“Stroke is a devastating condition that can be lethal and leaves most patients disabled or speech-impaired for life,” said lead study author Erik Prestgaard, a doctor with Oslo University Hospital, in a phone interview. “Prevention is important,” he added, “and patients can directly change their fitness.”

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The men in the study were enrolled in 1972-1975 at ages 40 to 59. At the start and again seven years later, their cardiorespiratory fitness was assessed with a bicycle test and with measurements of blood pressure and heart rate. Researchers then followed the men’s health for about 24 years through medical records and national registries.

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Based on the men’s cardiorespiratory fitness trend between the initial assessments seven years apart, about 39 percent of them “remained fit” (which means they maintained an average fitness level throughout). Another 39 percent “remained unfit” (started out below average and stayed there). Eleven percent “became unfit” (dropped from above average), and another 11 percent “became fit” (improved from below average to above average).

Overall, 199 men had strokes, with the highest risk seen among those who became unfit.

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The average age of first stroke was 73 in both of the unfit groups, 75 in the “remained fit” and 77 in the “became fit” groups, the study team said.

Men who had higher fitness levels while younger and became unfit had twice the stroke risk as men who remained fit, the study also found. Similarly, those starting with low fitness levels who became fit cut their stroke risk in half compared with those who stayed unfit.

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“You would expect that fitness would reduce the risk of stroke, but we were surprised by the large reduction,” Prestgaard said. In further analyses, he said, “each small improvement in fitness helped.”

Future studies should confirm these findings using better measures of cardiorespiratory fitness than were available in the 1970s, the authors said.

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The recently updated “Physical Activity Guidelines for Americans” says adults should do at least 150 to 300 minutes of moderate-intensity activity per week and muscle-strengthening activities at least two days per week.

“Fitness recognizes no age, and it’s never too old to start exercising,” said Peter Kokkinos of Veterans Affairs Medical Center of Washington, D.C. “This study shows that exercise benefits last for years, and you can see the difference all the way to 25 years,” he said in a phone interview. “Early changes have lasting effects.”