Additional Risk Factors

Does he or she smoke?

• At lower risk: Nonsmokers, former smokers
• At higher risk: Current smokers

Why smoking matters: Evidence is growing that smoking raises the risk of developing Alzheimer's disease by as much as 50 percent. In late 2007, Dutch researchers who tracked almost 7,000 people age 55 or older for 7 years reported that current smokers were more likely to develop dementia than people who had never smoked or had quit. Interestingly, the effect was more pronounced for people who did not have the APOE-e4 gene (the type known to be most vulnerable to the disease). Smoking didn't raise the already elevated risk of Alzheimer's in those who had the APOE-e4 gene.

It's thought that smoking damages the cardiovascular system and causes oxidative stress, both conditions that are associated with Alzheimer's.

You may have heard that smoking protects against dementia -- which is a myth. This persistent idea grew out of flawed studies and because fewer people with Alzheimer's were smokers. But in fact smokers, tend to have shorter life spans and are less likely to live to the advanced ages at which Alzheimer's most often strikes.

5. Does he or she have diabetes?

• At lower risk: No history of diabetes
• At higher risk: Those with type 2 diabetes

Why diabetes matters: People with type 2 diabetes have at least double the risk of developing Alzheimer's compared with people without diabetes. The risk was 65 percent higher for diabetics in 2006 data from the ongoing Religious Orders Study of priests and nuns. Some studies have found that the higher the blood sugar levels, the higher the dementia risk.

The mechanics behind the Alzheimer's-diabetes link aren't certain, but this is an area of intensive research. The leading explanations:

• High blood sugar may cause vascular problems. It's known that diabetes can cause cardiovascular problems and strokes; reduced blood flow to the brain may cause small-vessel damage there that leads to vascular dementia (a type of dementia that can appear with Alzheimer's).

• Insulin resistance leads to inflammation that can damage the brain. Insulin resistance occurs in diabetics when their cells can't use insulin properly to move glucose from the blood to be used for cell energy. The pancreas then makes extra insulin to compensate, which builds up in the blood and creates inflammation, which damages brain cells.

• Insulin resistance in the brain disrupts the proper formation of neuron connections. The autopsied brains of people with Alzheimer's always show amyloid plaques, clumps of the protein beta amyloid, which ruin brain cell connections. It's been discovered that this substance destroys the brain cells' receptors for insulin, which is used by the brain to make new memories. This results in dementia and memory loss.

As evidence of a clear biochemical link between the two conditions grows more compelling, some are calling Alzheimer's disease "type 3 diabetes."

There's also a likely genetic link between diabetes and Alzheimer's. People with diabetes who also have the APOE-e4 gene (the type that places a carrier at highest risk for Alzheimer's) were two-and-a-half times more likely to develop the disease, according to a 2008 National Institute on Aging study.

6. Does he or she have a history of being overweight?

• At lower risk: Low BMI in midlife
• At higher risk: Overweight or obese in midlife

Why weight matters: Being overweight or obese as measured by body mass index is well associated with an increased risk of developing dementia. Men with the leanest body mass index (BMI) in their late 40s and mid-50s were the least likely to develop Alzheimer's in a 20-year study of more than 7,000 Swedes; those who were heaviest were most likely. This same study also later found that women who were overweight at 70 were more likely to develop Alzheimer's in the next 10 to 18 years.

People who are overweight are more likely to have related health problems that are also linked to Alzheimer's, such as hypertension, high blood pressure, cardiovascular disease, and diabetes.

A related risk factor is if someone was previously overweight but has had unexplained weight loss recently. Weight loss associated with dementia may begin six to ten years before other symptoms become obvious. Research isn't clear whether this is because of declining ability to prepare meals or is a function of the disease process.