TUESDAY, July 19 (HealthDay News) -- More than half of Alzheimer's cases globally could be prevented if modifiable risk factors such as depression, obesity and smoking were eliminated, either with lifestyle changes or treatment of underlying conditions, new research suggests.

Even reducing the level of risk factors by a modest amount could prevent millions of cases of the memory-robbing illness, the researchers said. For example, a 25 percent reduction in seven common risk factors -- including low education, obesity and smoking -- could prevent up to 3 million Alzheimer's cases around the world and up to half a million in the United States alone, the study found.

The new research is being presented Tuesday at the Alzheimer's Association International Conference (AAIC) in Paris and will also be published online July 19 in The Lancet Neurology.

A second study, also being presented at AAIC, identified several "resiliency" characteristics that might help keep the aging mind healthy. Strengthening these factors could also help prevent Alzheimer's, researchers said.

"The idea here is to get a better bead on exactly how we can start untangling what the risk factors are, so that we cannot only treat and modify Alzheimer's but also start talking about prevention of Alzheimer's," said Mark Mapstone, associate professor of neurology at the University of Rochester Medical Center. "The field is working very hard [to figure out] what these risk factors are so we can start heading this disease off before it starts."

Mapstone was not involved with either study.

The first group of researchers, led by Deborah Barnes of the University of California San Francisco (UCSF), revisited prior epidemiological studies on links between Alzheimer's and seven key risk factors: low education levels, smoking, low physical activity, depression, hypertension during mid-life, obesity and diabetes.

They estimated that, together, these risk factors account for 17 million cases of Alzheimer's worldwide (about half of the estimated 34 million cases of dementia globally) and 3 million of the 5.3 million estimated U.S. cases.

Some factors appeared to have a greater impact on Alzheimer's risk than others. The UCSF team estimated that worldwide, 19 percent of Alzheimer's cases were attributable to low education, 14 percent to smoking, 13 percent to physical inactivity, 10 percent to depression, 5 percent to mid-life hypertension, 2.4 percent to diabetes and 2 percent to obesity.

In the United States, 21 percent of cases could be traced to low physical activity, 15 percent to depression, 11 percent to smoking, 8 percent to mid-life hypertension, 7 percent to mid-life obesity, 7 percent to low education and 3 percent to diabetes.

Healthy circulation in the brain is thought to be key to keeping the mind sharp, and numerous studies have tied common heart risk factors, such as obesity or hypertension, to an increased risk for dementia. But the researchers stressed that the risk factors included in this analysis have not been shown to actually cause Alzheimer's, only to be associated with it.

One expert agreed with that. "Links have been seen where the presence of a particular risk factor is associated with a higher likelihood of having Alzheimer's and these associations are not necessarily causal," said Dr. Marc L. Gordon, chief of neurology at Zucker Hillside Hospital and an Alzheimer's researcher with the Feinstein Institute for Medical Research in Manhasset, N.Y., who was not involved with the research.

In a second report presented at the meeting, researchers found that older individuals who had less stress, anxiety, depression and trauma -- even in the face of tragic events or circumstances such as a life-threatening illness -- were less likely to develop Alzheimer's.

A team led by Dr. Susanne Steinberg of the University of Pennsylvania tracked levels of what they called "resilient cognition" in 136 American adults aged 65 or older.

After three years, the ability of these individuals to maintain relatively low levels of stress, anxiety, depression and trauma, even when faced with stressful situations, was closely tied to maintaining healthy thinking ability, the researchers reported.

Steinberg and colleagues believe that key to this resilience is an ability to cope, to ask for help, and to maintain a positive attitude and take action even in the face of adversity. Conversely, people who had experienced physical neglect as children, or who scored poorly on tests meant to gauge suicidal thoughts, tended to have poorer cognitive performance, the researchers said.

More information

There's more on this condition at the Alzheimer's Association.