New cases of Alzheimer's disease and other forms of dementia have been waning in a number of developed countries, including the United States, in recent decades, several new studies indicate.

A study in the U.S. has found the rate of diagnoses of dementia has declined 44 percent compared with rate seen in the late 1970s, while another study that included England, the Netherlands and Sweden along with the U.S. showed similar reductions in diagnoses.

Those two studies, along with one on older Germans that found similar trends, were presented in Copenhagen at the Alzheimer's Association International Conference.

"This is some good news," Dean Hartley, director of science initiatives for the nonprofit association, said of the studies.

In the United States, around 5.2 million individuals suffer from Alzheimer's disease, considered the most common type of dementia, the association reports.

The U.S. study found that in addition to declining rates of diagnoses, the average age of patients in which doctors were finding dementia has risen, from 80 in 1978 to 85 in 2006.

"For those who get the disease, it may come later in life, which is a good thing," says Dallas Anderson, epidemiology chief at the National Institute on Aging. "Getting the disease in your 80s or 90s is a very different than getting it in your early 70s."

A leading candidate for the cause of the drop in dementia rates is improved cardiovascular health, the researchers said.

In recent decades people's cholesterol levels and average blood pressure have improved while rates of smoking, stroke and heart disease have declined, says Claudia Satizabal of the Boston University School of Medicine, who directed the U.S. study.

"What's good for the heart is also good for the brain," Hartley says, suggesting a healthy heart and blood vessels do a better job of delivering energy and oxygen to brain cells.

"The results bring some hope that perhaps dementia cases might be preventable, or at least delayed," she says.

Increased knowledge about maintaining health available to the public has also been suggested as a reason for lower diagnosis rates, the researchers said.

"For an individual, the actual risk of dementia seems to have declined," due to more education and control of health factors, says Dr. Kenneth Langa, a University of Michigan expert on aging.

The researchers pointed out that the opposite seems to be happening in poorer countries that are falling behind in healthcare and education, and dementia in many of those countries is on the rise.