The researchers also found that the average age at dementia onset was 11 years older in the "high fitness" group than in the "medium fitness" group, and the most pronounced risk reduction was seen among those with the highest fitness: "High compared to medium fitness decreased the risk of dementia by 88%", the researchers wrote.
"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", Hörder said.
Over the course of 44 years, each participant was tested for dementia six times.
Staying fit in middle age can slash women's risk of dementia by nearly 90 per cent. Some of the women were placed into the low-fitness category because they couldn't finish the test due to high blood pressure, chest pain or other problems.
She added,"More research is needed to see if improved fitness could have a positive effect on the risk of dementia and also to look at when during a lifetime a high fitness level is most important".
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.
A Swedish study of 191 women checked their fitness levels and then tracked them for 44 years.
She said limitations of the study included the relatively small number of women involved, all of whom were from Sweden, so the results may not be applicable to other groups.
Your stamina - or the time it takes for you to reach peak exhaustion during exercise - can be tied to your dementia risk, and a new study reveals how.
There are 850,000 people living with dementia in the United Kingdom and set to be over 1million by 2025.
The study measured the women's cardiovascular fitness based on an exercise test.
Regardless of which type is diagnosed, each person will experience their dementia in their own unique way.
The study did not show a direct cause-and-effect between exercise and dementia, and the results are not particularly surprising, but the difference in dementia risks was highly significant.
"There's a very strong connection between cardiovascular health - so the health of your heart and your circulatory system - and the health of your brain", Fargo said.
Only five per cent of the fittest women in the study went on to develop dementia in later life.
As a person's age increases, so does the risk of them developing dementia.