When highly fit women developed the disease, it came on average 11 years later than among those with moderate levels of fitness - at the age of 90 instead of 79.
Dr Helena Horder from the University of Gothenburg in Sweden said, "These findings are exciting because it's possible that improving people's cardiovascular fitness in middle age could delay or even prevent them from developing dementia".
At the start of the study, 191 women between 38 and 60 years old were asked to cycle on a stationary bike until they were exhausted. Results showed that while those who were physically fit and those who were not lived just as long, the women who could ride an exercise bike at a fast pace for six minutes in the initial test had a much lower risk of developing dementia than those who could not perform, USA Today reports.
Scientists found that only five per cent of the highly fit women developed dementia, compared with 25 per cent of the moderately fit and 32 per cent of those with low fitness levels.
The results meant that those with greater fitness were 88 per cent less likely to develop dementia.
Overall, those who dropped out of the tests had dementia rates of 45 per cent in later life. "This might indicate that processes in the cardiovascular system might be ongoing many decades before onset of dementia diagnosis".
"We need to see research that builds on findings like this and drives progress towards practical, evidence-based strategies that could help people reduce their risk of dementia".
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Researchers conducted initial exercise tests on middle-aged women, and followed them for 44 years.
Dementia is the name for a group of symptoms caused by disorders that affect the brain, often leading to memory loss or other problems with brain function.
About 850,000 people in the United Kingdom have dementia as a whole, according to the UK's National Health Service.
For women, physical fitness in midlife may do more than give the heart a boost; it may also benefit the brain, a new study from Sweden suggests.
"However, this study does not show cause and effect between cardiovascular fitness and dementia, it only shows an association".
Midlife has always been thought to be a "sensitive period" for the risk of dementia, which affects one in six people over the age of 80 and an estimated 850,000 people in Britain.
Worldwide, about 50 million people have dementia, and there are almost 10 million new cases every year, according to the World Health Organization.