Being physically fit could hold back the advance of Alzheimer's disease, US researchers have suggested.
Their study, published in the journal Neurology, looked at 121 people aged over 60, around half of them in the early stages of the disease.
Those with Alzheimer's who were less fit had four times more signs of brain shrinkage than those who were fit.
The Alzheimer's Research Trust said other research showed exercise reduced the risk of dementia.
Some 700,000 people in the UK are living with dementia, with this number predicted to grow quickly over the next two decades, as the proportion of older people in the population increases.
Other studies looking at the relationship between dementia and exercise tend to focus on whether being active can reduce the risk of the condition developing in the first place.
Dr Jeffrey Burns, from the University of Kansas School of Medicine, said his was one of the first to look at whether exercise could affect the progress of the illness.
His volunteers underwent a treadmill test to see how fit they were and then their brains were scanned for shrinkage, which is one way of measuring the severity of their Alzheimer's.
While there was no relationship between brain size and exercise in people tested who did not have Alzheimer's, Dr Burns said the four-fold difference in those who did was evidence that exercise might help.
He said: "People with early Alzheimer's disease may be able to preserve their brain function for a longer period of time by exercising regularly and potentially reducing the amount of brain volume lost.
"Evidence shows decreasing brain volume is tied to poorer cognitive performance, so preserving more brain volume may translate into better cognitive performance."
Susanne Sorensen, head of research at the Alzheimer's Society, said: "Exercise increases blood flow to the brain, delivering oxygen and nutrients to brain cells.
"This is one possible explanation why dementia progresses slower in people who are physically fit.
"Exercise also reduces your risk of developing dementia so it's important to take regular exercise. A healthy heart means a healthy brain."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "This adds to previous research showing that exercise helps reduce the risk of dementia and slows down its onset.
"A balanced diet and regular exercise can improve the quality of life of older people with dementia, as well as those who do not have the condition."