Liz Szabo

USA TODAY

A long-running study has found that dementia rates fell steadily over the past four decades, most likely due to declining rates of heart disease.

Although the Framingham Heart Study involved just 5,200 people, its findings likely reflect a national trend, said co-author Sudha Seshadri, a professor of neurology at the Boston University School of Medicine and a senior investigator with the study. Other research also suggests that dementia rates are declining in the U.S. and other developed countries.

Rates of dementia in the new study decreased from 3.6% during the late 1970s and early 1980s to 2% during the late 2000s and early 2010s, a drop of 44%, according to the study published online Wednesday in The New England Journal of Medicine.

"It's very good news," said Dallas Anderson, an epidemiologist with the National Institute on Aging, part of the National Institutes of Health, which funded the study. "We're seeing one generation after another where the risk is going down."

Researchers found that the sharpest decline in dementia was in a type called vascular dementia, which is caused by damage to the blood vessels that carry oxygen to the brain.

Although rates of Alzheimer's disease also fell, this decline could have been due to chance, the paper said. Alzheimer's is the most common form of dementia and accounts for 60% to 80% of dementia cases. Dementia is a general term for memory loss and other intellectual abilities serious enough to interfere with daily life.

When people in the study did develop dementia, it occurred later in life, so that people were able to spend fewer years disabled, Seshadri said. The average age at which people were diagnosed with dementia increased from 80 during the early years of the study to 85 in the most recent period.

As the study progressed, fewer people in the study suffered from heart disease as well. Heart disease and strokes have long been linked to an increased risk of dementia.

The study suggests that care for people who suffered strokes also improved over the years. In the early years of the study, people who survived a stroke were nine times as likely as others to develop dementia. In the most recent period, people who had strokes were less than twice as likely.

"We’ve been preaching for years that what’s good for the heart is good for the brain," said Ronald Petersen, who directs the Mayo Clinic Alzheimer's Disease Research Center and the Mayo Clinic Study of Aging. "Maybe our efforts to watch our diet and exercise are having a spillover effect, leading to less dementia."

Dementia rates didn't fall for all participants in the study.

Dementia and heart disease rates only declined among people who had at least a high school diploma, according to the study. Research has long linked higher incomes and educational levels with better health, Seshadri said.

Education influences a person's health in countless ways, Seshadri said. People who are better educated are more likely to find good jobs with health insurance and to learn about ways to stay healthy, such as by exercising and avoiding tobacco. Better educated people may stay more mentally active in retirement.

But Seshadri notes that a person's educational level also reflects that person's childhood experiences. A person raised with poverty, crime and abuse could be less likely to graduate high school, as well as less likely to grow into a healthy adult.

The improving health seen in the new study could be fragile, said Seshadri, noting that obesity and diabetes increased over time. Both conditions harm the heart and could lead to more dementia in the future.

Even if dementia cases grow at a slower rate, the total number of people with the condition are certain to increase in incoming decades as the elderly population expands, Seshadri said.

About 5.3 million Americans have Alzheimer's today, and one in three elderly people die with dementia, according to the Alzheimer's Association. As the nation ages, the number of people over age 65 with Alzheimer's disease will increase 40% by 2025, to 7.1 million.

Seshadri said she hopes her study motivates people to keep their hearts and brains healthy for as long as possible.

"People may say, 'I don't mind getting a heart attack. It's a good way to go,'" Seshadri said. "But heart disease may not just damage your heart. It could cause dementia. And I don’t know of anybody who thinks dementia is a good way to go."