It really could be the survival of the fittest for women wanting to avoid dementia in old age.

A Swedish study, published in journal Neurology, found very physically fit women were nearly 90 per cent less likely to develop dementia compared to those who are only moderately fit during middle age.

Of the small proportion of fit women who did go to have dementia, they developed the disease an average of 11 years later than the less physically fit women, or at age 90 instead of 79.

"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," said the study's author Helena Horder at the University of Gothenburg, Sweden.

During the study, 191 women of an average age of 50 took a bicycle exercise test until they were exhausted to measure their peak cardiovascular capacity.

A total of 40 women met the criteria for a high-fitness level, 92 women were in the medium-fitness category; and 59 women were in the low-fitness category.

Many in the lowest-fittest category had their tests stopped because of high blood pressure, chest pain or other cardiovascular problems.

Over a follow-up period of 44 years, the women were tested for dementia six times. During that time, 44 of the women developed dementia.

Of those, five per cent of the highly-fit women developed dementia, compared to 25 per cent of moderately-fit women and 32 per cent of the women with low fitness.

The highly-fit women were 88 per cent less likely to develop dementia than the moderately-fit women.

The researchers noted the findings do not show cause and effect between cardiovascular fitness and dementia, only an association.

"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," said Ms Horder.