MONDAY, May 6 (HealthDay News) -- Families with exceptional longevity also appear to have later onset of dementia, a new study suggests.

Ultimately, the same percentage of people in families surviving to 90 and beyond are prey to Alzheimer's disease as others, but the progressive brain disorder tends to develop later in life, the researchers say.

"The goal of the study was to determine whether or not members of exceptionally long-lived families are protected against cognitive [mental] impairment consistent with Alzheimer's disease," said lead researcher Stephanie Cosentino, an assistant professor of neuropsychology at Columbia University Medical Center in New York City.

The answer: Yes, except for the oldest of the old.

Although the reason for the delayed onset of dementia in the very old isn't clear, Cosentino said "there may be specific genetic pathways related to preserved cognition in these families." Alzheimer's disease is the most common cause of dementia among the elderly.

Cosentino's team followed more than 1,800 participants (1,510 family members and 360 spouses as "controls") in the U.S.-Danish Long Life Family Study, which is evaluating genetic and non-genetic factors associated with extreme longevity.

For the current report, published online May 6 in JAMA Neurology, they looked at the onset of Alzheimer's disease among blood relatives within long-living families and compared that with similar data on their spouses.

Older family members, average age 88, had similar rates of mental decline as their spouses, Cosentino found. However, sons and daughters, average age 70, of exceptionally long-lived people had less than half the risk of Alzheimer's disease than their similarly aged spouses, she said.

"Overall, a higher proportion of family members than their spouses were dementia-free until age 90," Cosentino said. "After 95 years of age, however, exceptionally long-lived individuals had a high prevalence of dementia, pointing to a delayed onset of mental impairment in families with exceptional longevity."

An important component of achieving extreme old age is the delayed onset of dementia, Cosentino said. About 5.4 million Americans are estimated to have Alzheimer's disease, most of them older than 65 years.

Another expert said other studies have shown similar protection against heart disease and cancer in long-lived families.

"This finding suggests that factors that protect the cardiovascular system and perhaps factors that ward off cancer could be at work in the brain to prevent Alzheimer's," said Dr. Sam Gandy, associate director of the Mount Sinai Alzheimer's Disease Research Center in New York City.

This new study dovetails perfectly with papers showing the essential role of inflammation in Alzheimer's dementia, Gandy said. "And some of the factors that cause Alzheimer's . . . share enormous similarity with blood vessel inflammatory diseases that probably contribute to cardiovascular disease and cancer," he said.

Overall, the study found that 38.5 percent of participants developed Alzheimer's disease. The risk was slightly reduced in men and women from long-lived families compared with their husbands or wives.

Moreover, the risk was further reduced among their sons and daughters, but did not carry over to nieces and nephews, the researchers found.

More information

For more information on Alzheimer's disease, visit the Alzheimer's Association.