After an average follow-up of 17 years, about 4,050 participants died. Of those deaths, 164 were attributed to dementia (72vascular dementia and 92 Alzheimer's disease). Of those 164 deaths, 123 of the people were in the low-fitness group, 23 were in the middle-fitness group, and 18 were in the high-fitness group.
People in the high- and medium-fitness groups had less than half the risk of dying as those in the low-fitness group, the researchers concluded.
The study appears in the February issue of the journal Medicine & Science in Sports & Exercise.
"These findings support physical-activity promotion campaigns by organizations such as the Alzheimer's Association and should encourage individuals to be physically active," study author Riu Liu said in a journal news release.
"Following the current physical-activity recommendations from the American College of Sports Medicine will keep most individuals out of the low-fit category and may reduce their risk of dying with dementia," Liu added.
Liu conducted the study as part of her dissertation at the University of South Carolina. She is now a postdoctoral fellow at the U.S. National Institute of Environmental Health Sciences.
While deaths in the United States associated with heart disease, breast cancer and stroke have declined in recent years, deaths related to dementia and Alzheimer's rose 46 percent between 2002 and 2006, according to the release.
The study doesn't prove that exercise will prevent dementia, however. Other factors may also come into play.