How fit someone is in midlife may predict how their brain will have aged decades in the future, according to research published today in the journal Neurology.
In the study, which spanned more than 20 years, researchers saw that middle-aged people who had poor fitness ended up with smaller brain volume than their fitter peers.
Typically, the brain shrinks at a rate of about 0.2 percent a year in older age. Scientists have linked dementia and cognitive decline with an acceleration in this process. Fortunately, research has also shown that the brain is still plastic late in life, and that exercise can help older people stave off brain atrophy. In one study, for example, people between 55 and 80 years old were able to increase the volume of their hippocampus, a memory-related region, by 2 percent after one year with an aerobic walking group.
Less is known about how a person's fitness throughout life might relate to later brain aging. To find out, researchers asked 1,583 healthy people with an average age of 40 to run on a treadmill until they were too exhausted to continue or had reached a certain heart rate. To get a picture of each person's fitness level and exercise capacity, the researchers logged the amount of time people spent on the treadmill, as well as how their heart rate and blood pressure went up as they started to run.
Two decades later, the researchers repeated the treadmill test and took MRI scans of the participants' brains. The researchers discounted those participants who developed heart disease or began taking beta-blocker medications to treat high blood pressure. Of the 1,094 remaining participants, those who'd been in poor shape in middle age had smaller brain volume than people who'd had better fitness. This difference persisted even after the researchers controlled for other factors like smoking.
"During the treadmill test, people with about 17 beats per minute higher heart rate or 14 units higher diastolic blood pressure had smaller brains later in life, at a rate that we would say was equal to one year accelerated aging," said coauthor Nicole Spartano, a postdoctoral fellow at the Boston University School of Medicine.
The older participants also took a second, shorter treadmill test so the researchers could check their heart rate and blood pressure again. An exaggerated surge in people's heart rate, which signals poorer fitness, was associated with a smaller brain. But this time their blood pressure response to exercise didn't have any association with brain size.
When the researchers repeated the analysis with the whole pool, this time including people with heart disease and those on blood pressure medication, the relationship between fitness and brain volume was more pronounced.
"Fitness may be especially important for prevention of brain aging in people with heart disease or at risk for heart disease," Spartano said.
This study showed a link between poor fitness in middle age and reduced brain volume later in life; the results don't tell whether the first directly causes the second.
"But from other studies we know that training programs that improve fitness may increase blood flow and oxygen delivery 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."
Genetics can also contribute to a person's fitness level, which the researchers did not examine in this study. In future, the team plans to track exercise and brain size at more frequent points, and to see whether the results translate to a more diverse population (this study included mostly white people of European ancestry).
"Instead of just using snap-shots of fitness or brain structure and function, we hope to be able to look at changes in these lifestyle factors and how they relate to changes in brain structure and function over time," Spartano said.