Up to half of Alzheimer's disease cases could potentially be prevented if people pursue education, quit smoking, exercise more and make other lifestyle changes, a new review suggests.

Researchers in the U.S. used a mathematical model to estimate the effect of seven risk factors for Alzheimer's disease: smoking, depression, low education, diabetes, too little exercise, and obesity and high blood pressure in mid-life.

Psychiatry Prof. Deborah Barnes and Dr. Kristine Yaffe of the University of California, San Francisco, estimated the population attributable risk — the proportion of people with Alzheimer's cases that can be attributed to a risk factor, assuming there is a cause-and-effect relationship.

Holocaust survivor Betty Stein, 92, plays ping pong at a program for people with Alzheimer's and dementia in Los Angeles. The program aims to raise heart rates and blood flow to the brain, and exercise participants mentally as well as physically since physical inactivity may contribute to cases of the disease. Lucy Nicholson/Reuters

Low education potentially contributed to the largest proportion of cases of Alzheimer's worldwide, the reviewers concluded in Tuesday's online issue of the journal Lancet Neurology.

It could be that education and mental stimulation throughout life lowers the risk of Alzheimer's by helping people to build up a reserve that allows them to keep functioning at a normal level despite changes in the brain, the researchers said.

"Education, even at a young age, starts to build your neural networks," so being deprived of it means less brain development, Barnes said.

The second largest number of cases could be attributed to smoking, perhaps through vascular disease.

Physical inactivity potentially contributed to the largest proportion of cases in the U.S. and the third largest worldwide, the model suggested.

Research limitations

The researchers pointed to several limitations of the research, such as the assumption of cause-and-effect relationships, none of which have been proven.

Also, many factors contribute to Alzheimer's, and it isn't known whether removing one will actually lower incidence of the disease.

The estimates might not apply to individual countries or communities, the researchers cautioned.

Barnes and Yaffe did not consider other potential risk factors such as diet, in part because so few studies have looked at it.

"The estimates reported in the review could be regarded as only theoretical because all the projected effects are based on observational studies," Laura Fratiglioni of the Aging Research Center at the Karolinska Institute in Stockholm said in a journal commentary.

Several intervention studies underway in Europe could help inform people about postponing the onset of Alzheimer's disease late in life, she said.

The review was funded by the Alzheimer's Association, the National Alliance for Research on Schizophrenia and Depression, the University of California, San Francisco School of Medicine and the U.S. National Institute on Aging.

Results of the study were also presented Tuesday at the Alzheimer's Association International Conference on Alzheimer's Disease in Paris, which runs through Thursday.

With files from The Associated Press