Washington, Sep 1 : People who smoke are at a greater risk of developing Alzheimer's disease and other forms of dementia than people who have quit or have never smoked, a new study has found.

Researchers at the Erasmus Medical Center in Rotterdam, the Netherlands, have shown that smokers over the age of 55 were 50 percent more likely to develop dementia than their non-smoking counterparts.

Study author Monique Breteler, MD, PhD, and colleagues followed nearly 7,000 people age 55 and older for an average of seven years. Over that time, 706 of the people developed dementia.

Smoking could affect the risk of dementia through several mechanisms, according to Dr. Breteler.

"Smoking increases the risk of cerebrovascular disease, which is also tied to dementia. Another mechanism could be through oxidative stress, which can damage cells in the blood vessels and lead to hardening of the arteries. Smokers experience greater oxidative stress than nonsmokers, and increased oxidative stress is also seen in Alzheimer's disease," Dr. Breteler said.

Oxidative stress occurs when the body has too many free radicals, which are waste products produced by chemical reactions in the body.

"Antioxidants in the diet can eliminate free radicals, and studies have shown that smokers have fewer antioxidants in their diets than nonsmokers," Dr. Breteler said.

The researchers also looked into how smoking affects the risk of developing Alzheimer's disease for people who have the gene that increases the risk of Alzheimer's, called apolipoprotein E4, or APOEa4.

They found that smoking did not affect the Alzheimer's risk for people who had that gene. But people who did not have the gene had a 70 percent higher risk of Alzheimer's if they smoked.

Smoking could cause small strokes, which in turn damage the brain and cause dementia, Dr. Breteler said.

The study is published in the September 4, 2007, issue of Neurology(r), the medical journal of the American Academy of Neurology. (With inputs from ANI)