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Women who were reasonably fit when they were middle-aged were much less likely to develop dementia than women who struggled to exercise even a little bit, researchers reported Wednesday.

The fittest women were 90 percent less likely to later develop dementia than those who were the least fit, the team at Gothenburg University in Sweden found.

In this case, being highly fit meant everyday exercisers, not athletes.

The study suggests not only that exercise can prevent dementia, but probably also shows that people destined to develop dementia are already showing some physical symptoms decades earlier.

"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 Helena Hörder of the University of Gothenburg. "However, this study does not show cause and effect between cardiovascular fitness and dementia.”

The findings fit in with a growing body of evidence that exercise can preserve brain cells and might prevent dementia, or at least delay it.

“You don’t need to be a marathoner to achieve the ‘highest’ level of fitness described in this study.”

And they were not talking about super-fitness, but a very average ability to ride a bike for a few minutes, said Laura Baker of the Wake Forest School of Medicine in North Carolina, who is conducting her own studies into fitness and dementia.

“You can hear yourself breathing and you are starting to sweat and your heart rate is just getting up,” said Baker, who was not involved in the Swedish research.

Hörder’s team worked with data on 191 women who are part of a bigger Swedish study following people over their whole lives.

They were about 50 on average when they took an exercise test in 1968. They rode exercise bikes to see how fit they were. They’ve been followed and regularly examined ever since — some for as long as 44 years.

Over that time, 44 of the 191 women developed dementia.

Their fitness in middle age predicted their later risk of developing dementia, the team reported in the journal Neurology.

Those who could not even finish the test had a 45 percent chance of being diagnosed with dementia decades later. Those considered highly fit had just a 5 percent risk. Women of medium fitness had a 25 percent risk.

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“What is good for the heart really does seem to be good for the brain also,” Nicole Spartano, who conducts similar research at Boston University, wrote in an editorial.

“You don’t need to be a marathoner to achieve the ‘highest’ level of fitness described in this study,” Spartano told NBC News. “I would go so far to say that is very possible for even completely sedentary women to achieve this ‘high’ fitness level.

Baker agreed.

“The low group only could get as high as 80 watts in a challenge test. The best they can do is only what most people could do in the early parts of a warmup,” Baker said. “These people are just really out of shape.”

The fittest women could do 120 watts or more, which is what people put out at the start of a normal workout, Baker said. “If you can talk very easily, not really sweating — that’s 80-120 watts,” she said. A watt measures the power created during an activity.

“Above 120 watts would not be the highly trained athlete by any means. It is your everyday exerciser.”

More than 5 million Americans have Alzheimer’s, and this is expected to snowball as the population ages. There’s no cure, and treatments do not work well. Drugs such as Aricept, known also as donezepil, and Namenda can reduce symptoms for a time but they do not alter the course of the disease.

However, studies have begun to show that exercise can help, even after people already have symptoms.

Exercise beats medication

One study found that patients who walked at a moderately brisk pace for an hour, three times a week, not only did better on memory and attention tests, but they lost weight and lowered their blood pressure.

Baker said exercise works more broadly than any single medication ever could.

“It increases blood flow, not only while you are exercising but while you are at rest,” she said.

“It opens up the vessels. It helps keep the pipes clean so you get better blood flow."

Exercise lowers blood pressure, improves cholesterol levels and ramps up the immune system. It can slow the brain shrinkage seen with aging and can prevent the blockages and tangles of brain cells that mark Alzheimer's. And it does not take much, Baker said.

“I get asked all the time, ‘Is it too late to start?’ Our study so far says no, it’s not too late. You can start any time.”

“We do know that you don’t have to exercise at really high levels. You don’t have to exercise like an athlete,” she said.

Twice a week is not quite enough, because the body must start over again each time, but three sessions a week is good, she said.

Baker is running a trial now in which people with mild cognitive impairment — subtle changes in memory and thinking — are trying treadmill exercise as therapy.

“They are all sedentary to begin with,” Baker said. “We can improve cognition.”

And there is no such thing as being too old, she said.

“I get asked all the time, ‘Is it too late to start?’ Our study so far says no, it’s not too late. You can start any time.”

And people who are middle-aged and find it hard to walk around the block might take it as a warning. The Swedish researchers found those who had to be taken off the exercise bikes before they finished the test because of chest pain or some other signal of trouble had the highest rates of dementia.

“This indicates that adverse cardiovascular processes might be going on in midlife that seem to increase the risk for dementia,” they wrote.

Baker practices what she preaches.

“I exercise four to five times a week,” she said – using both an elliptical trainer and lifting some weights.

“It’s not just for brain health,” Baker added. “It is for stress management and release. If I don’t exercise, I feel like I am not at my best.”