updated 7/22/2011 9:35:48 AM ET 2011-07-22T13:35:48

Up to half of Alzheimer's disease cases worldwide are potentially attributable to seven preventable risk factors, a new study suggests.

The findings show that Alzheimer's cases could be reduced if people quit smoking, increased their physical activity, enhanced their mental activity, controlled their blood pressure and diabetes, and managed their obesity and depression.

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Globally, an estimated 33.9 million people have Alzheimer's, but that number could be reduced by 3 million if there was a  25 percent reduction in all seven of these risk factors, the researchers found . A 10 percent reduction in these seven factors could prevent 1.1 million cases, they said. The number of worldwide cases is expected to triple over the next 40 years.

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"Given the current absence of disease-modifying treatments, as well as increasing awareness that symptoms develop over many years or even decades, there has been growing interest in identification of effective strategies for prevention of [Alzheimer's disease], wrote Deborah Barnes and Kristine Yaffe, researchers at the University of California, San Francisco (UCSF).

The top 3 factors

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In this study, the authors reviewed previous research that examined factors that predispose people to developing Alzheimer's. They identified seven factors that were potentially within a person's control to change.

The researchers said that enhancing mental activity could make the biggest difference in developing Alzheimer's. That's because the study showed "low educational attainment" was the factor that impacts the largest portion of Alzheimer's cases worldwide. They defined low educational attainment as not completing higher education, having a low IQ or not participating in mentally stimulating leisure time activities, and found it contributes to 19 percent of Alzheimer's cases, or 6.5 million cases worldwide.

The factor contributing to the second-highest number of cases was smoking, which contributes to 14 percent of cases, or 4.7 million cases worldwide, the study showed.

Physical inactivity contributed to 13 percent of worldwide cases and was the third-largest factor. However, it was the highest contributor to cases is the U.S. — contributing to 21 percent, or 1.1 million cases. [Related: 6 Foods that Are Good for Your Brain ]

"What really mattered was how common the risk factors were in the population. In the U.S.A., about a third of the population is sedentary, so a large number of Alzheimer's cases are potentially attributable to physical inactivity," said Barnes, a professor of psychiatry at UCSF.

"Worldwide, low education was more important, because so many people throughout the world are illiterate or are not educated beyond elementary school," Barnes said. "Smoking also contributed to a large percentage of cases because it is unfortunately still really common."

The findings suggest that smoking cessation initiatives and public health initiatives to increase physical activity levels throughout life could have a dramatic decreasing effect on the number of Alzheimer's cases, the researchers said.

A note of caution

Other experts noted that while these seven factors may contribute to the development of Alzheimer's, none of the factors have been shown to cause the disease.

"Accumulated evidence from epidemiological research strongly supports a role for lifestyle and cardiovascular risk factors in the pathogenesis and development of dementia. However, none of these factors has been proven to have a causal relation specifically with [Alzheimer's disease]," wrote researchers Laura Fratiglioni, from the Karolinska Institute in Sweden, and Chengxuan Qiu, from the Stockholm Gerontology Research Center, in an editorial accompanying the new study.

However, the findings suggest "that preventive and therapeutic interventions have great potential," and that interventions should be carried out in high-risk populations, they said.

The results of the study were presented yesterday (July 20) at The Alzheimer's Association 2011 International Conference on Alzheimer's Disease (ICAD), held in Paris. The study was simultaneously published online in the journal The Lancet Neurology.

Pass it on: If you live in the U.S., becoming more physically active may be your best bet for preventing Alzheimer's disease.

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