Jul 19, 2011

Lowering Risk Factors Might Prevent Millions of Cases of Alzheimer's Disease

"In our study, what mattered most was how common the risk factors were in the population," said Barnes. "For example, in the U.S., about one third of the population is sedentary, so a large number of Alzheimer's cases are potentially attributable to physical inactivity."

Alzheimer's Reading Room

Deborah Barnes
This study is the first known analysis that tries to quantify and compare how risk factors are associated with Alzheimer's. According to the authors, seven conditions or behaviors account for up to half of the 35 million cases of Alzheimer’s around the world.

The authors identified the following factors that increase the risk for Alzheimer's: diabetes, high blood pressure, obesity, smoking, sedentary behavior, depression and low educational level. These conditions are modifiable.

Reducing these seven risk factors by 25 percent could mean 3 million fewer cases of Alzheimer’s worldwide, including 500,000 in the U.S.

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Model Shows Lowering Risk Factors May Prevent Millions of Alzheimer's Cases

Deborah Barnes, PhD, MPH, Associate Professor of Psychiatry at the University of California, San Francisco and Mental Health Research PI at the San Francisco Veterans Affairs Medical Center, and colleagues used mathematical modeling to calculate "population attributable risks" (PARs) for potentially modifiable Alzheimer's risk factors to project the potential impact of risk factor reduction on Alzheimer's prevalence in the U.S. and worldwide.

PARs are used to estimate the percentage of cases of a given disease that are potentially attributable to, or caused by, various risk factors. PARs take into consideration both the strength of the association between the risk factors and the disease as well as how common the risk factors are.

They found that roughly half of Alzheimer's cases may potentially be attributable to modifiable risk factors. Together, seven modifiable risk factors contributed to as many as 17 million Alzheimer's cases worldwide and nearly 3 million cases in the U.S.

  • Alzheimer's Disease International, in their World Alzheimer Report 2010, determined that there are 35.6 million cases of dementia worldwide.
  • 5.4 million Americans are living with Alzheimer's disease, according to the Alzheimer's Association 2011 Alzheimer's Disease Facts and Figures.

The scientists calculated PARs for diabetes, mid-life hypertension, mid-life obesity, smoking, depression, low educational attainment and physical inactivity. (Dietary factors were not considered due to heterogeneity of definitions and lack of data on prevalence.) The researchers then estimated the total number of Alzheimer's cases currently attributable to each risk factor individually and all seven risk factors combined in the U.S. and worldwide.

Finally, they calculated the number of Alzheimer's cases that could potentially be prevented by 10 percent and 25 percent reductions in prevalence of the risk factors.

At AAIC 2011, the researchers reported the proportion of Alzheimer's cases worldwide that are potentially attributable to each of the seven risk factors:

- low education 19 percent
- smoking 14 percent
- physical inactivity 13 percent
- depression 11 percent
- mid-life hypertension 5 percent
- mid-life obesity 2 percent
- diabetes 2 percent

And specifically in the U.S.:
- physical inactivity 21 percent
- depression 15 percent
- smoking 11 percent
- mid-life hypertension 8 percent
- mid-life obesity 7 percent
- low education 7 percent
- diabetes 3 percent

Together, the seven potentially modifiable risk factors contributed to roughly 50 percent of Alzheimer's cases worldwide (51 percent, 17.2 million) and in the U.S. (54 percent, 2.9 million).

"We were surprised that lifestyle factors such as physical inactivity and smoking appear to contribute to a larger number of Alzheimer's cases than cardiovascular diseases in our model," said Barnes. "But this suggests that relatively simple lifestyle changes such as increasing physical activity and quitting smoking could have a dramatic impact on the number of Alzheimer's cases over time."

According to Barnes' calculations, a 10 percent reduction in all seven risk factors could potentially prevent 1.1 million Alzheimer's cases worldwide and 184,000 cases in the U.S. over time. A 25 percent reduction in all seven risk factors could potentially prevent more than 3 million Alzheimer's cases worldwide and 492,000 cases in the U.S. over time.

"In our study, what mattered most was how common the risk factors were in the population," said Barnes. "For example, in the U.S., about one third of the population is sedentary, so a large number of Alzheimer's cases are potentially attributable to physical inactivity. Worldwide, low education was more important because literacy rates are lower or people are not educated beyond elementary school. Smoking also contributed to a large percentage of cases because it is unfortunately still very common."

Barnes says the estimates make an important assumption – that there is a causal relationship between the risk factors examined and Alzheimer's disease. "The next step is to perform large-scale intervention studies to really find out whether changing these risk factors will lower the risk of developing Alzheimer's over time," Barnes said.

The study results will be published online in The Lancet Neurology.



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Original content Bob DeMarco, the Alzheimer's Reading Room