"I was not surprised that there was an association, but I was surprised that it was such a strong association between the group with highest fitness and decreased dementia risk," said Helena Hörder, a professor in the Department of Psychiatry and Neurochemistry at the University of Gothenburg in Sweden, who was first author on the study.
The new study involved 191 women in Sweden, 38 to 60 years old, who completed an ergometer cycling test to evaluate their cardiovascular fitness.
During the test, the women's workload was measured, based on how much weight or resistance could be added to the bicycle before they became fatigued.
"The level that you are so exhausted that you have to interrupt the test is a measure, in watts, of your work capacity," Hörder said. "Cardiovascular fitness or endurance can also be tested in a submaximal test where you don't push the person to maximal capacity."
Based on their crude peak workload, the women were separated into three groups: Fifty-nine were in the "low fitness" group, 92 were "medium fitness," and 40 were "high fitness."
Those cycling tests were conducted in 1968, and the women were followed over a 44-year period until 2012. During that time, the researchers tracked the women's health, taking a close look at who was diagnosed with dementia and who was not.
The researchers found that among all of the women, 44 of them (or 23%) developed dementia from 1968 to 2012. Yet among those who interrupted their cycling test at submaximal workload, that percentage jumped to 45%.
"Many of those who interrupted the test at submax, very low watt level, probably had indications for a poor cardiovascular health status," Hörder said. "This might indicate that processes in the cardiovascular system might be ongoing many decades before onset of dementia diagnosis."
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.
The study had some limitations, including that it involved a small sample of women in Sweden. More research is needed to determine whether similar findings would occur in a larger, more diverse group.
Also, the findings were not causal. So more research is also needed to determine whether improved fitness could have positive effects on dementia risk and when in life a high fitness level is most important.
"One of the missing pieces of a study like this -- and really the weakness in the literature to date -- is that the kinds of studies that we have mostly seen are association studies. These are studies of correlations, and they can't necessarily talk about causality," said Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association in Chicago, who was not involved in the new study.
Still, he said, "the picture that is really emerging from the literature is a picture about the importance of fitness in midlife, not just old age, when it comes to protecting your brain health and preventing or delaying Alzheimer's disease and other dementias."
"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.
"The reason for that is because the brain actually is what we would call a highly vascularized organ, meaning that your brain has many blood vessels," he said. "The demand for nutrient-, oxygen-rich blood in the brain is very high compared to other organs, and so anything a person can do to increase their cardiovascular fitness level is likely to have positive benefits on brain health."