A modest reduction in seven modifiable risk factors for dementia, including smoking, obesity, sedentary lifestyles, and midlife high blood pressure, could have a huge impact, says Deborah Barnes, PhD, associate professor of psychiatry at the University of California, San Francisco.
Even a 25% reduction in the seven risk factors, which also include depression, diabetes, and low education, could prevent 3 million cases of Alzheimer's disease worldwide and nearly half a million in the U.S. alone, she tells WebMD.
The number of Alzheimer's cases around the world is expected to triple to 106 million by 2050, Barnes says. With no effective treatment to reverse the course of the relentless disease, prevention is key, she says.
The findings were presented here at the Alzheimer's Association International Conference and published online by the journal Lancet Neurology.
Alzheimer's Risk Factors
Using data from previously published studies, the researchers first identified the seven most common risk factors for Alzheimer's disease, which together account for up to half of the 35 million cases of Alzheimer's worldwide.
Assuming a causal relationship between each risk factor and dementia, the researchers calculated just how strongly each factor affects risk. Then they took into account the total number of people affected by each risk factor, both worldwide and in the U.S. alone.
All seven factors increase the risk of Alzheimer's to a similar degree, by about 80% to 90%, the study showed. But their prevalence varied widely.
Worldwide, low education -- specifically, not finishing secondary school -- had the biggest impact on Alzheimer's cases, accounting for 19% of cases.
"Education is probably a very weak proxy for lifelong cognitive activities, but we had to make do with the data we had," Barnes says.
While she is not suggesting people go back to school at age 70 (although some do), Barnes say that challenging the mind with crossword puzzles and other mental activities may help to prevent Alzheimer's.
Another 14% of cases worldwide were attributed to smoking, 13% to physical inactivity, 10% to depression, 5% to midlife hypertension, 2% to diabetes, and 2% to obesity.
In the U.S., however, lack of exercise was the No. 1 problem, contributing to 21% of preventable cases of Alzheimer's disease.
Sedentary lives contribute to three of the other risk factors -- diabetes, obesity, and hypertension, Barnes says.
Depression had the second biggest impact on Alzheimer's cases in the U.S., accounting for 15% of cases, followed by smoking, at 11%. Also, 8% of cases were attributable to midlife hypertension, 7% to midlife obesity, 7% to low education, and 3% to diabetes.
The major limitation of the study was the central assumption that the seven risk factors in the analysis cause Alzheimer's, which has not been proven. Also unknown is whether removal of a risk factor would result in fewer cases of dementia.
Asked if setting up programs to help people change their lifestyles would be prohibitively expensive, Barnes says, "You couldn't put a price on it. But it gives us a little bit of hope about things we could do now [to thwart] the epidemic that is coming our way."
Anti-smoking campaigns with "edgy ads that appeal to kids" have had a huge impact in keeping youngsters from lighting up, Barnes adds.
Ronald Peterson, MD, director of the Mayo Alzheimer's Disease Research Center at the Mayo Clinic in Rochester, Minn., says, "Assuming each risk factor is causal, and we don't know that, only that there is an association, the point [of the study] is well taken.
"If we ratchet down some of the risk factors, we will have an impact [on Alzheimer's disease]. I don't think we'll cut cases by 50%, but we don't have to be passive either," he tells WebMD.