By Morgan Manella Special to CNN
(CNN) -- Poor physical fitness in middle age might be associated with a smaller brain size later on, according to a study published in an online issue of Neurology.
Brains shrink as people age, and the atrophy is related to cognitive decline and increased risk for dementia, a researcher said, and exercise reduces that deterioration and cognitive decline.
In this study, more than 1,500 people at an average age of 40 and without dementia or heart disease took a treadmill test. Twenty years later, they took another test, along with MRI brain scans. The study found those who didn't perform as well on the treadmill test -- a sign of poor fitness -- had smaller brains 20 years later.
Among those who performed lower, people who hadn't developed heart problems and weren't using medication for blood pressure had the equivalent of one year of accelerated brain aging. Those who had developed heart problems or were using medication had the equivalent of two years of accelerated brain aging.
Their exercise capacity was measured using the length of time participants could exercise on the treadmill before their heart rate reached a certain level. Researchers measured heart rate and blood pressure responses to an early stage on the treadmill test, which provides a good picture for a person's fitness level, according to the study author Nicole Spartano, a postdoctoral fellow at the Boston University School of Medicine.
Physical fitness is evolving as a significant factor related to cognitive health in older age. A study published in May 2015 found that higher levels of physical fitness in middle-aged adults were associated with larger brain volumes five years later.
This study shows that for people with heart disease, fitness might be particularly important for prevention of brain aging, Spartano said.
"We found that poor physical fitness in midlife was linked to more rapid brain aging two decades later," she said. "This message may be especially important for people with heart disease or at risk for heart disease, in which we found an even stronger relationship between fitness and brain aging."
The researchers also found that people with higher blood pressure and heart rate during exercise were more likely to have smaller brain sizes 20 years later. People with poor physical fitness usually have higher blood pressure and heart rate responses to low levels of exercise compared to people who exercise more, Spartano said
"From other studies, we know that exercise training programs that improve fitness may increase blood flow and oxygen to the brain over the short term," Spartano said. "Over the course of a lifetime, improved blood flow may have an impact on brain aging and prevent cognitive decline in older age."
The study suggests promotion of physical fitness during middle age is an important step toward ensuring healthy brain aging.
"The broad message," Spartano said, "is that health and lifestyle choices that you make throughout your life may have consequences many years later."
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