Education has long been thought to protect against the ravages of brain diseases like Alzheimer’s. Numerous studies seemed to suggest that the more educated were less likely to develop dementia.

But a large new study finds little difference between people with a high school diploma and those with a Ph.D. when it comes to staving off the damage to brain cells caused by dementing diseases or the rate at which mental decline progresses, once it starts.

“It’s been a longstanding idea that education might be one of those things that allows a person to tolerate these kinds of brain pathologies,” said the study’s lead author, Robert S. Wilson, a professor of neurological and behavioral sciences at the Rush University Medical Center in Chicago. “We found that the more pathology you find in the brain, the faster the cognitive decline was.”

That doesn’t mean that education has no impact, Wilson said. Education’s effects on the brain are like weightlifting’s impact on the muscles. So the more education a person has, the thicker certain areas of the brain become and the more connections between cells — synapses — there are.

“Those with higher education begin older age with a higher level of cognitive function," Wilson told NBC News, "so if they decline cognitively at the same rate as everyone else, they are more likely to die from something else as compared to someone who started at a lower level of education.”

And that explains why rates of dementia seemed to be lower in highly educated people than in others, Wilson said. But if a person lives long enough for cognition to begin to deteriorate, higher levels of education will not slow the rate at which people’s thinking skills and memory decline.

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For the new study, published Wednesday in the journal Neurology, Wilson and his colleagues analyzed data from the Religious Orders Study, which involves older Roman Catholic clergy members from across the U.S., and the Rush Memory and Aging Project, which involves older people from Chicago and its suburbs. Volunteers from both studies are evaluated annually for any changes in cognition. They also agree to donate their brains for autopsy when they die.

The 2,889 volunteers had an average of 16.5 years of education, an average age of 77.8 and no signs of dementia when they entered the two studies. By the end of Wilson’s study, with an average of eight years of follow-up, 1,044 volunteers had died and 696 had been diagnosed with dementia. Brain autopsies had been completed on 752 of the volunteers by the time Wilson and his colleagues started their analysis.

The researchers divided the volunteers into three groups based on their level of education: 12 or fewer years, 13 to 16 years and 17 or more years. Even though it had been many years since volunteers had finished their schooling, it was clear that more education translated into higher scores on tests of thinking and memory at the beginning of the study. Nevertheless, once cognition started to fail, education had no bearing on the rate of decline, the study found.

Wilson suspects that while past education had no impact on the rate of decline, learning new things in older age would make a difference. “It’s been previously shown that people who are more engaged in more cognitively stimulating activities in old age seem to tolerate the pathology [from brain diseases] better than others,” he said. “They decline less quickly than those who have not been as cognitively active.”

Determining what can slow mental decline is the big question on everyone’s mind, said Sandra Weintraub, a professor of psychiatry, neurology and psychology at the Mesulam Center for Cognitive Neurology and Alzheimer’s Disease at Northwestern University.

“People are grasping for any kind of direction on how to preserve brain health,” said Weintraub, who is involved with Northwestern’s “SuperAger” study, which follows people over 80 who have exceptional memories and cognitive abilities. “Nobody wants to get Alzheimer’s or lose cognition.”

The SuperAger study has found that some people — even those who turn out to have brains riddled with toxic proteins — seem to remain mentally intact, but it’s not yet clear why that is.

One thing that has been shown to keep people mentally sharp, Weintraub said, is physical activity. In fact, a study published last week found that people between 20 and 67 who started a program of aerobic exercise saw significant increases in mental function and an increased thickness in a brain region called the frontal cortex.

“Whatever you do that’s good for your heart is good for your brain,” Weintraub said. Another example of that bit of wisdom is another new study, which found that when people achieve dramatic improvements in blood pressure, it appears to help stave off dementia.

Exercise for the brain also seems to pay off, said Prashanthi Vemuri, an associate professor of radiology at the Mayo Clinic in Rochester, Minnesota. “There are lots of studies showing that bilingualism really delays dementia,” Vemuri said. And that’s true even if the second language is learned later in life, she added.

Wilson's study did show that having a higher level of education seemed to have a small, but significant, effect on the number of mini-strokes a person experienced, Vemuri said. And even though the effect is small, it may be important, since “people who have greater vascular disease are more likely to develop dementia,” she added.

Ultimately, Vemuri said, while more education may not slow cognitive decline once it starts, it does appear to delay the start.

And for many people, that may be long enough.