"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," she said.
Researchers studied a group 191 women between 38 and 60 years old in Sweden over a 44-year period of time (1968-2012). The participants were initially asked to complete an ergometer cycling test to evaluate their cardiovascular fitness.
"The level that you are so exhausted that you have to interrupt the test is a measure, in watts, of your work capacity," Hörder told CNN. "Cardiovascular fitness or endurance can also be tested in a submaximal test where you don't push the person to maximal capacity."
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Based on their performance in the initial testing, the women were divided into three groups: 59 were classified as "low fitness", 92 were "medium fitness" and 40 were "high fitness." The researchers then tracked the women until 2012, closely examining which groups developed symptoms of dementia and which did not.
In total, 23 percent developed some form of dementia in the proceeding decades. However, the percentage was significantly higher in the low and medium fitness groups when compared to those in the high fitness category.
"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," Hörder said. "Many of those who interrupted the test at submax, very low watt level, probably had indications for a poor cardiovascular health status. This might indicate that processes in the cardiovascular system might be ongoing many decades before onset of dementia diagnosis."
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Although the results are significant and appear to align with previous research, there were some limitations, such as the relatively small number of subjects, the fact that fitness level was only measured once, and the lack of filtering for other risk factors for dementia.
Future research, the scientists said, should look at a larger and more diverse sample, analyze physical fitness more often and also address other potential factors.
"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," Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association in Chicago, who was not involved in the new study, 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,” he added.
Approximately 5.4 million Americans are currently living with Alzheimer's disease, the most common form of dementia. According to the Centers for Disease Control and Prevention, it is sixth leading cause of death among American adults.
Globally, the World Heath Organization estimates that nearly 50 million people suffer from dementia, with 10 million new cases every year.
The Alzheimers Association suggests people should quit smoking, eat a healthy diet, get enough sleep, stay socially engaged, challenge their minds by reading or playing games and take care of their heart health to reduce their risk of cognitive decline.
Read the full study at n.neurology.org.