Problems with walking and balance may be the first sign of Alzheimer's disease, say US researchers.
The first signs of Alzheimer's may be physical
A study of 2,288 elderly people found that such physical symptoms were associated with an increased risk of developing dementia.
Researchers from the University of Washington said they believed that exercise could help to stall the progression of the disease.
Their study appears in Archives of Internal Medicine.
Previous research has suggested that exercise may reduce the risk of Alzheimer's - possibly by boosting blood flow to the brain. Researchers monitored patients every two years for signs of physical and mental decline.
At the start of the study, none of the participants had any sign of dementia but after six years 319 individuals had developed dementia - 221 of them had Alzheimer's disease.
Those with good physical performance scores at the start of the investigation were three times less likely to develop dementia than those with poor scores.
The researchers assessed physical function using a variety of tests.
The first indicators of future dementia appeared to be problems with walking and balance. A weak hand grip was a later sign.
Study leader Dr Eric Larson said: "We were surprised to find that physical changes can precede declines in thinking.
"If confirmed, this study might also help explain the association of physical exercise with a reduced risk of dementia, suggesting that exercise, by improving and maintaining physical function, might benefit cognitive function through a connection between the two."
A previous study by Dr Larson had shown that regular exercise reduced the risk of dementia and Alzheimer's disease by up to 40%.
He added: "Physical and mental performance may go hand in hand, and anything you can do to improve one is likely to improve the other."
Harriet Millward, deputy chief executive, Alzheimer's Research Trust said anything that helped to diagnose Alzheimer's at an early age would be useful.
"This is the first study relating dementia to physical function rather than intensity or regularity of exercise.
"It suggests that people who can't do some physical tasks as well may be at higher risk of developing dementia.
"It is too early to say whether some of the tests the researchers used, such as gait or handgrip, might be used to help diagnose Alzheimer's," she added.
Dr Susanne Sorensen, head of research at the Alzheimer's Society said: " "The studies findings are constant with the Alzheimer's Society's message that leading a healthy life may reduce your risk of developing dementia," she added."