. There may be other Alzheimer risk genes in addition to APOE-e4 that as of now remain unknown.

Reduced risk gene: APOE-e2

While APOE-e4 is a major risk factor in developing the disease, this variant of the apolipoprotein E gene seems to confer some protection from Alzheimer’s disease. It is currently unknown why APOE-e2 is protective against Alzheimer’s disease while APOE-e4 increases risk

Familial Alzheimer’s: APP and the presenilins

There are three known genes that, if inherited in a mutated version, virtually assure a person will develop Alzheimer’s disease. The genes are called amyloid precursor protein (APP), presenilin 1 (PS1) and presenilin 2 (PS2). When one of these genes is inherited, the resulting disease is called “familial Alzheimer’s disease,” as it tends to affect several family members in every generation. This type of Alzheimer’s disease also tends to have an early onset, with symptoms first appearing between the ages of 30 and 60. These genes, and thus familial Alzheimer’s disease, are responsible for most cases of early-onset Alzheimer’s disease. There are several hundred families worldwide affected by familial Alzheimer’s disease.

APP is normally found sticking out of the cell membrane of neurons and appears to help these cells grow and survive. However, when the APP protein is cut on the outer surface of a neuron, it generates a protein fragment known as the beta-amyloid protein. This protein tends to aggregate in large clumps called plaques that are one of the hallmarks of Alzheimer’s disease. Inheriting a mutated version of APP somehow increases the production of beta-amyloid protein to the extent that the beta-amyloid aggregates together and forms plaques early in life, leading to early-onset Alzheimer’s disease. The presenilin genes PS1 and PS2 are normally involved in cutting APP to form the beta-amyloid protein. When inherited in a mutated form, either of these genes can lead to high levels of beta-amyloid protein and plaques resulting in the early onset of Alzheimer’s disease.

There are genetic tests available to determine if you have any of these genes. Talk to your doctor if you are concerned about increased genetic risk for Alzheimer’s disease.

Head injury

Some studies indicate a link between serious head injury and increased risk of Alzheimer’s disease. In general, the greater the trauma experienced by the head, the greater the increase in Alzheimer’s risk. Researchers are still trying to uncover more specifics about the link between head injury and Alzheimer’s, including a mechanism for why injury might cause the disease. One idea is that an injury does not so much cause Alzheimer’s disease as accelerate its arrival.

Poor heart health

Many of the same factors that increase the risk of heart disease may also increase the risk of Alzheimer’s disease. High blood cholesterol levels, high blood pressure, and type 2 diabetes all can increase your risk of Alzheimer’s. These factors all increase the risk of vascular disease in the brain and decrease the output of the heart, both of which may cause sensitive brain tissue to die for lack of sustenance. Maintaining your physical fitness, eating healthy, and controlling your heart disease risk factors all can help lower your risk of Alzheimer’s disease.

Lack of mental engagement

Exercising your mind is another way to reduce Alzheimer’s disease risk. Several studies suggest that remaining mentally active throughout your life, especially in your later years, reduces the risk of Alzheimer’s disease. Doing crossword puzzles, playing trivia games, doing math in your head, all are fun and easy ways to help decrease your Alzheimer’s risk. Simply engaging in satisfying social relationships can help as well.

Low education

Some studies have found an association between higher levels of education and reduced risk of Alzheimer’s disease. The theory behind this finding is that the more you exercise and enrich your mind, the more active your neurons will be and the stronger will be the connections between them. It is unclear whether this means that people with higher education levels get the disease less often, or if it is just harder to detect the disease in people who have more nimble minds to begin with.

Toxicity

Some researchers believe that exposure to toxic chemicals may be to blame for many cases of Alzheimer’s disease. Several years ago, scientists announced that some Alzheimer’s disease patients had high levels of the metal aluminum in the brains, which led some to conclude that exposure to aluminum might cause the disease. However, further studies did not support this claim and now most researchers no longer believe aluminum is responsible for any cases of Alzheimer’s disease. While it is certainly possible that some other chemical or pesticide increases Alzheimer’s disease risk, none has yet been identified.