The dramatic increase in obesity and diabetes in the United States over the past few decades, coupled with lengthening life expectancies, has led health officials to predict that rates of dementia will also skyrocket.

Some experts estimate that the number of Americans with Alzheimer’s disease and other forms of dementia will reach 7.1 million by 2025 — a 40 percent increase over the 5.1 million older Americans affected by such diseases today. 

But a new study, published Wednesday in the New England Journal of Medicine (NEJM), suggests that the rate of new cases of dementia in the U.S. may actually be decreasing.  Using data from the long-running Framingham Heart Study, researchers have found that the incidence of dementia has declined significantly over the past 30 years — although only in people with a high school diploma or more.

The study also found that people with at least a high school education were more likely to have more indicators of good heart health. That finding suggests that improvements in physical health through lifestyle changes and better health care can help prevent or delay cognitive decline.

“Our study offers cautious hope that some cases of dementia might be preventable or at least delayed,” write the authors of the study.

Study details

Participants in the Framingham Heart Study undergo numerous medical tests and examinations every two years. Since 1975, those tests have included ones that look for signs of dementia. In the current study, researchers analyzed data collected from more than 5,200 of the study’s participants during four time periods — or what the researchers call “epochs” — between the late 1970s and the early 2010s.

When they compared the incidence of dementia across those epochs, they found that, compared to the first epoch, the incidence of dementia declined by 22 percent during the second, by 38 percent during the third, and by 44 percent during the fourth.

The decline was due mostly to lower rates of vascular dementia, which is caused by problems with the flow of blood to the brain — usually the result of a series of small strokes. The study found no significant decline in the incidence of Alzheimer’s disease, whose cause is unknown but appears to be related to an abnormal accumulation of proteins in the brain.

Possible causes

The observed decline in vascular dementia might be explained, according to the researchers, by a reduction in circulation-related risk factors, especially smoking and high blood pressure, among the study’s participants. 

Indeed, the study found that the rate of circulation-related diseases, such as stroke and congestive heart failure, declined significantly among Framingham participants over the 30 years of the study.

The study’s findings also suggest that improvements in medical treatments may be contributing to the decline in dementia. In the late 1970s and early 1980s, people who had a stroke were nine times more likely to develop dementia than those who had not had a stroke. By the early 2000s, patients who had a stroke were only two times more likely to develop dementia.

The reasons for the association between education and a lower risk of dementia is not clear, but it may have to do with healthier lifestyle behaviors and/or better access to health care.

The study also found that improvements in heart health have occurred over the past three decades only among people with at least a high school education.

Limitations and implications

This new research comes with several important caveats. The scientists were not able to determine whether changes in lifestyle — specifically changes in diet and exercise habits — might explain the study’s results. Furthermore, most of the study’s participants were white and suburban. It’s not known, therefore, whether the findings would also apply to other population groups.

Another study, however — one published Wednesday in the journal Alzheimer’s and Dementia — suggests that dementia rates have, indeed, been decreasing among African-Americans in recent decades.

“We are expecting an explosion of dementia over the next 50 years, with devastating consequences both on a personal level and on a society level because our population is aging,” says Dr. Sudha Seshadri, a professor of neurology at Boston University and the Framingham Heart Study’s senior investigator, in a video released with the new study. “If we can, however, bend the arc of risk so that people get it later, closer to the natural lifespan, then we will be reducing the individual as well as the societal burden of dementia.”

We’re still going to see increasing numbers of people developing dementia over the next 30 years, she acknowledges. “But at the same time,” she adds, “and for the same reason, it’s important to invest more resources in understanding what we’re doing right — why the mean age of dementia is increasing in Framingham — so that we can promote whatever we’re doing right.”

FMI: You’ll find an abstract of the study on the NEJM website, but the full study is behind a paywall.