One in seven Americans over 70 has some form of dementia, according to a national study.

The population-based study included men and women from all regions of the U.S. Researchers found that about 3.4 million people, or 13.9 percent of the population, age 71 and older have a form of dementia. Click here to read more on this study

Researchers compiled data from 856 men and women in the Aging Demographics and Memory Study conducted in 2002 by the University of Michigan Institute for Social Research (ISR) and Duke University Medical Center. It was funded by the National Institute on Aging.

Trained nurses and neuropsychology technicians visited participants at their homes around the country, where they examined them using a diagnostic test similar to one used in most memory evaluations.

Researchers interviewed family and friends to find out how the participants functioned in their day-to-day activities and took DNA tests using a cheek swab to check for any presence of an APOE e4 allele — a gene linked to an increased risk of developing Alzheimer’s disease.

The information then was reviewed by a team of Duke physicians and psychologists who found that the prevalence of dementia increased by 5 percent in those 71 to 79 and by 37.4 percent of those 90 and older.

Of the 2.4 million people reportedly suffering from dementia, 9.7 percent of the population 71 and older also had Alzheimer’s disease, one of the most common causes of dementia, according to researchers.

In fact, Alzheimer’s disease accounted for most of the cases of dementia, particularly in those 90 and older, who comprised 79.5 percent of total cases, compared with 46.7 percent of those participants in their 70s.

Researchers also found the presence of one or two of the APOE e4 alleles in participants, as expected, which may have increased their risk of developing Alzheimer’s.

Overall, researchers found that Alzheimer’s disease accounted for approximately 69.9 percent of all dementia cases, while vascular dementia, often caused by stroke, accounted for 17.4 percent of total cases.

Researchers then compiled the estimated cases of dementia from earlier studies of those 71 and over with those from other studies on people 60 to 70, which resulted in a new estimated total of 3.8 million people with dementia, including more than 2.5 million with Alzheimer’s.