The state of the evidence reflects in part the long time it took before researchers even realized that Alzheimer’s was a disease, said Dr. Richard J. Hodes, director of the National Institute on Aging. Until the mid-1980s, many thought dementia was a normal part of aging, and so serious studies of its causes and prevention did not really begin until then. Scientists have spent the years since searching for factors that might affect risk, checking data from other studies to see if, for example, diet or blood pressure or years of education might be associated with the disease.

In the meantime, doctors are in a bind. Should they tell people to do things like walk briskly or eat vegetables  activities that might someday be shown to protect against Alzheimer’s and that certainly cannot hurt? Or should they wait for absolute proof, confirmation that a diet or a drug or an exercise regimen prevents Alzheimer’s?

The Alzheimer’s Association tells people to exercise, challenge themselves mentally, remain socially engaged and keep their hearts healthy. Such measures can only help, says Dr. Maria C. Carrillo, a senior director of the organization.

But, she said, “The Alzheimer’s Association certainly agrees that there is not enough evidence to say anything definitive about the prevention of Alzheimer’s disease and any kind of intervention.”

Of course, Dr. Hodes said, there are many reasons to follow practices to improve general health. But, he said, researchers have to be careful about implying that any measures will protect against this degenerative brain disease.

“We don’t know that yet,” Dr. Hodes said.

Rating the Quality

Dr. John W. Williams Jr., head of the Duke group that evaluated the studies, thought the task would not be too arduous. He expected relatively few studies and clear results.

To its great surprise, the Duke group discovered a vast amount of literature on Alzheimer’s prevention. Instead of coming up empty on many topics, Dr. Williams said, “We came up empty on very few.”