A total of 100 people with an average age of 92 years old took part in the study published in the journal Neurology. Researchers followed them for 12 to 14 years, carrying out between two to four brain scans and two to 13 neuropsychological exams on each person. Volunteers were excluded from the study if they had signs of dementia at the start.

The brain scans detected what are known as amyloid plaques that are linked with Alzheimer’s disease. The researchers also collected information on variables which might affect the participants’ brain health, including lifestyle choices, their job, health, sleeping habits, hobbies, and whether they experienced depression.

By the time most people reach their 90s, amyloid plaques have built up in their brains, and not having them is the exception. The team set out to understand how some people who these plaque are able to maintain their cognition in their ninth decade. In the study, 66 percent of the participants had them, even if they did not have Alzheimer’s symptoms.

One finding the team made was that those with a form the APOE gene called ε2 were six times more likely to avoid amyloid plaque build-up in their 90s. Less amyloid was also linked to lower pulse pressure.

“The APOEε2 finding was particularly interesting,” co-author Beth E. Snitz, associate professor of neurology at the University of Pittsburgh in Pennsylvania told Newsweek. That’s because another form of the APOE gene, ε4, is thought to increase the risk for amyloid plaque build-up and for Alzheimer’s disease.

“This finding nicely shows that sometimes what best predicts a healthy outcome in aging is not simply the absence of a risk factor, but rather a specific ‘protective’ factor,” she said. “This could in turn lead to new treatment ideas, including ‘drug-able targets’ for development and testing.”

The team also found that participants who scored the best on the cognitive tests at the start of the study were more likely to maintain these faculties by the end.

A good test score was also linked to maintaining thinking skills despite having beta-amyloid plaques, as was never having smoked.

Good cardiovascular health also seemed to be protective against amyloid build-up, as well as life satisfaction, and having a paid job at the start of the study.

The authors acknowledged the results may have been skewed by including the sorts of people who would commit to a long-term study. In addition, the majority of the participants were white, men, highly educated, and in good health at the beginning of the research. So this may mean the results may not relate to other populations.

Another draw-back was that some of the data was collected a long time ago and collected for different purposes other than answering the study’s questions. “So we had to make do with what information happened to be available,” said Snitz.

She said: “Both genetic and lifestyle factors are important to understanding how some people live well into their 90s with intact thinking and memory abilities. Understanding this kind of resilience may help identify ways to prevent dementia.

“Also, the study reinforces some things we knew already—like the importance of good cardiovascular health in preventing Alzheimer’s disease, as well as the idea that a build-up of ‘cognitive reserve’, i.e., developing strong reasoning and other cognitive skills earlier in life, likely can help buffer against the effects of brain disease or injury later in life.”

Keeping intellectually and socially engaged as we age “are good bets as protective factors,” said Snitz.

In an editorial accompanying the study also published in Neurology, Dr. Claudia H. Kawas and María M. Corrada of the Institute for Memory Impairments and Neurological Disorders University of California, Irvine, who did not work on the paper said: “The results of this exploratory study are potentially important in terms of our scientific understanding about maintenance of cognitive health with aging, and they add to the current literature of resistance and resilience, particularly for the oldest-old.”

“Further research is necessary to understand the biological underpinnings and factors that contribute to this resilience,” they wrote.