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We don’t know what actually causes Alzheimer’s, but there are many factors that result in a person having a greater or lesser risk of developing the disease. Risk factors for Alzheimer’s include age, family history, genetics, gender, general health and various lifestyle factors.

Age is the most important risk factor for Alzheimer’s. Older people are far more likely to develop the disease than young people. One in 100 people aged 60-64 years who don’t have a close family relative with Alzheimer’s can expect to develop the disease. This increases to a 1 in 25 chance for people aged 70-74 years, 1 in 5 for those aged 80-84 years, and 1 in 2 or 3 for people aged 90-94 years. It is estimated that a quarter of people over 85 years of age have Alzheimer’s.

Family history is the second main risk factor in developing Alzheimer’s. If a person has a parent or sibling with Alzheimer’s, the risk is 2-3 times higher. A third of those with Alzheimer’s have a parent, brother or sister with the disease. The risk increases if two generations have Alzheimer’s, such as a parent and sibling, or parent and grandparent on the same side of the family, and if they both had Alzheimer’s before the age of 65 years.

Genetics are another factor. Two genes connected with familial Alzheimer’s have been identified so far: a “risk gene” called Apolipoprotein E and a “deterministic gene”. There may be up to a dozen more risk genes associated with the disease. Everyone has two Apolipoprotein E genes. There are three types of these genes, known as types 2, 3 and 4. A person can have two of the same type, for example, 3, 3, or two different types, for example, 2, 4. Those with a type 4 are at greater risk of Alzheimer’s at a younger age, especially people with two type 4s. Type 2 carries the least risk. Researchers are trying to work out why all this is the case.

The deterministic gene is associated with early onset Alzheimer’s, a much rarer form of the disease. A person has a 1 in 2 chance of getting this from age 30-60 years if a parent has it. Early onset is entirely familial, unlike the more common late onset Alzheimer’s, which can be familial or non-familial.

People with Down syndrome are at higher risk of developing Alzheimer’s. This can occur from around the age of 40 years. Most of them will eventually develop the disease if they live long enough. These people are at greater risk due to their additional chromosome. This gives them an extra copy of the gene for the protein that causes amyloid build-up, a feature of Alzheimer’s.

Gender is a risk factor in developing Alzheimer’s. Women have a slightly higher risk, even allowing for the greater proportion of women among the older population. Women taking hormone replacement therapy are less likely to get Alzheimer’s, according to some studies, although one study found an increased risk in women with low oestrogen levels.

Severe head injuries are thought to increase the risk of Alzheimer’s in later life. A study found that people who are unconscious for fifteen or more minutes from a head injury are more prone to develop the disease than those with no head injury. Other studies have been inconclusive. Curiously, head size can make a difference to whether a person might get the disease. People with small heads are at more risk.

Heart disease, stroke, diabetes, high blood pressure and high cholesterol may increase the likelihood of getting Alzheimer’s. The connection between brain and heart is an important one. With every heart beat, nearly a quarter of the blood pumped from the heart goes to the head. Thus people with a poor vascular, or blood vessel, system are at greater risk of developing the disease. This can come about through smoking, diabetes, hypertension or diet.

Diabetes may be linked directly to Alzheimer’s. A symptom of both is amyloid deposits appearing in the pancreas and the brain respectively. Research into a possible relationship is ongoing.

High blood pressure is often associated with Alzheimer’s, as it can damage the brain’s blood vessels and reduce oxygen. This could upset nerve cell circuits, which may be important to cognitive skills.

Research suggests high cholesterol levels mean a higher risk of Alzheimer’s. Logically, this means that people who take cholesterol reducing drugs will be less likely to get the disease. Some studies into the use of statins, the most common drug taken to lower cholesterol, have found a reduced risk while others have been inconclusive. Other research has shown that the animo acid homocysteine may increase the risk of the disease.

Food high in trans and saturated fats raises the chances of Alzheimer’s. High fat consumption can increase the risk of diabetes and hypertension, which in turn may increase the risk of developing the disease. A study of about 3,700 older people in Chicago found that those who ate a high fat diet were more likely to suffer cognitive decline associated with Alzheimer’s. Good nutrition, on the other hand, is thought to reduce the likelihood of the disease.

Studies suggest anti-inflammatory drugs such as ibuprofen, indomethacin and naproxen may lower the risk of Alzheimer’s, although clinical tests have not shown a link so far. Any reduced risk may be offset by side effects including stomach irritation and gastrointestinal bleeding.

A link between aluminum accumulations in the brain and Alzheimer’s was first reported in 1965. Studies have found that people exposed to aluminum in antiperspirants, antacids and drinking water have a greater risk of developing the disease.

A postmortem study in New York found that depression is a risk factor in Alzheimer’s. The study compared the brains of people who had Alzheimer’s and a known history of depression with those who weren’t diagnosed with depression. People with depression had more brain plaques and tangles, which are associated with Alzheimer’s.

Education level is sometimes considered a risk factor for Alzheimer’s. Studies have found that people with a higher level of education are less likely to get the disease. This may be due to the fact that more education means greater use of the brain, and those who have undertaken more courses of study are more likely to use their brain in other areas of their lives, including as they get older. Research has shown that a person with an active brain is less at risk of Alzheimer’s.

Scientists believe that a combination of risk factors rather than any single factor is what may ultimately increase or decrease a person’s likelihood of developing Alzheimer’s. Thus if a person is high on the risk scale for a number of factors, such as family history, heart disease and high fat diet, then all of these factors rather than just one of them may contribute to the person eventually having Alzheimer’s.